NCRP REPORT No. 81
Carbon-14 in the Environment Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION A N D ...
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NCRP REPORT No. 81
Carbon-14 in the Environment Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION A N D MEASUREMENTS
Issued May 15, 1985 National Council on Radiation Protection and Measurement
7910 W O O D M O N T AVENUE
/
BETHESDA, MD. 20814
LEGAL NOTICE This report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its reports. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this report, nor any person acting on the behalf of any of these parties (a) makes any warrantly or representation, express or implied, ith respect to the accuracy, completeness or usefulness of the information contained in this report, or that the use of any information, method or process disclosed in this report may not infringe on privately owned rights; or (b) assumes any liability with respect t o the use of, or for damages resulting from the use of, aliy information method or process disclosed in this report.
L i b r a r y of Congress Cataloging i n Publication D a t a National Council on Radiation Protection and Measurements. Carbon-14 in the environment. (NCRP report; no. 81) "Issued May 15, 1985." Bibliography: p. 74 Includes index. 1. Carbon-Isotopes-Environmental aspects. 2. Carbon-Isotopes-Physiological effect. 3. Carbon-Isotopes. I. Title. 11. Series. QH545.C37N38 1984 628.5 84-29586 ISBN 0-913392-73-1
Copyright O National Council on Radiation Protection and Measurements 1985 All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews.
Preface Catbon-14, with a half-life of 5.7 x lo3 years, is produced naturally by cosmic ray interactions in the atmosphere. This source has been augmented by anthropogenic sources such as nuclear weapons testing and, to a lesser extent, emissions from nuclear power plants. The important human exposure to 14C is internal, as the result of inhalation of atinospheric air and ingestion of food. The metabolism and kinetics of radiocarbon in the human body follow those of ordinary carbon. A fraction of the carbon introduced into the body is, including any I4C, retained as protein, fat, carbohydrate and other materials until equilibrium is reached. This report considers the importance of 14Cas a potential source of local and worldwide radiation exposure. The available information on 14Cis examined with regard to its physical properties, sources, distribution in the environment, and sampling and measurement. The behavior of 14Cin biological systems, projected impact, dosimetry, and waste management is also examined. The dose rate to man from 14Cis evaluated for naturally produced "C, for the amount of 14Cproduced by nuclear weapons testing, and for the projected amounts of 14C produced as the result of nuclear power given certain scenarios. Projections of human exposure to I4C from the various sources are compared as to significance. The present report is one of a series of reports produced by the Task Groups of Scientific Committee 38 on important radionuclides, pmduced both naturally and man-made, which are perceived as having an impact, real or potential, on the exposure of man. The Council has noted the adoption by the 15th General Conference of Weights and Measures of special names for some units of the Systime International &Unites (SI) used in the field of ionizing radiation. The gray (symbol Gy) has been adopted as the special name for the SI unit of absorbed dose, absorbed dose index, kerma, and specific energy imparted. The becquerel (symbol Bq) has been adopted as the special name for the SI unit of activity (of a radionuclide). One gray equals one joule per kilogram; and one becquerel is equal to one
second to the power of minus one. Since the transition from the special units currently employed-rad and curie-to the new special names is expected to take some time, the Council has determined to continue, for the time being, the use of rad and curie. To convert from one set of units to the other, the following relationships pertain: 1 rad = 0.01 J kg-' = 0.01 Gy 1curie = 3.7 x 101Os-l= 3.7 x 10'' Bq (exactly). The present report was prepared by the Council's Task Group on 14Cof Scientific Committee 38 on Waste Disposal. Serving as Chairman of Scientific Committee 38 was: Merril Eisenbud Institute of Environmental Medicine New York University Medical Center Tuxedo, New York
Serving on the Task Group were: A. Allen Moghissi, Chairman Institute for Regulatory Science Alexandria, Virginia Members Philip W. Krey Department of Energy New York, New York
John R. Totter Oak Ridge National Laboratories Oak Ridge, Tennessee
John M. Matuszek Radiological Sciences lnstitute Center for Laboratory and Research Albany, New York
Robert W. van Wyck Consolidated Edison New York, New York
Lester Machta National Oceanic and Atmospheric Administration Rockville, Maryland
NCRP Secretariat-Thomaa Fearon E. Ivan White
The Council wishes to express its appreciation to the members for the time and effort devoted to the preparation of this report. Warren K. Sinclair President, NCRP Bethesda, Maryland January 15, 1985
Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction a n d Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Properties of I4C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Sources of 14C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Natural 14C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Atmospheric Nuclear Weapons Tests . . . . . . . . . . . . . 3.4 Nuclear Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 I4CProduction in Light Water Reactors . . . . . . . . 3.4.3 14CProduction in Fast Reactors . . . . . . . . . . . . . . . 3.4.4 14CProduction in Graphite Moderated Reactors . 3.4.5 14C Production in Heavy Water Reactors . . . . . . . 3.4.6 Release Estimates in Nuclear Power Industry . . . 3.4.7 Reduction of Releases . . . . . . . . . . . . . . . . . . . . . . . . 3.4.8 Interpretation of Release Estimates . . . . . . . . . . . . 4 . Distribution of 14C in t h e Environment . . . . . . . . . . . . . . 4.1 Distribution of Carbon a n d Carbon- 14 i n the Biosphere . . . . . . . . . . . . . . . . ...................... 4.1.1 Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Terrestrial Biosphere . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Oceans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Reliability of Reservoir Estimates . . . . . . . . . . . . . . . 5 Sampling a n d Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Collection of I 4 C From Air . . . . . . . . . . . . . . . . . . . . 5.2.2 Collection of 14CFrom Water . . . . . . . . . . . . . . . . . 5.2.3 Collection of Biota and Soil Samples . . . . . . . . . . . 5.2.4 Collection of Urine . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Sampling for 14CParticles . . . . . . . . . . . . . . . . . . . . 5.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Sample Combustion . . . . . . . . . . . . . . . . . . . . . . . . .
. .
.
5.3.2 Solid Source Counting . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Internal Gas Counting . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 Liquid Scintillation Counting . . . . . . . . . . . . . . . . . 5.3.5 Direct Ion Mass Spectrometry . . . . . . . . . . . . . . . . 6.3.6 Laser Absorption Spectroscopy . . . . . . . . . . . . . . . . 6.3.7 Isotopic Enrichment . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Presentation of I4C D a t a . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Behavior of 14Ci n Biological Systems . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Carbon-14 Uptake a n d Retention-Ingestion . . . . . . 6.3 Carbon-14 Uptake a n d Retention-Inhalation . . . . . 6.4 DNA Incorporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Carbon-14 i n Human Food . . . . . . . . . . . . . . . . . . . . . . 6.6 Concentration Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Kinetics of Localized Releases on Vegetation . . . . . 7 Projected Radiation Doses from '"C . . . . . . . . . . . . . . . . . . 7.1 Environmental Models . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Compartment Models . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2 Projected Environmental 14C Specific Activity from Nuclear-Power Products . . . . . . . . . . . . . . . . . 7.2 Dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Steady-State Specific-Activity Dosimetry Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Dose t o Man'from 14C i n t h e Environment . . . . . . . . 7.3.1 Natural Carbon-14 . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2Weapons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 Nuclear Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.WasteManagement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Nuclear P o w e r Reactors . . . . . . . . . . . . . . . . . . . . . . . . . 8.1-1 Removal and Disposal . . . . . . . . . . . . . . . . . . . . . . . 8.2 Institutional Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Waste Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Mobility of 14C Following Shallow-Land Burial . . 8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . APPENDIX A: Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The NCRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NCRP Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
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1. Introduction and Summary This report summarizes the available information on 14Cin terms of its physical prop rties, sources, distribution in the environment, sampling and analys~s,biology, projected impact, dosimetry, and waste management; and considers and evaluates its importance as a potential source of local and worldwide radiation exposure. Carbon-14 is produced naturally by cosmic ray interactions in the atmosphere. This natural source has been augmented by anthropogenic sources which include fallout from nuclear weapons testing and, to a lesser extent, emissions from nuclear power reactors. The specific activity of the atmospheric 14Cis continually reduced by the combustion products of fossil fuels and other sources which release stable carbon to the atmosphere diluting I4C concentrations. This is known as the Suess effect (Suess, 1955). Naturally produced I4C is formed by the reaction of neutrons of cosmic ray origin in the upper atmosphere with nitrogen, oxygen, and carbon. The reaction of neutrons with 14Nis the predominant source. The rate of production and the resulting concentration of cosmogenic 14C are functionally related to the variation in cosmic-ray flux and energy spectrum. The average production rate of cosmogenic 14C is estimated to be 0.038 MCi/y leading to a current global inventory of about 3.8 MCi in the atmosphere (UNSCEAR, 1977). The detonation of thermonuclear devices introduced an estimated 9.6 MCi of "C into the atmosphere and the current atmospheric inventory from both sources above is about 13.4 MCi. Carbon-14 is also produced in nuclear reactors as a result of absorption of neutrons by nitrogen, carbon, or oxygen present as components of air, coolant, moderator, structural materials, fuel, or impurities. Section 3 presents estimates of 14C production for a Boiling Water Reactor (BWR), Pressurized Water Reactor (PWR), Graphite Moderated Reactor (GMR), and Liquid Metal Fast Breeder Reactor
e
(LMFBR). Approximately 95 percent of exchangeable carbon resides in the oceans. In the atmosphere, the predominant form is carbon dioxide. Carbon-14 released to the environment enters the carbon cycle of exchangeable carbon reservoirs (i.e., atmosphere, terrestrial biosphere, 1
2
/
1. INTRODUCTION AND
SUMMARY
ocean, ocean sediment, and organic shale) and is subject to the same exchange kinetics as stable carbon except for the oceanic buffer factor and fractionation. The kinetics of uptake, retention, and elimination of humans exposed to carbon-14, either by inhalation of 14C02or by ingestion of food containing 14C,follow those of the carbon containing compound. Only a small fraction of ingested carbon is retained by the body in the form of protein, fat, or carbohydrates. Most of the ingested carbon is eliminated largely as C02 or urea. Inhaled 14C02rapidly equilibrates with the air in the lung, and enters many organic components of body tissue. The ingestion pathway is the primary route for 14C incorporation. The specific activity of 14C02(pCi/gC) in the human body .will be the same as that observed in environmental material under static conditions. ,Estimates based on static equilibrium with present environmental 14Clevels indicate that the human radiation dose is now on the order of 1.5 mrem/y. When a significant localized release occurs, the dynamics of 14C must be considered. Calculations based on a dynamic, localized release model (Killough and Rohwer, 1977)indicate that, under unusual conditions, local vegetation near a source may have a specific activity up to three-fold that expected from the global average 14C02. Section 7 presents a mathematical model based on a series of compartments, each of which represents a reservoir of exchangeable carbon. This model was used to predict (see Fig. 7.4) the I4C specific activity from nuclear power production as a function of time. These predictions indicate that the contribution of atmospheric I4C from fallout will continue to decrease with time and that the nuclear power contribution will increase but will remain at a level two orders of magnitude less than natural 14C. Fig. 7.6 shows whole body dose equivalent for measured and predicted levels of atmospheric 14Cfor the period 1955 to 2000. This data reflects a decrease of annual dose equivalent from present to the year 2000 because of the expected combustion of significant quantities of fossil fuels which will dilute atmospheric I4C with additional stable carbon.
Properties of 14C The earth contains about 1.6 x loz4g of carbon. However, only 4.1 lo1' g exchange among the atmosphere, the oceans, and the biosphere. Roughly 95 percent of the exchangeable carbon with which the airborne I4C will interact resides in the oceans. In the atmosphere, virtually all of the carbon is in the form of carbon dioxide; globally, less than 1 percent is in the form of carbon monoxide, methane, formaldehyde, and other molecules. Carbon appears in nature as one of several isotopes in the indicated percentages: I2C, 98.9 percent; I3C, < 1.1 percent; and 14C,<< 0.1 percent; other carbon radioisotopes, << 0.1 percent. Hydrocarbons and carbon monoxide, react with OH radicals in the atmosphere and are converted to carbon dioxide. Conversion times may be days to weeks for carbon monoxide and 1 to 10 years for methane (NAS-NRC, 1977). On the other hand, carbon dioxide is converted to carbon monoxide by photons from the ultraviolet end of the solar spectrum, mainly in the upper stratosphere. In theory, the equilibrium between carbon monoxide and carbon dioxide favors carbon dioxide by a factor of about 1,000 or more up to altitudes of at least 40 km. Observational evidence to about 13 km indicates over 300 mL/m3 of carbon dioxide to less than 0.1 mL/m3 of carbon monoxide. The reaction of carbon dioxide in the photosynthesis process or its absorption through the air-water interface is associated with an isotope effect. In every case, the heavier isotope of carbon reacts slower. Fractionation factors are: for air to water, 0.972; water to air, 0.955; air to biota, 0.904; and water to inorganic carbon, 0.966 (Fairhall and Young, 1970). The isotopic fractionation factors represent the relative equilibrium ratios of I4C to 12Cin each of the respective media. Other than fractionation a t these interfaces, 14Cand 12Cbehave alike. Carbon-14 was undetected in nature until after its systematic preparation in the laboratory. Ruben and Kamen (1940) and Kamen and Ruben (1941) produced a carbon isotope of mass number 14 by bombarding a graphite probe, enriched in 13C,with low energy deuterons (3 to 4 MeV). Carbon-14 was later identified in nature by Libby et al. (1955). Carbon-14 decays to I4N by emission of a beta particle with a X
3
/
4
2. PROPERTIES OF 14C
maximum energy of 0.156 MeV (100 percent) and an average energy of 0.045 MeV (Lederer et al., 1978). Table 2.1 presents some of the important nuclear properties of 14C together with those of the other carbon isotopes. The half-life of 14Chas been measured by absolute counting together with mass spectrometric determinations. Table 2.2 presents a sum-
TARLE 2.1-Properties of carbon isotopes
Nuclide
Isotopic Mass
Decay
10.01700 11.01114 12.00000 13.00335 14.00324 15.00940
i9+
(nC = 12~00000~
l0C llC 12C 13C 14C
lGC
Mode
p+ stable stable
68-, r
Maximum Energy
(MeV)
1.9 0.98 -
0.156 9.8 (83, 5.3 ( 7 )
Natural Half-life
Abundance (%)
191 s 20.4 min 5730 y 2.3 s
-
98.89 1.11 10-la
-
16C 16.00963 --0.74 s 'This value is based on the atmospheric concentration of "C of 6 pCi/gC. Carbon is fossil materials, ocean sediments or geologically aged materials contain no 'C.
TABLE 2.2-Summary Half-Life (Years)
103 - 105 21,000 4,000 4,700 f 470 5,300 f 800 5,100 f 200 7,200 500 6.400 f 200 6,100 200 5,580 f 45
+
*
+
5,589 f 75 5,513 + 165 6,360 f 200 5,360
* (200)
5.568 5,745 5,680 5,730 f 40
-
of
the published values for the half-life of 14C References
Ruben and Kamen (1941) Langsdorf and Purbrick (1945) Reid et aL (1946) Nonis and Inghram (1946) Norris and Inghram (1948) Yaffe and Gmnlund (1948) Miller (1947); Brown and Miller (1947) Hawkings et aL (1948); Hawkings et aL (1949) Engelkemeir et al. (1949); Engelkemeir and Libby (1950) Jones (1949) Miller et al. (1950) Miller (1947); Brown and Miller (1947); Miller et (2.1. (1950) Miller (1947); Brown and Miller (1947); Manov and Curtis (1950) Libby (1955) Hughes and Mann (1964); Mann et al. (1961) Olsson et al. (1962) Godwin (1962)
2. PROPERTIES OF "C
/
5
mary of the published values for the half-life of 14C.The most probable value for the half-life of 14Cis 5730 y (Lederer e t al., 1968). However, the Sixth Radiocarbon Conference held at Pullman, Washington on June 11, 1965 (Berger and Suess, 1979) reconsidered the question concerning the half-life that would be recommended for use in expressing radiocarbon dates. The Conference recommended that for use in expressing radiocarbon dates, the previously used value of 5568 y (Libby, 1955) be retained. This was to avoid the confusion that would arise with the many published dates and in recognition of the fact that there are discrepancies between the radiocarbon chronology and other chronologies that would not be corrected by a change in half-life (Johnson, 1965). The Tenth and Eleventh Radiocarbon Conferences (Radiocarbon, 1980; 1983) have not changed this convention. A convention of reporting 14Catmospheric data has been instituted to distinguish data prior to atmospheric nuclear weapons testing. Time prior to 1950 is denoted as the time in years prior to 1950 followed by the initials B.P. ("before present").
3. Sources of 14C 3.1 Introduction Carbon-14 is formed naturally in the upper atmosphere by reaction of neutrons of cosmic ray origin with nitrogen and, to a lesser extent, with oxygen and carbon. Large amounts of 14Chave also been released to the atmosphere as a result of nuclear weapons testing. All nuclear reactors produce 14C due to capture of neutrons by nitrogen, carbon, or oxygen present as components of the fuel, moderator, structural hardware, or in impurities. The neutron-induced reactions are: 13C(n,y)'4C;I4N(n,p)l4C;l5N(n,d)I4C;"j0(n,3He)14C;170(n,a)14C.The oxygen reactions will occur in any reactor containing heavy-metal oxide fuels or water as a coolant. The (n,-y) reaction in 13C will be important in carbide fuel reactors, while the (n,p) and (n,d) reactions will occur in all reactors containing nitrogen as an impurity in the fuel, coolant, or structural materials. Most of the I4C formed in the coolant and moderator of light water reactor, and in the deuterium oxide moderator and annulus gas of heavy water reactors, will be converted to a gaseous form and will be released from the reactor site. On the other hand, 14C formed in the fuels or in the graphite of high temperature gas cooled reactors (HTGR) will be converted to a gaseous form at the fuel reprocessing plant, primarily as carbon dioxide. Both sources will be released to the environment unless special equipment is installed to collect and convert them to a solid for long-term storage or disposal. Carbon-14 is also produced commercially for preparation of labeled materials used in medical or biological tracer research.
3.2 Natural 14C Natural 14C is formed primarily from 14N in the upper atmosphere by capture of neutrons produced by cosmic-ray interactions. The production rate of cosmogenic I4C is functionally related to variations in the cosmic-ray flux and energy spectrum. The production rate of secondary neutrons and consequently of natural 14C increases with 6
3.2 NATURAL "C
/
7
altitude and reaches a maximum at a pressure altitude of between 120 to 75 millibar (15 to 18 km). However, because of the distribution of the mass of air with height, half of the natural 14C resides in the stratosphere and half in the troposphere. Averaged over the 11-year sunspot cycle, the global production rate of cosmogenic 14Chas been estimated to be 0.038 MCi/y (UNSCEAR, 1977). Fig. 3.1 provides the time history of 14Csince 1700 in dated tree rings. Records earlier than 1700 yield fluctuations of the same general type and magnitude as observed in Fig. 3.1 between 1700 and 1850. These variations have an amplitude of about 10 percent of the mean value and are generally inversely related to solar activity. According to Stuiver (1961), galactic primary cosmic rays, principally high energy protons that generate 14C-producing neutrons in the stratosphere, are more likely to be scattered out of the neighborhood of the earth's orbit during periods of increased solar activity. Stuiver and Quay (1980) have also attempted to relate changes in the 14C production to variation in geomagnetic field intensity. After about 1900 A.D., the 14Cto 12C ratio in tree rings exhibits a clear-cut decrease caused by the dilution of 14Cby stable carbon due to the combustion of fossil fuel during the industrial era, commonly known as the Suess effect (Suess, 1955). After 1950, the concentration of 14Cin the environment rose sharply due to nuclear weapons testing. Cosmic ray 14C is produced largely in the stratosphere. Mixing between the stratosphere, the troposphere, and the surface or mixed layers of the oceans is characterized by exchange times of the order of years to tens of years, much shorter than the half-life of 14C.However, transfer into and out of the deeper portions of the oceans takes hundreds to thousands of years. Thus, the deep oceans will have a lower I4C to I2C ratio than the atmosphere. The surface ocean is depleted by about four percent (apparent age of about 300 years) while the deep ocean by about 17 percent (apparent age of about 1350 years). The depleted 14Cin the surface waters relative to air results from two processes. First, a slow exchange of deep water with the surface results in a dilution of surface 14Cwith 12C.Second, the fractionation of 14C relative to 12Cfrom air to water of 0.972 and 0.955 from water to air will produce depletion of 14Cin all water relative to air even with rapid exchanges between all reservoirs. Cosmogenic 14Coxidizes soon after formation. According to MacKay et al. (1963), the main product is initially 14C0. This is confirmed by limited measurements showing significant 14Cactivity in atmospheric CO. Other attempts to measure 14C in stratospheric CO have been unsuccessful by Hagemann et al. (1965). His balloon sampling tech-
DATE
Fig. 3.1 Fludtuation of specific activity of "C during the last three centuries. The A1'C scale expresses the deviation of the "C activity (pCi/gC) from an age-corrected oxalic acid standard as a fraction of lo3 (per mil, O/oo). The full curve is drawn smoothly through averages of these data obtained for each period of solar activity. The sources of data (distinguished by different symbols) are: 0, Lerman et al., 1970, Northern Hemisphere; W, Lerman et a l , 1970, Southern Hemisphere; 0, Suess, 1965; Houterman et aL, 1967, Northern Hemisphere; A, Stuiver, 1969, Northern Hemisphere (from Ekdahl and Keeling, 1973).
3.3 ATMOSPHERIC NUCLEAR WEAPONS TESTS
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9
nique, however, may have been too insensitive to detect the 14C0from cosmic ray production. Carbon-14 also appears in methane and other hydrocarbons in ground level air indicating some exchange of carbon atoms among various molecules. Since CO and hydrocarbons are formed from fossil fuel combustion, the fraction of manmade pollutants can be estimated from the I4C activity in the gas (Rosen and Rubin, 1965; Currie and Klouda, 1982).
3.3 Atmospheric Nuclear Weapons Tests
An estimated 9.6 MCi of '"C have been introduced into the atmosphere through the detonation of thermonuclear devices. This is the sum of the vertical bars from major tests which appear in Fig. 3.2. These inventories are based on an estimate of 0.02 MCi/megaton of total energy for air bursts and half that value for ground detonations (Machta et al., 1963). Virtually all of this radioactivity is believed to be in the form of I4CO2,the only form in which it has been detected. Most of the 14Cfrom weapons tests has been directly inserted into the stratosphere, as illustrated in Fig. 3.3. The tropospheric and stratospheric inventories of bomb 14Cbased on figures such as that of Fig. 3.3 for the period March 1955 through July 1969 are shown in Fig. 3.2 (Telegadas, 1971). These latter inventories are slightly lower than those estimated from the power of the nuclear detonations but lie within the uncertainty of the two estimates. If the concentration per elemental mass unit or the specific activity (pCi 14C/gC) is the same in the atmospheric compartments, the tropospheric inventory will contain about four-fifths of the entire atmosphere (stratospheric mass = l/4 trophospheric mass). Thus, Fig. 3.2 shows that after the major nuclear testing period in 1961-1962, the stratosphere contained more 14Cthan did the troposphere, but within a few years the inventory is reversed; and after many years of atmospheric mixing, the tropospheric inventory will become about four times that of the stratospheric inventory if no further atmospheric nuclear tests occur. Fig. 3.4 illustrates a few examples of 14C measured in ground level air over the globe. Geographical differences are small. By the end of 1970, the bomb I4Cin air had been reduced to about half of the cosmic ray background concentration of about 6 pCi/gC. The transfer of bomb 14C from the stratosphere to the troposphere, biosphere, and oceans can be approximated by a compartment model. Calculations of the long-term fate of the 9.6 x lo6 Ci of 14Cbased on such a model appear
10
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SOURCES OF " C
Fig. 3.2 The time history of nuclear weapons tests produced "C inventories in the troposphere and in the stratosphere (up to about 30 km), assuming no change in the height of the troposphere and a non-varying concentration of atmospheric "C02. Vertical bar denotes the total "C injected into the stratosphere during a testing perind whose duration is denoted by the solid horizontal bar on the abscissa (based on Telegadas, 1971).
graphically in Fig. 3.5. By the year 2500, the air concentration of 14C from weapons tests (0.056 pCi/gC) will have been reduced to one percent of the adjusted (for new fossil fuel 12C)cosmic ray 14C specific activity (4 pCi/gC) based on a 12C02atmospheric concentration of 475 mL/m3. 3.4 Nuclear P o w e r 3.4.1 Introduction
Carbon-14 is produced in nuclear power reactors from absorption of neutrons by carbon, nitrogen, or oxygen which may be present as components of coolant, moderator, structural materials, fuel, or as
3.4 NUCLEAR POWER
NORTH OECEYBER 1964. FEBRU4RY
/
11
SOUTH 1965
Fig. 8.3 A cross section from pole to pole showing concentrations of man-made "COZ. Heavy solid and dashed lines are schematic or average tropopause positions. Dots and crosses indicate locations near which samples have been collected. The difference
between crosses and dots reflects the laboratory performing the radiocarbon analyses. Concentrations are expressed in 106 I4C atoms/g air. The samples a t altitudes above about 20 km were collected aboard balloons. All samples collected in the time interval September-November 1963 have been averaged. The thin lines (solid and dashed) are subjectively interpolated and extrapolated isolines of "C concentration from the obsewed data (from Telegadas, 1971).
impurities. These neutron-induced reactions and cross sections are shown in Table 3.1 for Light Water Reactor, Liquid Metal Fast Breeder Reactor, and High Temperature Gas Cooled Reactor neutron spectra.
8961 mold . p i e p u ~ P!38 ~s
~JWXO 96.0
'(6861) 70 7J lesaS uo Pawq 9L6I puOLa9 f(9L6T)70 73 1vpKN rnog UaV? 9L6T 0% an098 passaidxa ulea3o aql jo SW8M aalejrns aq? u! pue araqdsomqe aw u! U O ~ J E ~ O P-S ~ B 'B!a ~
3.4 NUCLEAR POWER
/
13
Fig. 3.5 Computed tropospheric specific activity of "C due to the injection of 9.6 MCi by weapons tests. After about the year 2500 A.D., most of the reduction in specific activity in the atmosphere will be due to radioactive decay.
TABLE 3.1-Effective cross sections (in barns. lo-'' cm2) for formation -of "C
Reaction No. 1
2 3 4 5
Reaction
Cmassection'
13C(n,r)'4C 14N(n,p)I4C 16N(n,d)14C 160(n,'He)I4C "O(n. 'He)14C
0.9 mb 1.8 b 0 0 0.235 b
LWR LMFBR spectra --spectra
1.00 mb 1.48 b 0 0 0.183 b
0.5 pb 12.6 mb 1.0mb 0.05 pb 0.12 mb
HTGR
epectra
0.42 mb 1.02 b 0 0 0.11 b
" For 2000 m/s neutrons.
The energy-dependent cross sections are collapsed into a single, effective cross section which applies to the neutron spectrum of each type of reactor. The values listed are taken from Davis (1979). Reaction (1) is important primarily in an HTGR or other reactor which has a high carbon content. Reactions (2) and (3) will occur in reactors containing nitrogen as an impurity or as a constituent in the fuel, coolant, or structural materials. Reactions (4) and (5) will occur in reactors containing heavy-metal oxide fuels or water as the coolant. and the The fractional isotopic abundance of 1 7 0 is 3.9 x thermal cross section is about '/7 of 14N. Therefore, there is a 20,000
I
14
/
SOURCES OF "C
to 1 ratio of I4Cproduction in favor of nitrogen for thermal reactions. This ratio extends to about 180,000 to 1 in favor of the I4N reaction over the I3C reaction. Since reactor materials and impurities differ significantly from one type of reactor to another, 14Cproduction rates will vary accordingly but will be strongly influenced by the amount of nitrogen available for reaction. Production of I4C must be determined for each reactor type and requires evaluation of materials and impurities, particularly nitrogen, within the reactor core. 3.4.2
I4CProduction in Light Water Reactors
3.4.2.1 Formation in Fuel. Carbon-14 is formed primarily by reac-
tions (2) and (5) in the fuel. The quantity of 14Cproduced in fuel from reaction (2) will depend on the amount of nitrogen contained within it. A survey of analytical data from five fuel manufacturing companies shows that the nitrogen content in LWR oxide fuel varies from 3 to 50 rg/g with a median value of about 25 rg/g (Davis, 1979). The production corresponding to a nitrogen concentration of 25 pg/g averaged for both BWR and PWR light water reactors with a fuel burnup of 27,500 (BWR) or 33,000 (PWR) MW(t)d/MgU, is 16 Ci/ GW(e)y (Davis, 1979). The quantity of 14Cformed from reaction (5) can be calculated on the basis of the stoichiometry of U 0 2 and the abundance of 170in normal oxygen. On this basis, I4Cproduction in LWR oxide fuel, with the same fuel burnup, is calculated to be about 0.1 Ci of 14C/MgU which corresponds to a production rate of about 3.5 to 4 Ci/GW(e)y (Davis, 1979). 3.4.2.2 Formation in Core. Hardware Core structural materials include stainless steel support hardware, Zircaloy cladding, and nickel alloys used as fuel and fuel bundle hardware. Table 3.2 describes the quantities of these metals in a BWR and PWR core, an estimate of the range of contained nitrogen, and the amount of 14C produced from reaction (2). Calculated 14C,production is based on burnup of 27,500 MW(t)d/MgU in four year for a BWR and a burnup of 33,000 MW(t)d/ MgU in three year for a PWR (Davis, 1979). The ranges of 14C production calculated for a BWR and PWR, taking into consideration variation of nitrogen content in stainless steel, are 52 to 72 Ci/GW(e)y, and 30 to 43 Ci/GW(e)y, respectively. However, a large fraction of the stainless steel hardware in core-support structures is subjected to a neutron flux which is lower than the core average. Therefore, the calculated 14C formation in stainless steel listed in Table 3.2 is conservatively high.
3.4 NUCLEAR P O W E R
TABLE 3.2-Production Material
/
15
of "C in core hardware of light-water reactors Concentration of material in core (kg/MgU)
Concentration of nitrogen in metal in core material (g/MgU)
Total calculated "C production Ci/GW(e)ya
Boiling-Water React09 Zircaloy-2 (Grade RA-1) 304 stainless steel Inconel-X
316 50 3.4
<25.3 50-80
17.4 34.655.1 0.0 Total range, 52-72
Pressurized Water Reactorc Zircaloy-4 (Grade RA-2) 302 stainless steel 304 stainless steel Inconel 718 Microbrace 50
235 4.2 37.1 12.8 2.6
G18.8 4.2-6.7 37.1-59.4
9.5 2.1-3.4 18.8-30.0 0.0 0.12 Total-range. 30-43 -
-
-
0.2
" Based on thermal-to-electric efficiency of 0.33, corresponding to 40.2 MgU/GW(e)y for a PWR. ORICEN calculations assume 18.823MW(t)/MgU 4 years in reactor, to 27,500 MWd/MgU; 2.6 wt % as U. 'ORICEN calculations assume 30.0MW(t)MgU, 3 years in reactor, to 33,000 MWd/ MgU, 3.3 wt 9% as U.
3.4.2.3 Formation in Cooling Water. Oxygen of the cooling water, dissolved nitrogen, and nitrogen-containing chemicals in the water are sources of 14C. A calculation of the quantity of 14C produced from reaction 5 in oxygen in the cooling water of both BWRs and PWRs, assuming a ratio of 12,000 g-atoms of O/MgU (corresponding to the density of water at 300eC), indicates that about 5 to 6 Ci of I4C/ GW(e)y are produced from this source (Davis, 1979). Nitrogen dissolved in cooling water of BWRs and PWRs varies significantly depending upon operational parameters. Nitrogen dissolved in BWR cooling water varies from one mL/kg up to 25 mL/kg depending on whether Np is utilized as a cushioning gas in the pressurizer during start up and the quantity of nitrogen purged prior to full power operation. Carbon-14 production from reaction (2) with nitrogen dissolved in cooling water is calculated to be less than one Ci/GW(e)y for a BWR (RubIevskii et al., 1973) and ranges from one to five Ci/GW(e)y for a 1000 MW(e) PWR based on previous assumptions for fuel burnup. Carbon-14 production from reaction (2) in nitrogen containing compounds added to the cooling water for chemical treatment varies depending upon the type of reactor and the treatment utilized. Some current design 1000 MW(e) PWRs add about two L of hydrazine
16
/
SOURCES OF "C
solution to the cooling water to scavenge dissolved oxygen during start up (an operational rule of thumb is to add about one L of 35 percent solution for each ppm of dissolved oxygen). This treatment will add about 240 g of nitrogen to the core of the reactor leading to a production of about three Ci/GW(e)y for a large PWR based on the same assumptions as utilized for core formation of 14C. 3.4.2.4 Summary of 14CFormtion in L WRs. Estimated "C production in LWRs as determined above is summarized in Table 3.3. The formation in PWRs is estimated to range from about 60 to 80 Ci/ GW(e)y and for BWRs from about 80 to 100 Ci/GW(e)y. Other estimates of 14Cproduced in pressurized water reactors and boiling water reactors range from a low of about six to more than 100 Ci/GW(e)y. These estimates are based on direct measurements of releases (Blanchard et al., 1976; Fowler and Nelson, 1979; Kahn et al., 1971; Kunz et al., 1974; Bray et aL, 1977; Ride1 et al., 1976; Schiittelkopf, 1977; Schriber e t al., 1980; Kijig, 1980; Arndt et al., 1980; Winkelman et al., 1982) and calculations (Kelly et al., 1975; Magno et al., 1974 ERDA, 1975; Bray et d, 1977; NEA, 1980; Bonka, 1980). Lower estimates generally result from direct measurements a t plant sites which reflect releases of 14Cformed primarily in cooling water. Calculated production values generally include the much greater formation in core hardware. Chemical analysis of reactor effluents has shown that a significant fraction of "C released from two PWRs is in the form of methane and ethane (Kunz et al., 1974) and from a BWR is in a chemical form other than COz (Kunz et al., 1975; Wahlen and
TABLE3.3-Summon, of "C formation in linht water reactors -
Reactor
Trpe
PWR
-
Fuel
- -- -
-
-
m g e of
Source of Formation
Formation Matrix
"C Formed (Ci/Gw(e)y)
-
4 16
0 2
N Cooling Water
6
0 2
NP(dissolved) N (added) BWR
Core Hardware
N (impurity)
Fuel
0 2
1-5 3 30-43
Total
60-80
Total
4 16 6 <1 52-72 80-100
N Cooling Water
0 2
Core Hardware
N (impurity)
N2 (dissolved) --
3.4 NUCLEAR
POWER
I
17
Kunz, 1978; Sweibach et al., 1978). Winkelman et al., 1982 have summarized 14C releases from German power and research reactors for the period 1978 to 1981. The annual release from PWRs was 5.6 Ci/GW(e)y and that from BWRs was 13.5 Ci/GW(e)y. Detailed measurements reported by these authors indicate that 14C released from PWRs was overwhelmingly in a chemical form other than Con. Data reported by Rublevskii et al. (1973) show substantially greater (300 Ci/GW(e)y) release levels which deviate significantly from other reported values. The reasons for the increased releases are not known, but may be due to the use of nitrogen as the blanket or pressurizer gas in water cooled reactors (Davis, 1979). Since these practices are not used in modern or operating LWRs, Rublevskii's data have not been considered in arriving at release estimates in this section. 3.4.2.5 Estimate of Release to the Environment. In estimating releases of 14C to the environment, it is assumed that the quantity formed in core hardware wiU remain with the metal. Furthermore, because of other activated radioactive products in the hardware, it is assumed that the metal will be disposed of in a manner which will prevent release of 14C. Calculated formation rates in Table 3.3 show that 14Cproduced in LWRs, excluding the amount produced in core hardware, ranges from 30 to 34 Ci/GW(e)y for PWRs and about 26 Ci/GW(e)y for BWRs. If it is assumed that most 14Cin cooling water is released at the reactor site, the quantity released from a PWR is estimated to be about 10 to 14 Ci/GW(e)y and that from a BWR is about six Ci/GW(e)y. For the purposes of this section, a value of 10 Ci/GW(e)y will be assumed for each. The corresponding amount estimated to be released at the fuel reprocessing facility is about 20 Ci/GW(e)y for each type of reactor. 3.4.3
14CProduction in Fast Reactors
Most fast reactors presently planned or operating are of the liquid metal fast breeder type which utilize sodium asthe coolant. Therefore, estimates of 14Cformation are based on this type ofteactor design and more specifically on Clinch River Breeder Reactor (CRBR). This reactor is designed to use a mixed oxide fuel (uranium and plutonium) and the primary structural material of the core will be 316 or A-286 stainless steel (Davis, 1979). Carbon-14 will be formed from oxygen as part of the oxide fuel and from impurities in the fuel as well as in the structural material. 3.4.3.1 Formation in Fuel and Core Hardware. Detailed calculations of 14Cformation in the fuel from oxygen as part of the oxide fuel and
/
18
-...
SOURCES OF "C TABLE 3.4-Production of "Cin Clinch River Breeder Reactor -~
Location - -.-. - -Fuel
Core Hardware .. --
Source
Production in Core (Ci/GW(e)y)
Oxygen Nitrogen Nitrogen . (impurity) -
0.13 6.12' 12.8
"Based on an average concentration of 25 pg/g nitrogen in fuel.
from nitrogen as an impurity are presented by Davis (1979). These data also include an estimate of formation from nitrogen impurity in the stainless steel core hardware. A summary of the results of these calculations, presented in Table 3.4, shows that about 6 Ci/GW(e)y would be formed in the fuel, mostly from the nitrogen impurity; and about 13 Ci/GW(e)y would be formed in the core hardware from the nitrogen impurity. These estimates are similar to those of Blanchard et al. (1976). 3.4.3.2 Estimate of Release to Environment. Based on the experience with LWRs i t is assumed that 14Cformed in core hardware will remain with the metal and will not be released to the environment. Similarly, it can be assumed that most of the 14C formed in the fuel will be released t o the environment during fuel reprocessing. On this basis, release of 14C from LMFBRs is estimated to be small a t the reactor site and about six Ci/GW(e)y at the fuel reprocessing plant. 3.4.4
14C
Production in Graphite Moderated Reactors
Data on the production of I4C in graphite moderated reactors (primarily gas cooled and high temperature gas cooled) are presented in Bonka et al. (1973); Brooks et al. (1974); Kelly et al. (1975); Magno et al. (1974); ERDA (1975); and Davis (1979) and show a range of from 20 to 1000 Ci/GW(e)y. Production rates of about 350 Ci/GW(e)y for an advanced gas cooled reactor and about 300 Ci/GW(e)y for Magnox reactors, most of which occurs in the graphite moderator from an assumed impurity of 1 0 pg/g nitrogen, are estimated by Kelly et ul. (1975). A theoretical production of. 160 Ci/GW(e)y for an HTGR, most of which appears in the cladding, is estimated in ERDA (1975). That reference also indicates that there is a 99 percent retention of 14Cin both the graphite and fuel particles and that only about two Ci are expected to be released during reprocessing. A production rate of about 120 Ci/GW(e)y for high temperature gas cooled reactors is estimated in Brooks et al. (1974). A comprehe'nsive review of estimates of release from GMRs is presented by Kelly et al. (1975). In view of
3.4 NUCLEAR POWER
/
19
the wide range of estimates for 14C releases from graphite moderated reactors, a value of 300 Ci/GW(e)y is selected as an upper estimate for the purposes of this section, most of which is assumed to be associated with the fuel and may be released during fuel recovery operations.
3.4.5 14CProduction in Heavy Water Reactors Data on the formation of 14C in Heavy Water Reactors (HWR) is limited (Kelly et al., 1975; Koehl, 1976) and apply to reactors of the CANDU type. The data show that for this type of reactor most of the 14Cis produced from reaction of "0 and neutrons (reaction (ti), Table 3.1) in the moderator and is estimated to be about 450 Ci/GW(e)y. Similar amounts are estimated to be produced in the annulus gas from reaction of 14Nand neutrons (reactions (2), Table 3.1) if nitrogen is utilized, but this source is reduced to insigificant levels if nitrogen is replaced with COz. Table 3.5 summarizes release estimates for the CANDU type HWRs from Koehl (1976). For the purpose of this section, a value of 650 Ci/GW(e)y for HWRs has been selected based on these data and assumes that one half of the reactors utilize CO2 as the annulus gas. It is also assumed that most of this 14Cwill be released at the reactor site.
3.4.6 Release Estimates in Nuclear Power Industry To simplify the task of estimating 14C release from nuclear power plants, they have been categorized into four general types: 1) Light Water Reactors-including both PWRs and BWRs; 2) Fast Reactors-mainly LMFBRs; 3) Graphite Moderated Reactors; and 4) Heavy Water Reactors. Release values for each of these four reactor categories are summarized in Table 3.6. The values adopted are based on release estimates developed in
--
TABLE 3.5-"Cproduction estimate for HWRs production rate, Ci/GWfe)y -
Reactor Type
Annulus Gas-Nt Annulus Gas COz
-
-
-
In Moderator and Coolant
In Fuel
In Annulus Gas
Total
430 430
1
430 0.02
861
1
Average
431 646
20
/
SOURCES OF "C TABLE3.6-Estimated "C release rates from the nuclear fuel cycle (Ci/Gw(e)~)
Reactor Type
At Reactor Site
LWR FR GMR HWR
A t Fuel Reprocessing Site
Total
20 10 300 -
30 10 300 650
10 -
650
TABLE3.7-Nuclear power production estimates through 1990" -GW(e)y For Period Reactor Type
Fast Heavy Water Graphite Moderated Light Water Total
TZgg 1971-1975 h 1 1 36 19 57
1 11 38
107 157
1976-1980
1981-1985
3 26 53 402 484
9 49 64 803 925
1986-1990
10 51 65 939 1065 -
Total
24 138 256 2270 2688
"Based on 70% operating factor.
Sections 3.4.2 through 3.4.5. LWRs with an assigned value of 30 Ci/ GW(e)y include both PWRs and BWRs since release estimates for each approximate this level. Fast Reactors were assigned a value of 10 Ci/GW(e)y which approximates the six Ci/GW(e)y estimated for LMFBRs. An estimate of the operating history of each known power reactor in the world operating prior to December 1975 was obtained from the World Directory of Power Reactors (1976). This reference was also used to develop a projection of the operation of these reactors through 1990 and to project operation of all new reactors scheduled to become operational by that date. These projections were then used to develop Table 3.7, which shows an estimate of the world-wide cumulative GW(e)y nuclear power production through 1970 and for each five-year interval thereafter through 1990 for each of the reactor categories. Projections from 1988 to 1990 are based on the same rate of power production as given in the World Directory of Power Reactors (1976) for the years 1986 and 1987. The table projects a total accumulated reactor operation of about 2700 GW(e)y through 1990 and that more than 80 percent of the power ~roductionwill be from LWRs. These estimates are consistent with other projections (Nuclear Engineering International, 1984). Table 3.8 shows a projection of 14Crelease estimates based on the Ci/GW(e)y values adopted for each reactor category in Table 3.6 and the power production estimates presented in Table 3.7. The data show
3.4 NUCLEAR POWER
TABLE 3.8-Amount ReactorType
Fast Heavy Water Graphite Moderated Light Water Total
of "C
/
21
RCi) estimated to be released through 1990
TE~gh
1971-1975 1976-1980 1981-1985 19861990
-
-
-
0.7 10.8 0.6 12.1
7.2 11.4 3.2 21.8
16.9 15.9 12.1 44.9
0.1 31.8 10.2 24.1 66.2
0.1 33.2 19.5 28.2 81.0
Total
0.2 89.8 76.8
68.2 235.0
that about 235 kCi of 14C are projected to be released through 1990 from the nuclear power industry with about 40 percent from operation of HWRs, 30 percent due to GMRs, and 30 perc6nt attributed to LWRs. The average release rate for each year between 1981 and 1990 is projected to be about 15 kCi which is 40 percent of the average annual natural production rate of 40 kCi reported in UNSCEAR (1977). 3.4.7
Reduction of Releases
3.4.7.1 Minimization of Production. As previously discussed, 14C production rates are strongly influenced by the amount of nitrogen available in reactor systems. Data presented in Table 3.5 show that 14Cproduction in HWRs of the CANDU type is reduced by 50 percent, if COz rather than nitrogen is used as the annulus gas. Additional steps to minimize availability of nitrogen are described by Davis (1979). Any step that will reduce nitrogen content in reactor fuel, coolant/moderator, or other materials will reduce I4C production and should be given first consideration in any plan to reduce releases of 14C to the environment. However, this step is subject to chemical constraints in reactor coolants. 3.4.7.2 Determinution of Release Point. Potential mitigation of releases of 14Cproduced in nuclear reactor systems requires consideration of the quantity and point of major releases. Table 3.9 combines release estimates for each general type in Table 3.6 with power production data in Table 3.7 to illustrate projected total 14C releases through 1990 by release point. The data show significant differences in release sites for each general reactor type. Production of 14C in HWRs occurs mainly in the moderator (and annulus gas if nitrogen is used). Since moderator and annulus gases are normally released a t reactor facilities, the primary release point for 14C should be at each reactor site. In contrast, 14C production in GMRs is in the fuel and graphite fuel blocks and releases
/
22
SOURCES OF "C TABLE 3.9-Primary -
Reactor Type
sources of reactor "C releases through 1990
CW(e)y Through 1990"
Source of Relesseb Ci/CW(e)y At At Reactor Reprocessing aite
Fast Heavy Water
Graphite Moderated Light Water
24
-
138
650 10
256 2270
site
Total est. "C Releases Through 1990 (kCi)
-
10
-
300 20
90
77 Total
68 235
" Data from Table 3.7. Data from Table 3.6.
for this type of reactor are projected to be at the fuel reprocessing facility. For LWRs, one-third of the 14Cproduction is in the moderator coolant and should be released at the reactor site while about twothirds is produced in the fuel and should be released at the reprocessing facility. Measured releases reported by Blanchard et al. (1976) and Kahn et al. (1971) indicate that the quantity of 14C emitted to the atmosphere from LWRs is from 30 to 1000 times the quantity released in liquid effluents. Based on the limited data available for other reactor types, it is reasonable to assume that the major fraction of 14C will appear in airborne releases rather than in liquid effluents (NEA, 1980).
3.4.8 Interpretation of Release Estimates
The above estimates of 14Cproduction and releases from the nuclear power industry are considered to be reasonable estimates from presently available data. However, it should be recognized that the experimental data from which the estimates are made are limited. Estimates of future releases should also be considered to be tentative projections because of (1) the present uncertainty in development of the nuclear power program; (2) the present uncertainty of fuel availability, fuel recovery, and waste disposal plans; and (3) uncertainty due to major changes in the projections which could result from steps to mitigate such releases during either power production, fuel element recovery operations, or both. Lastly, no release estimates have been made for reactors operated for weapons materials or military purposes, some of which started operation in the 1940s. These reactors have probably released and will continue to release 14C although the magnitude of these releases remains unknown.
3.5 LABELED COMPOUNDS
/
23
3.6 Labeled Compounds The amount of I4C produced for incorporation into chemicals used in medical or biological research is unavailable. The actual figures are regarded as privileged information by the industries which produce these compounds. Information available from the U.S. Department of Energy places the probable production as greater than 100 but less than 500 Ci annually. In a survey of radionuclides shipped to medical schools, hospitals with more than 500 beds, and universities and colleges with more than 5,000 students, Anderson et al. (1978) found that about 36 Ci of 14C were received in 1975 by the 686 respondents. The surveyed institutions were estimated to account for one-third of all sources of 14C, leading to the conclusion that about 108 Ci were received by all institutions. Wastes containing about 26 Ci of 14Cwere shipped from responding institutions to shallow land burial sites during the same year indicating that about 28% of 14C was disposed elsewhere and probably locally. The annual increase in waste from all institutions was about 14 percent. The 1975 data provided by Anderson et al. (1978) extrapolated to 1978 would indicate a value of about 160 Ci which can be compared to the value of 221 Ci estimated by Eisenbud (1980) for the same year. It must be assumed that all of the 14Cfrom these sites will become available to the biosphere in the course of a period of time short with respect to the half-life of 14C. Estimates of doubling time for the increase in 14C labelled compounds range from 7 to 40 years depending upon the assumptions of use of 14C.The contribution of labelled compounds to the global 14C inventory is, however, so small that the uncertainty on doubling time is not significant.
Distribution of 14C in the Environment 4.
4.1
Distribution of Carbon and Carbon-14 in the Biosphere
The biosphere has been alternatively described as the living film of the earth-that part of the earth inhabited by living things (Vallentyne, 1971)-or as the totality of all plants and animals. As pertains to the exchangeable reservoirs of 14Con the earth, the former description has more relevance. An identification of the exchangeable carbon reservoirs in the biospheric sense will indicate the relative importance of these compartments. A convenient grouping of these reservoirs includes the atmosphere, the terrestrial biosphere, the ocean, the ocean sediment, and the organic shale. A diagrammatic model of the global carbon cycle (Baes et al., 1976), which illustrates the interaction of these reservoirs and their subgroups, is shown in Fig. 4.1. Similar box models of the carbon cycle (Killough and Emanuel, 1981; Reiners, 1973; Bolin, 1970; Machta, 1973) have been proposed that have different size and exchange rates of the various carbon reservoirs. The critical uncertainty in Fig. 4.1 and in most models is the exchange rate between the surface and the deep ocean. Oeschger et al. (1975) made a significant improvement in estimating this exchange rate by using a diffusion model rather than a simple compartment model. Broecker et al. (1980) concluded that the box-diffusion model of Oeschger and his associates provides an excellent fit to the average distributions of natural and bomb-produced radiocarbon. Nuclear testing 14C has been traced to study the exchange of CO, between atmosphere and ocean (Nydal et al., 1984), an important part of any model. Except for the atmosphere and the inorganic ocean, the carbon and radiocarbon contents of these reservoirs and their exchange rates are now well known. Table 4.1 summarizes the estimates of total carbon and the natural 14Ccontents in these reservoirs made by a number of investigators. The radiocarbon estimates are based on the assumption that the atmospheric concentration of natural 14Cprior to the industrial revolution was about 6 pCi/gC. This is probably the best estimate derived from the values in Table 4.1. 24
VOLCANOES, FUMAROLES A N 0 HOT SPRINGS 0.01 - 0 . 0 5
THERMOCLl?4E
DEEP OCEAN
- 4 bm LIMIT 6.600.000 C OF WHICH -lQOOO OR 0.15% I S FOSSIL FUELS
W~ATHERINGON LAND SURFACES
.38
LEGEND GPP, GROSS PHOTOSYNTHETIC PRODUCTION NPP, NET PRIMARY PRODUCTION Rg GREEN PLANT RESPIRATION
.
Rh , HETEROTROPMIC RESPIRATION F, FIRES ( r r c r p t fossll fuels) ANTHROPOGENIC FLUXES
----
Fig. 4.1 The carbon cycle (pool sizes in lo1*kg, fluxes in lo1*kg y-') (Baes et d , 1976).
2 w
s a
e
d
3 %
8 w
Q % a m
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In
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4. DISTRIBUTION OF CARBON-14 IN THE ENVIRONMENT
U
U
U
" U t
U
I-' u
I-' U
U
U
u' U
U
4.1 DISTRIBUTION OF CARBON AND CARBON-14
/
27
In geologic time, the ocean sediment and the organic shale are part of the carbon cycle. However, in the 8000-year-mean-lifetime of a I4C atom, those compartments are sinks. The radiocarbon associated with the precursors of organic shale has long since decayed away. Some radiocarbon reaches the ocean sediment, but because of the 108-year residence time of carbon in the ocean sediment (Broecker, 1973) none is recycled to other reservoirs. Consequently, no estimates of the exchangeable carbon in the ocean sediments and organic shale are given in Table 4.1, although the available data on the natural 14C content of the ocean sediment is shown when reported. Carbon-14 from nuclear weapons tests has entered the global carbon cycle in significant quantities with respect to natural 14Cproduction, and its measurement is leading to more reliable estimates of the content and exchange rates of the various reservoirs. The radiocarbon from the nuclear power industry has been negligible, but that which may be released to the environment will also distribute itself with the same exchange rates. The ratio of 14Cto total carbon in the reservoirs interfacing with man is important, because it relates to the human population dose from radiocarbon. A brief description of some of the reservoirs follows. 4.1.1
Atmosphere
The atmosphere is one of the few reservoirs in which the carbon content is reasonably well known (see Table 4.1). Man's activities increased the COPconcentration from an assumed value of 270 to 295 mL/m3 in 1860 to 331 mL/m3 in 1974 (see Figure 4.2). The annual cycle at Mauna Loa in Fig. 4.2 predominantly reflects the photosynthetic cycle of the Northern Hemisphere, although the seasonal melting of sea ice, sea surface temperatures and seasonal fluctuations in the burning of fossil fuel make contributions. Fig. 4.3 shows the annual production of C 0 2from fossil fuel combustion since 1860. Almost 60% of the anthropogenic COz produced since 1860 can be accounted for in the atmosphere by the trend illustrated in Fig. 4.2 (Keeling et al, 1982). However, the quantitative distribution of the remaining 40% among the other carbon reservoirs is not clear.
4.1.2
.Terrestrial Biosphere
The terrestrial biosphere consists of that part of the land masses which support life. Baes,et al. (1976), as is shown in Fig. 4.1, divide the carbon content of the terrestrial biosphere into two compartments:
28
/
4. DISTRIBUTION O F CARBON-14 I N T H E ENVIRONMENT
Fig. 4.2 Atmosphere carbon dioxide concentration a t Mauna Loa Observatory (data from Keeling et al. [I9821 and private communication). Note scale starts a t 310 ppm or mL/m3.
the living compartment containing 680 x 10" kg and the dead compartment containing 1080 x 10" kg. Whittaker and Likens (1973), as is shown in Table 4.2, provide a similar estimate of 827 x 10'' kg for the living component of the terrestrial biosphere by summing the plant masses of various land ecosystems. Table 4.2 also provides the plant masses for the maritime ecosystems and the net primary production for both the terrestrial and marine biosphere. The net primary production is the amount of carbon stored in plant tissue after respiration. Table 4.2 illustrates that continental plants contain essentially all of the world's living carbon, 90 percent of it residing in the forests, but only 66 percent of the global net primary production. Continental animals, including man and domestic livestock, contain only 0.02 percent of the global plant carbon (Whittaker and Likens, 1973). Baes et al. (1976), in Fig. 4.1, also subdivided the terrestrial biosphere into three regions, each with both a slowly and a rapidly changing carbon component: 1) Northern Woods, above 30°N latitude, with 540 x 10" kg of slowly exchangeable C and 75 x 1012kg rapidly exchangeable C, 2) Southern Woods, below 30°N latitude with 520 x 10" kg of slowly exchangeable C and 47 x 10'' kg rapidly exchangeable C, and
4.1 DISTRIBUTION OF CARBON AND CARBON-14
/
29
3) Non-Woods, summed over all latitudes, with 540 x 10" kg of slowly exchangeable C and 40 x 10" kg rapidly exchangeable C. The woods consist of forest, open woodlands, and wet thickets including woody swamps. The non-woods include grassland, desert or semidesert tundra, agricultural, urban, and suburban areas. Summing each compartment in these regions indicates that the carbon in the terrestrial biosphere can be grouped into 1600 x 10'' kg of slowly exchangeable carbon and 162 x 1012kg of rapidly exchangeable carbon. Several investigators have considered the role of the global freshwater system as a reservoir of carbon and "C. This system includes groundwater, inland seas, lakes, streams, and the ice and snow of the cold regions. Although ice and snow represent huge reservoirs of freshwater, they contain negligible amounts of CO2 (Langway et aL, 1965) and 14C (Grey, 1972). The largest part of the non-glacial freshwater system is the groundwater. In the upper 0.8 km of the earth's crust, the groundwater amounts to 4.9 x 10" L as compared to 0.22 x
30
1
4. DISTRIBUTION O F CARBON-14 IN THE ENVIRONMENT
TABLE4.2-Primary Production - and Biomavs Estimates for the Biosphere Total Net Primary
Production (10" kdv)
Total Plant Mass (10" kg)
Tropical rain forest Tropical seasonal forest Temperate evergreen forest Temperate deciduous forest Boreal forest Woodland and shrubland Savanna Temperate grassland Tundra and alpine meadow Desert scrub Rock, ice and sand Cultivated land Swamp and marsh Lake and stream Total continental Open ocean Upwelling zones Continental shelf Algal bed and reef Estuaries Total marine Full total
1018 L for the total of all the lakes, inland seas, and streams (Grey, 1972). Reiners (1973) and Grey (1972), as shown in Table 4.1, indicate that the freshwater system contributes little to the total exchangeable carbon or natural 14C.
4.1.3 Oceans For modeling purposes, the ocean can be divided into three compartments (Broecker, 1974; Stuiver, 1973). The surface layer, which extends to a depth of about 75 meters, is relatively well mixed and is approximately in equilibrium with atmospheric C02. The thermocline, which extends about 1000 meters below the surface layer, is a relatively stagnant zone stabilized by decreasing temperature and increasing density with depth. The deep ocean, which is the third and largest compartment, extends to a n average depth of 4 km and is isolated by the thermocline. The approximate volume of each compartment is Surface layer Thermocline Deep ocean
0.024 x lo9 km3 0.25 X lo9 km3 1.1 x lo9 km3
4.1 DISTRIBUTION
OF CARBON AND CARBON-14
1
31
The predominant form of carbon in the ocean is dissolved inorganic material. About 89 percent of the inorganic carbon in the ocean is in the form of the bicarbonate (HC03-) ion, about 10 percent is carbonate (C032-) ion, and about one percent is carbonic acid (i.e., dissolved C02 and H2C03). The total concentration of inorganic carbon is 2.0 mmol/ L in the surface water and 2.4 f 0.1 mmol/L in the deep ocean (Broecker and Li, 1970). The inorganic carbon contents of the ocean compartments, as shown in Fig. 4.1, are then 580 x 1012 kg in the surface layer, 6600 x 10'' kg in the thermocline, and 32,000 X 1012 kg in the deep ocean. The next most abundant form of carbon in the ocean is the marine detritus or dead organic matter. While the concentration varies widely, a reasonable mean of 0.1 mmol/L (Baes et al., 1976) provides a burden of 29 X lo1* kg of dead organic carbon in the surface layer and thermocline and 1620 x 1012 kg in the deep ocean. The living form of carbon in the ocean is overwhelmingly plant life, the phytoplankton. More than 90 percent of the basic organic material that fuels and builds life in the sea is synthesized by phytoplankton in surface waters (Isaacs, 1969). Table 4.2 shows that the carbon from marine plants is a relatively small fraction of the world's total plant carbon, but is responsible for 34 percent of the global net primary production. While Table 4.2 suggests that about 2 x 1012 kg of living carbon exists in marine plants, Baes et al. (1976) select half that value, as is indicated in Figure 4.1. Another reservoir of carbon in the ocean that is active in the relatively rapid regime of the carbon cycle is the carbonate solids, both suspended and in the surface sediments. These solids are primarily the CaC03 that is produced by biota in the surface layer and, after settling, is dissolved in the deep ocean. Consequently, the CaC03 sediments are limited to depths of four km or less. Baes et al. (1976) report that the carbon content of this carbonate pool is about 410 x lo'* kg. The carbon content of the calcareous ooze of the deep ocean sediments is enormous and is 3 x 10" kg. However, as indicated earlier, the deep sediments are a sink for 14Cand do not enter into an exchange equilibria of 14Cwith the surface ocean layer.
4.2 Fluxes
The fluxes (mass or activity flowing per unit time) in and out of the various carbon reservpirs along with the magnitudes of the reservoirs dictate the distribution of carbon or radiocarbon injected into the
32 / 4. DISTRIBUTION OF CARBON-14 IN THE ENVIRONMENT atmosphere or any one of the other reservoirs. Baes et al. (1976) illustrate (Fig. 4.1) that, while C02 from volcanic activity and other geothermal venting was important over the long history of the earth, it is now small (- 0.04 x 1012 kgC/y) compared to the rate of fossil carbon burning (presently 5 x 1012 kgCjy). Although the anthropogenic flux of C is, in turn, small compared to many of the other fluxes in the carbon cycle, its impact on the increasing atmospheric burden of carbon dioxide is clearly seen in Fig. 4.2. Baes et al. (1976) assumed that a balance exists between the total of the fluxes of carbon into and out of each pool. For the terrestrial system:
-
( N P P ) = R h + I: (kgC/~)
(4-1)
where (NPP) = net primary production R h = heterotrophic respiration, or respiration from organisms that do not involve photosynthesis (kgC/y), and F = flux from non-fossil fires (kg/C). Then the mean residence time, T, of a carbon atom in the living biomass reservoir is,
where C = the size of the carbon reservoir. Therefore, from Fig. 4.1 the mean residence times of the rapidly and slowly exchanging carbon reservoirs in the .terrestrial biosphere are 5 and 64 years, respectively. A similar calculation (from Table 4.2) for the marine biosphere suggests a residence time of about one month. There are but two significant fluxes of carbon to the ocean. One is the weathering of limestone by the reaction, CaC03 + C02 + H20
+ Ca2++ 2 HC03-
(4-3)
which, according to MacKenzie and Garrels (1966), yields only 0.38 x 1012kgC/y. The other is the solubility of C02 in the ocean water which is mainly dictated by the concentration of C032- in the water. C02 (g)
+ C032- + H 2 0 + 2 Hc03-
(4-4)
It is the interrelationship of Eqs. (4-3) and (4-4) that controls the transfer of CO2 between the atmosphere and the surface layer of the ocean. From chemical equilibrium considerations, Keeling (1973b) has shown that, at the present time, the C02 concentration in the atmosphere will increase by a factor of nine before the total carbon concen-
4.2 FLUXES
/
33
tration in the surface layer of the ocean will double. This buffering factor of the surface ocean is related to the ratio of the sum of the concentration of C032- but increasing the concentration of HC03twice that of COB'-, thereby increasing the buffer factor. Consequently, with increasing concentration of atmospheric COz, the capacity of the surface ocean to respond is reduced and'is limited by the availability of C03'- ion. Keeling (1973b) and Oeschger et al. (1975) have estimated from Suess effect 14C measurements and other assumptions that about 13 percent of the atmospheric reservoir of C02exchanges with the surface layer of the ocean each year, which represents about 15 percent of the COPreservoir in the surface layer. As indicated in Fig. 4.1, this amounts kg of carbon per year. to an equilibrium flux of about 90 x the COz reservoir in the surface layer. As indicated in Fig. 4.1, this amounts to an equilibrium flux of about 90 x 1012 kg of carbon per year. These estimates of the uptake capacity of surface ocean water do not consider the effect of circulation and mixing with the deep ocean water. The circulation of the oceans has been described graphically by Broecker (1974). As a ~ e s u lof t photosynthesis that consumes C02 and shifts Eq. (4-4) to the left, the surface layer has relatively large C032concentrations and is supersaturated with CaC03. This surface water absorbs COz from the atmosphere, and as it cools in the Arctic Ocean and Norwegian Sea, it descends to the deep ocean. This water traverses the deep Atlantic and Indian Oceans and finally reaches the Pacific Ocean. Along the way, respiration and decay processes produce C02 and, with increasing depth, shift Eq. (4-4) to the right. As a result, water at intermediate depths is saturated with CaCOa and the deep water is unsaturated. From the distribution of 14C in the oceans, compartment model calculations indicate that only two to eight percent per year of the volume of the surface wak'r is transported to the deep ocean (Broecker and Li, 1970; Oeschger et al., 1975). Consequently, transfer to the deep ocean by these models does not drastically increase the rate by which the surface water can absorb Con from the atmosphere. Oeschger et al. (1975) deviati from the simple compartment model concept and invoke eddy diffusion as a transport mechanism from the bottom of the mixed surface layer (75 m) to the deep ocean. Adopting an eddy diffusion coefficient of 4.0 x lo3 m2/y, they increase the rate of exchange with the deep ocean and adequately reproduce the concentration of bomb 14Cin the surface layer of the ocean with time and its profile with depth. As a result of this greater flux from the atmosphere to the deep
34
/
4. DISTRIBUTION OF CARBON-14 IN
THE ENVIRONMENT
ocean and subsequently to the surface water, Oeschger et al. (1975) have also been able to reproduce the integrated increase in atmospheric CO2 concentration since 1860 and its annual trend since 1960. They calculate that, of the cumulative anthropogenic input of Cop up to 1970, 54 percent has remained in the atmosphere, which is in good agreement with the observations; and that 46 percent has been taken up mostly by the oceans. An intrinsic property of the model by Oeschger et al. (1975), which is lacking in the conventional models, is that the rate of transfer of COz from the surface layer to the deep ocean is dependent upon the rate of change of COz in the atmosphere.
4.3 Reliability of Reservoir Estimates Inspection of Table 4.1 indicates that the reservoirs that can be easily and representatively sampled are the ones whose total carbon and radioactive carbon contents are most accurately known, notably the atmosphere and the inorganic carbon in the ocean. Estimates, especially the earlier ones, of the total carbon of the smaller and less homogeneous reservoirs sometimes vary by factors of 10 or more. However, the most recent estimates have nqrrowed the range of variability. Because of the preponderance of the ocean contribution, the total of the world's exchangeable carbon and 14C is known with good precision. The mean and standard deviation of the world's exchangeable carbon from all 12 studies, excluding the ocean sediment, is (40.9 +. 2.2) x 1015 kg. Similarly, the mean of the three estimates of the total 14Cis 241 & 14 MCi, including the relatively small amounts in the ocean sediment. Even if major revisions are made in the estimated contents of some of the uncertain reser+oirs, the integrated 14C and total carbon in all of the exchangeable radiocarbon reservoirs will not likely change by more than 10 percent. Dividing the global inventory of natural 14C(241 MCi) by the surface area of the earth provides a hypothetical disintegration rate per unit area of 1.8 atoms/cm2 s. At equilibrium, the production rate should equal this disintegration rate. Estimates of the natural 14Cproduction rates have ranged between 1.3 and 3.1 atoms/cm2 s (Kouts and Yuan, 1952; Ladenburg, 1952; Anderson, 1953; Libby, 1955; Soberman, 1956; Craig, 1957; Hess, 1960; La1 and Peters, 1962; Lingenfelter, 1963; Korff, 1968; and Light et d., 1973). Lingenfelter and Ramaty (1971) have adjusted for the inverse relationship between solar activity and I4C production during the past 200 years and calculate an average production of 2.33 atoms/cm2 s.
4.3 RELIABILITY OF RESERVOIR ESTIMATES
/
35
O'Brien (1979) has calculated cosmogenic isotope production as a function of geomagnetic field strength and solar modulation along with spallation yield values based on the formulation of Silberberg and Tsao (1973a, 1973b).O'Brien's estimate of the current radiocarbon production rate is 1.75 atoms/cm2 s.
5. Sampling and Analysis 5.1 Introduction
Problems associated with analysis of 14C in sampling, chemical processing, counting, data processing and quality assurance are common to the analysis of most other materials. Appropriate information may be found in Dixon and Massey (1969), Crow et al. (1960), NCRP Report No. 47 (NCRP, 1976) and NCRP Report No. 58 (NCRP, 1978). There are, however, peculiarities to the 14Canalysis requiring particular attention. The discovery of 14C in nature by Libby and co-workers (Libby, 1955) has led to the development of radiocarbon dating as an archaeological tool. The rigorous requirements in sensitivity and precision necessary to obtain meaningful radiocarbon dates motivated dating laboratories to develop specific sampling, chemical processing, counting and data processing methods for the determination of 14C activity (Maddock and Willis, 1961; Michael and Ralph, 1971; Rafter and Grant-Taylor, 1972; Perlman et al., 1972).
6.2 Sampling
5.2.1 Collection of
14CFrom
Air
The following discussion deals primarily with those techniques that would be most generally applicable for collection of modern radiocarbon samples. Samples of 14C02 in ground-level air can be collected by exposing NaOH solutions in dishes or trays (Olsson and Stenberg, 1967; Walton et al., 1970) for several days. This method is the simplest and least expensive to set up, requiring only reasonable care to minimize intrusion of dust, precipitation, and other foreign matter. A simple roof with a mesh screen to allow good air circulation is ordinarily sufficient. The chief drawback to this sampling method is the extended time period required to collect a measurable quantity of Cop. Collection time for ground-level air samples may be shortened by 36
5.2 SAMPLING
/
37
pumping air through a molecular sieve (Young and Fairhall, 1968). Molecular sieve units were first developed for aircraft and balloon sampling of the upper atmosphere (Ashenfelter et al., 1972; Steinberg and Rohrbough, 1962; Fergusson, 1963a; Fergusson, 1963b) where the amount of water vapor is low. Water vapor decreases the absorption efficiency of the molecular sieve for C02. Extending the collection time to a t least 30 minutes and using a large excess of molecular sieve can overcome this difficulty (Young, 1967). Drying the air with a silica gel column, then cooling on glass beads at -70°C prior to adsorption of COz on molecular sieve has also proved a successful solution (Drobinski et al., 1965). Collected COZ must be degassed from the molecular sieve by heating the trap to 380°C (Young and Fairhall, 1968) or 500°C (Drobinski et al., 1965). Winkelman et al. (1982) have provided a detailed sampling train for collection of CO2 from ambient air and reactor effluents using a molecular sieve. Their method is routinely applied for assessment of l4CO2from German power reactors. Degassed CO2 may be purified and counted directly as a gas, collected in Ba(OH)z/BaC12 solution for counting a s BaC03, or absorbed in a trapping solvent for liquid scintillation counting. Care must be exercised to ensure that cross-contamination of samples does not occur due to inadequate degassing of the molecular sieve before each sample collection. Collection of COa from air into NaOH solution offers the advantage of being a single-pass system. Therefore, there is little likelihood of cross-contamination of samples. Regardless of the method used for collection of ground-level air samples, care must be taken in the selection of stations to avoid contamination of samples with COP originating from other sources such as local fossil fuel combustion (Walton et al., 1970). Some isotope fractionation may occur in samples collected by either of the absorption methods described above. The effect is, however, of concern to radiocarbon dating and is of little or no significance in studies concerned with estimating 14Cpopulation dose (Levin, 1978). Compounds other than C 0 2 in air have been sampled by a number of investigators. Kung et al., 1975; Kung et al., 1974; Matuszek et al., 1974; Husain et al., 1979 and Schriber et al., 1980, have sampled organic compounds around low-level waste disposal sites and in reactor effluents. Grab samples of 2 to 4 L are used for sampling and subsequent gas chromatographic separation of various organic compounds. It is often desirable to separate 14C02from all other carbon containing compounds. In this case, sampling trains described for tritium by Moghissi et al. (1970) and Lamberger et al. (1976) may be modified. In these cases, COs is dissolved in a basic solution. Subsequently, the sample is oxidized in the presence of catalysts (see section 5.3 for
38
1
6. SAMPLING AND ANALYSIS
further details) at appropriate temperatures. Schriber et al. (1980) applied this method to the sampling of reactor effluents.
5.2.2
Collection of 'TFrom Water
Extraction of 14C02from water samples can also provide measurable samples as long as sufficient water is processed and a sensitive counting technique is used. Linick and Suess (1972) found that seawater samples could not be stored in 55-gallon drums for longer than a few weeks. They and others measuring radiocarbon levels in the sea extract COz on board ship. Sample volumes vary from as little as 60 L, using modified beer kegs (Fairhall et al., 1972) to 200 L, using standard oceanographic samplers (Ostlund et al., 1974; Roether, 1974). The 14C activity is stripped from the acidified water samples as COz and collected either a BaC03 or on molecular sieve for shipment to the laboratory.
5.2.3 Collection of Biota and Soil Samples The requirements for sampling soil and biota for 14C analysis are similar to those for many other radionuclides. NCRP Report No. 47 (NCRP, 1976),dealing with tritium, contains a detailed description of sampling for soil and biota applicable to 14C analysis with minor modifications. The two major requirements are freezing the sample as soon after collection as possible and closure of the sample container to avoid cross contamination (Williams and Linick, 1975).
5.2.4 Collection of Urine Because only a few milliliters of urine are needed for an analysis a single voiding is sufficient. If the sample is collected for occupational monitoring the subjects should remove any protective clothing and wash their hands prior to the sample collection. For collection of environmental samples these precautions are usually unnecessary. Samples that are kept for sometime prior to analysis should be stored at reduced temperature and should be preserved with an aqueous solution of "merthiolate" (Moghissi and Lieberman, 1970).
5.2.5 Sampling for 14CParticles In certain industrial, medical, and research facilities, particles may be present in the air and on work surfaces. Particles are collected from
5.3 ANALYSIS
/
39
air by the usual filtration. The contamination of work surfaces may be assessed using a "smear" or "wipe" test. Usually a filter paper is used for this test. The counting system used will determine the selection of filter paper. If liquid scintillation is used, the filter paper must be either soluble in the scintillation liquid or must be light conductive in liquid scintillation medium. Several 14C measurements for samples collected at nuclear power reactors (Kunz e t ab., 1975; Kunz e t al., 1974; Matuszek et d . , 1974; Kahn et d . , 1974; Arndt et al., 1980) have been made on grab samples of gaseous or liquid effluents. Distillation of 14C from reactor liquid effluents is quantitative only after oxidation with NaOH/KMn04 (Krieger and Gold, 1973).
5.3
Analysis
Specific separation methods for 14C02or other 14C species are, for the most part, closely related to the sampling or measurement method used. Counting principles described for low-level radionuclide measurements in general (NCRP, 1977; NCRP, 1978; Sugihara, 1961) apply also for I4C measurements. Blank and control samples are particularly important for 14Canalysis.
5.3.1 Sample Combustion The choice of sample combustion depends upon the nature and the size of the sample and the counting method used. The techniques described for tritium (NCRP, 1976) are also valid for 14C.Because liquid scintillation is often the counting choice, the combustion methods are often described in association with liquid scintillation counting (Rapkin, 1974; Wagner and Winkelmann, 1974; Rauschenbach and Simon, 1974; Burleigh, 1974; Cooper, 1974; and Fox, 1980). Combustion techniques may be categorized into wet oxidation, oxygen flask, oxygen bomb and tube furnace combustion. The wet oxidation technique is based on the reaction of organic compounds with strong oxidizing agents. Mahin and Loftberg (1970) have reviewed wet oxidation techniques. Although several other wet oxidation techniques have been developed since that time, the limitations of wet oxidation make them of little usefulness for all but most specialized applications. Oxygen flask techniques have been in wide application since their
40
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6. SAMPLING AND ANALYSIS
introduction in the 1950's. However, their popularity has been significantly declining since the introduction of automated tube furnace techniques. An outstanding oxygen flask technique is described by Rauschenbach and Simon (1974). This technique built around the standard 500 to 1000 mL Erlenmeyer flask may be automated for routine applications. The oxygen bomb technique is based on the combustion of a sample in a n enclosed container capable of withstanding the temperature and pressure associated with spontaneous ignition of a sample. Oxygen bomb techniques are not useful for routine applications except for large samples. An example of a n oxygen bomb technique capable of oxidation of up to log of sample is described by Moghissi et al. (1975). Tube furnace techniques go back to Liebig in 1831. The sample is heated in the presence of oxygen and the combustion products are oxidized to COa H20, and other gases in the presence of a catalyst. NCRP No. 47 (NCRP, 1976) contains an excellent review of this subject. Gacs et al. (1980) have provided an additional method capable of separating 14C from other radionuclides which may be present in biological samples.
5.3.2
Solid Source Counting
The earliest radiocarbon dates were obtained by counting elemental carbon on cylindrical screen-wall Geiger counters. Solid CaI4CO3spread on stainless steel planchets (Krieger and Gold, 1973) has been used in association with thin-window 6-counting for evaluation of 14Clevels in liquid effluents from nuclear power plants (Kahn et at., 1974). However, results obtained by counting solid sources are likely to be imprecise due to large self-absorption corrections required for counting CaC03or BaC03 (Uzhovaskaya and Lur'e, 1974). The uneven dispersion of a solid source slurried on a planchet would also introduce significant errors. Counting of "infinitely thick" samples is generally not useful for low-level radiocarbon measurements, because of the required large sample size and the .low counting efficiency.
5.3.3 Internal Gas Counting Internal gas-proportional counting has replaced solid source counting as a popular technique for low-level counting of 14C,particularly for radiocarbon dating. Various investigators differ in their choice of counting gas, using either COP, CHI, or C2H2. Several papers and
5.3 ANALYSIS
/
41
reviews contain information describing the steps necessary to obtain high purity 14C02required for counting, including the required conversion of the sample to CH, or C2H2(ICRU, 1972; Rafter and GrantTaylor, 1972; Michael and Ralph, 1971; Maddock and Willis, 1961; Johnston, 1956; Buddneier et al., 1970; Long, 1965; Olson and Nikoloff, 1965). The sensitivity and precision of gas-proportional counting for I4C have been improved by the use of Delrin detectors (Oeschger et al., 1972, Oeschger and Loosli, 1975). Rise-time pulse discrimination serves to further reduce background (Oeschger and Loosli, 1975; Sudar et al., 1973). Harbottle et al. (1979) reported development of a small gas proportional counter with associated shielding for samples as small as 10 mg of carbon. These counters have sensitivities comparable to accelerators (see Section 5.3.5) for measurement of small samples, but counting times are very long (up to 70 days).
5.3.4 Liquid Scintillation Counting
The use of liquid scintillation counters for measurment of 14C has increased rapidly since 1965. Present photomultiplier tubes and solid state circuits have greatly reduced the background and increased the stability characteristics of liquid scintillation spectrometers over systems available before 1970. The presently available liquid scintillation counters are capable of detecting I4Cwith an efficiency of greater than 90%. The corresponding background would range from 5 to 25 counts/ min depending on the degree of the optimization of the system. A detailed discussion of liquid scintillation techniques is provided in other NCRP Reports (NCRP 1976, 1978) and by Bransome (1970), Dyer (1971), Crook et al. (1971, 1972), Crook and Johnson, (1974), Horrocks (1974) and Peng et al. (1980). The optimization of liquid scintillation counting for analysis of I4C in environmental samples resembles that for tritium. NCRP Reports Nos. 47 and 58 (1976,1978) contain details of the relationships among sample size, counting efficiency, and background count rate for optimized systems. Coincident with the improvement in commercially available liquid scintillation counting systems has been the development of an improved technique for conversion of COz to benzene (Noakes, 1963; Noakes et al., 1965) that has been incorporated into techniques for analysis of environmental samples (Johns, 1975), seawater samples (Mathews et al., 1973), surface water samples (Herbert et a l , 1973),
42
/
5. SAMPLING AND ANALYSIS
and atmospheric air samples collected on molecular sieve (White, 1971). Modifications of the technique are available for large samples (Coleman et al., 1972) and small samples (Polach et al., 1972). Techniques for conversion of 14C to other compounds soluble in liquid scintillator solutions have generally not been as well accepted as benzene synthesis. An intercomparison of radiocarbon dates led to the conclusion that benzene synthesis does not lead to measureable isotope fractionation (Tamers and Person, 1965). However, conversion of COP to C2H2is a step that may exhibit isotope fractionation. In comparison with benzene synthesis, it is generally simpler to measure 14C02trapped in scintillator-compatible solvents (Rowinska et al., 1975; Peterson, 1973; Stewart and Weir, 1973; Bosshart and Young, 1972; Murray, 1971; Moghissi et al., 1971; Gupta, 1966, Kelly et al., 1961). Analytical simplicity appears to have made direct absorption of CO2 the most widely used liquid scintillation technique for 14C analysis. The solubility of C02 in common scintillation solvents such as xylene is about 25 mL gas/mL of toluene in a deaerated system (Horrocks, 1968). The deaeration technique is described by Shupping et al. (1969). A more common method for the assay of 14C is the absorption of CO2 by a base that is subsequently dissolved in an appropriate scintillation mixture. A number of inorganic and a large number of organic bases have been used for this purpose. Hyamine, a long chain organic primary amine which was used extensively in the past, is only of historical interest today. Its limited capacity for CO2 absorption and its persistent phosphorescence and chemiluminescence have contributed to the search for better replacements. Phenethylamine in a mixture of ethanol and toluene (Woeller, 1961) can absorb a significant volume of C02 (40 mL of C02 gas/mL of solvent). Ethanolamine is also used as an absorbent for C02 (Jeffay and Alvarez, 1961). The absorption of C02 in both amines is somewhat slow. Methoxyethylamine is presently the most convenient absorbent for C02. Its effectiveness is demonstrated by its capacity to absorb over 1.5 L of COP in a 20 mL scintillation mixture and the solubility of both its carbamate and carbonate in toluene. A simple technique which can be readily adopted by most radiochemical laboratories requires the trapping of C02 in NaOH. There are a variety of methods for the assay of 14Cin the resulting sodium carbonate. Moghissi et al. (1971) introduced a technique consisting of the preparation of a detergent mixture (380g of triton QS44 per liter of triton N101) and adding this detergent mixture to xylene containing
appropriate scintillators in a volume ratio of 1 2 . Sodium carbonate is soluble in this mixture up to a concentration corresponding to 100 mL of COP. Smith et al. (1983) have used routinely a mixture of ethylene glycol monomethylether, toluene, 12N NaOH and Liquifluor, a commercial scintillation liquid (volume ratio 47.6:40:7.6:4.8). This mixture was capable of absorbing about 67 mL C02. An even simpler technique involves the conversion of sodium carbonate to barium carbonate by the addition of BaC12. After collection, drying, and weighing of the precipitate, Ba14C03is counted as a gel suspension in a liquid scintillator. This method has been used for measurement of 14C in reprocessing plant effluents (Schiittelekopf, 1977) and in gaseous reactor effluents (Riedel et al., 1978; Riedel and Gesewsky, 1977; Kahn et ctl., 1974; Krieger and Gold, 1973). Pfeifer et al. (1981) have improved the precision of gel suspension using Ca14C03. Their technique has been applied to the measurement of 14C releases from a TRIGA reactor (Tschurlovitz et al., 1983). The suspension of solids in liquid scintillation is associated with inherent inaccuracies. The non-uniform distribution of solids resulting from settling or other causes introduces inaccuracies in the measurement. In addition, because the size of the particles is not uniform and cannot be necessarily reproduced in various experiments, changes in light conductivity and self absorption introduce uncertainties in analytical results. Simple measurements can be also performed by liquid scintillation counting of hrectly solubilized biological materials (Horrocks, 1974; Shone et al., 1974; Ward, 1973; Wheeler and Strother, 1973; Simpson and Browing, 1973; Chapman and Marcroft, 1971). Direct solubilization of urine samples in various commercially available scintillation liquids is a common technique for urine bioassay (Moghissi et al., 1969; Smith et al., 1983). Moghissi et al. (1970) and Muse and Rao (1975) have discussed details of liquid scintillation counting of wipe samples. Significant improvements in the commercially available mixture have led to their application in all but specialized studies. Radioassay of compounds other than 14C02would require the design of a system capable of solubilizing or emulsifying the sample in the scintillation liquid. This topic has been extensively discussed by Horrocks (1974) and by the NCRP (NCRP, 1976). Similarly, intricacies of the determination of counting efficiency (quench correction), solvents, and solutes, and other details of liquid scintillation counting have been extensively discussed by Horrocks (1974) and by the NCRP (1976, 1978).
44
5.3.5
/
5. SAMPLING AND ANALYSIS
Direct Ion Mass Spectrometry
The measuring methods described in the previous section depend on indirect evaluation of "C content by counting the P radiation emitted when an atom of 14Cdecays. Only a small fraction of the 14C decays, so that for a three-day counting period typical of radiocarbon dating, only about a millionth of the 14C present is involved in the measurement. Oeschger et al. (1970) first suggested that the direct measurement of the 14C atoms in a sample could potentially improve radiocarbon measurement capabilities a million-fold over that obtained by measurement of @ decay, permitting extension of radiocarbon dating beyond that obtained by radiation counting. Routine mass spectrometry has proved inadequate for direct measurement (Anbar, 1978), because of interference from more abundant natural species, e.g., 14Ninterferes with I4C measurement and 12C, 160 and ''0 interfere with I4Co2spectrometry. Recent attempts a t direct measurement with high-energy particle accelerators have proved moderately successful (Muller, 1977; Nelson et al., 1977; Bennett et al., 1977; Muller et al., 1978; Bennett, 1979). High-energy accelerators provide resolution of 14C from contaminants, but the precision and sensitivity of the direct ion-counting systems are associated with some problems including contamination with extraneous 14C,variable background values due to memory effects from previous samples or from 14 C adsorbed onto the ion source, and variable ionization efficiency. Direct ion measurements are presently limited to those facilities having a large accelerator. Doucas et al. (1978) have designed a small van de Graaff accelerator which could be used as a dedicated dating instrument.
The newly developing technique of laser absorption spectroscopy appears to offer several advantages for environmental measurements of 14C,as well as for isotopic measurements of several other species, such as '%, 170,1 8 0 , and 15N (Schnell and Fischer, 1978; Lehmann et al., 1977). This method may prove useful for dynamic in situ evaluation of chemical processes in a polluted environment, such as those in a stack plume, a major roadway, or an urban area. Wahlen et al. (1977) found that laser absorption spectroscopy was sufficiently sensitive to perform 14C02measurements in physiological experiments where uptake, release, and isotopic exchange are studied in a closed system. They also suggested that laser absorption spectroscopy may prove
5.4 PRESENTATION OF "C DATA
/
45
useful for monitoring 14C discharged from nuclear-power reactors. Sensitivity may be enhanced by opto-acoustical detection (Lehmann et al., 1977), by enrichment (Wahlen et al., 1977), or by increasing the path length over which measurement is made.
5.3.7 Isotopic Enrichment Each of the measurement techniques described previously may benefit from methods for 14Cisotopic enrichment. A modern sample contains approximately one 14C atom in 1012 atoms of stable 12C, a ratio which is measurable by the techniques previously described. However, the ratio decreases by a factor of two for every 5730 years of sample age. Thus, a 60,000-year-old sample will contain only one 14Catom in 1015atoms of 12C,making radiocarbon dating very difficult regardless of the measuring technique. Grootes (1977) and Stuiver et al. (1978) have extended radiocarbon dating to 75,000 years using thermal-diffusion isotopic enrichment. Hedges (1978) has reported on an isotopic-enrichment technique using laser photolysis of selected carbonaceous compounds. Although laser photolysis may provide advantages such as smaller sample requirements, more rapid enrichment, and better yields than for thermal diffusion, the technique is as yet untested.
5.4 Presentation of 14C Data In addition to the common radiological units used to report environmental radioactivity, e.g., &i/mL, pCi/L, pCi/gC, or pCi/m3, 14Clevels may be reported relative to modern natural 14C levels (6 14C or A 14C).The conventions used to define 6 I4C and A 14C (Wigley and Muller, 1981; Williams and Linick, 1975; Rafter and Grant-Taylor, 1972; Broecker and Walton, 1959) vary somewhat from laboratory to laboratory, but the imprecision created by the different conventions is only about 5% of the actual value. Therefore, conversion of a "delta" value or a 14Cdate to a concentration value for 14Cwill err by at most 5% regardless of the convention used. The four references cited provide a comprehensive description of the conventions most commonly used by various dating laboratories. 5.5
Summary
Radiocarbon dating laboratories and certain environmental laboratories use internal gas-proportional counting because of its precision
46
/
5. SAMPLING AND ANALYSIS
and excellent sensitivity. Carbon dioxide is counted directly or converted to a hydrocarbon gas such as methane. Gas counting does not lend itself readily to automation and automated systems are not commercially available. The simplest and most popular technique for measurement of 14Cis liquid-scintillation counting. With care, liquid-scintillation spectrometry using benzene conversion can achieve high precision and excellent sensitivity. Instrumentally, the technique is simple, but chemical preparation of benzene requires considerable labor. The availability of reliable commercial instruments increases the attractiveness of the method. The sensitivity of liquid scintillation counting is adequate for most applications provided sufficient quantity of sample is available. The minimum limit of detection for 14C in liquid scintillation depends upon the counting efficiency, the background and the sample size. Typically, an unquenched sample of 14Ccan be counted in a liquid scintillation counter with an efficiency exceeding 90%. The corresponding background ranges between 5 to 25 counts/minute depending upon the sample size, the nature of the vial, and several other parameters. For environmental and occupational monitoring, the application of methoxyethyamine in a modern liquid scintillation counter would permit the introduction of 0.5 t o 0.8g of carbon in the liquid with a counting efficacy of 60 to 80% resulting in three to six counts/minute above background for a "modern" carbon sample. Wipe and urine samples will be counted with a comparable efficiency if the sample is not dark. Accelerated-ion counting of 14C is a promising technique requiring considerable improvement to achieve adequate precision. The small sample requirement for the direct counting of 14Cions appears to be its most desirable feature. Infra-red laser spectroscopy, combining the capability for dynamic in situ measurement with that for making measurements over long path lengths, may prove t o be the most useful method for evaluation of isotopic and chemical-exchange processes in the environment.
6. Behavior of Systems
14C
in Biological
6.1 Introduction This section is intended to be an overview of the behavior of 14C in biological systems and is not a detailed discussion of 14Cmetabolism and kinetics in the human body. Additional detail is available in the references contained in the section. A discussion of uptake, metabolism, excretion and dosimetry of 14C-labelledcompounds normally not encountered in the environment is also beyond the scope of this report. Two reports of the International Commission on Radiological Protection (ICRP, 1967, 1982) contain relevant information on metabolism of various 14C-labelledcompounds. Recent studies by Simmons et al. (1982) and Smith et al. (1983) contain additional useful information. The metabolism and kinetics of radiocarbon in the human body follow those of ordinary carbon. A fraction of carbon introduced into the body is retained as protein, fat, carbohydrates, and other materials. The remainder of ingested carbon is excreted unchanged or is metabolized to COz,urea or other metabolites. Accordingly 14Cis excreted following a multicomponent exponential function. The corresponding biological half times range from a fraction of an hour to several years. The ICRP suggests a biological half time of 40 days for 14C for dosimetric purpose as a conservative value.
6.2.
Carbon-14 Uptake a n d Retention-Ingestion
The dose rate to an individual from 14C intake depends on the specific activity of the food from each source and the amount of the ingested "C which is retained over the period under consideration. When food is eaten, a quantity of C02 and urea approximately corresponding to the carbon intake is excreted within the excretion within the next few hours. Even in growing children, only a small fraction of the ingested carbon is retained for a long period of time. An 8-year47
48
/
6. BEHAVIOR OF 14C IN BIOLOGICAL SYSlXMS
old boy consuming about 440 g of food daily (60 g protein, 60 g fat, 320 g carbohydrate) would increase in mass an average of about 4 kg/ y (Albritton, 1954). The daily food intake contains an average of about 216 g of carbon while his average daily added mass contains about 2.5 g of carbon. The net long-term retention in this case is about 1.2 percent of the intake. Part of the excreted carbon, however, comes from previously ingest~dcarbon. An exact determination of the quantity of exchange between newly ingested dietary components and components that have resided for some time as tissue protein, carbohydrates, or fat is difficult to make; and definitive experiments are lacking. Turnover times in the human are often unknown and are complex due to the interrelationship between the metabolic pathways of fat, carbohydrates, and protein. However, experiments with humans by Hellman et aL. (1953); Berlin et al. (1951, 1955); and Baker et al. (1954) are useful for making estimates of turnover rates. Carbohydrate is degraded chiefly to a 3-carbon fragment and then to a 2-carbon fragment, acetate ion, acetyl coenzyme A, or acetylphosphate from which it is converted to citrate and oxidized to COZ through operation of the citrate cycle enzymes (White et aL., 1973). Alternatively, the 2-carbon fragment can be reversibly converted to fatty acids. Proteins are converted to amino acids, some of which in turn are metabolized by routes that lead to glucose and subsequently to acetate or are directly converted to acetoacetate which is also a precursor of acetate. Thus, the major pathways leading to energy production from each source intermingle at the 2-carbon level, and anabolic pathways leading to retention of carbon for periods of time longer than 24 to 48 hours start a t the 2- or 3-carbon level. There are, of course, numerous special pathways, particularly those involving the amino acids that are required in the diet. These are not quantitatively important compared with daily energy production, but are important physiologically. The net process in the body will be the result of a large number of individual processes, each affecting the change in specific activity of most of the others, and is approximately describable by a combination of three or more first order reactions of tissue activity-concentration following cessation of supply of the isotope of carbon. The latter rate constants are sometimes used in estimating the dose from intakes of 14C in foodstuffs that are not continued long enough to reach a new higher activity-concentration equilibrium (see ICRP, 1967, 1982). Proteins in the body often have multiple functions but may be grossly divided into those whose major function is structural and those
6. CARBON-14 UPTAKE AND RETENTION
/
49
whose chief function is catalytic. As might be expected, many structural proteins show relatively long turnover times. However, plasma albumin (about 0.17 percent of body carbon) might, in a sense, be considered to be a structural protein, and it has a very short turnover time, about 7 to 11 percent being resynthesized daily corresponding to a half-time of disappearance of about seven days (Turner and Hulme, 1971). Hemoglobin is an example of a protein with a catalytic function and a relatively long turnover time, about 0.8 to 1.5 percent being synthesized daily. About 1.8 percent of the adult body-carbon is in hemoglobin (White et al., 1973). Many enzymes have relatvely rapid turnover rates, but each represents only a small fraction of the total body carbon. If all of the major proteins with short turnover times are added together, it will be seen that their daily renewal rate could account for a significant fraction of the daily protein intake. However, they are synthesized from a mixture of amino acids that results from destruction of performed proteins plus those that remain undegraded from the diet. Further, the nonessential amino acids are partly synthesized de novo from the acetate pool or from other derivatives of carbohydrate and the nitrogen of other amino acids. The emphasis has been on protein, but fat and carbohydrate turnover is also important. Fat is a major reservoir of carbon in the body and is also dynamically related to protein and carbohydrate. Different dietary customs determine the relative amounts of protein, fat, and carbohydrate in diets. In a diet containing 15 percent by mass of fat and 15 percent of protein, the energy contribution of the fat is about one half that of the 70 percent of carbohydrate and more than double that of the protein. As a first approximation, it may be of interest to estimate the time required to reach a specific activity equal to one-half the final equilibrium value, assuming (incorrectly) that each type of carbon source is freely interchangeable only with the corresponding type in the adult body. From values given for Reference Man (Table 6.1), the adipose tissues contain about 8,000 g of carbon; and protein contains about 6,700 g. Using a maximum value of eight percent net weight for liver glycogen and about one percent muscle glycogen, there is also about 300 g of storage carbohydrate carbon. If the intake is 300 g daily of food carbon divided so that 205 g arises from carbohydrates, 56 g from fat, and 39 g from protein, and the individual is in nitrogen balance, then the
50
/
6. BEHAVIOR OF ''C IN BIOLOGICAL SYSTEMS
TABLE6.1 Selected data from Reference Man" Organ Mass (g) Total carbon (g)
-
Total Body Skeleton: Cortical bone Trabecular bone Red marrow Yellow marrow L U ~ ~ S
Liver Kidneys Spleen Thyroid Testes G.I.Tract Stomach (wall) Small intestine (wall) Upper large intestine (wall) Lower large intestine (wall) Adipose fat Volume of air breathed (mL/day) Mass per day of dietary carbon (&/day)
7.0 x 1.0x 4.0 x 1.0 X 1.5 X 1.5 X 1.0 x 1.8 x 3.1 x 1.8 X 2.0 x
lo4
lo4
3.5 X
10' lo3 lo3 lo3 10' 102 10' 10'
1.6 x 2.5 X 5.5 x 1.3 X 1.3 X 9.5 X 1.0x 2.6 X 4.0 X 2.0 X 2.1 3.1
1.5 x 6.4 X 2.1 X 1.6 X 1.2 x
10' 10' 102 10' lo'
1.8 x 10' 7.4 X 10' 2.4 X 10' 1.9 X 10' 8.0 x lo3
10'
lo3
loa
103 16
10' lo2 lo2 lo2 10' 10' 10'
2.3 x 10'
3.0 x lb
'ICRP (1975)
equilibrium half-times are given by: 8,000 g
0.693 days = 99 days for fat, 56 g 6,700 g x 0.693 days = 119 days for protein, and 39 g 300 g x 0.693 days = 1 day for carbohydrate. X
205 g
The actual time course of increase in carbon specific activity of the larger number of discrete "poolsn is difficult to estimate from known parameters. The average may be taken to be that calculated by assuming all dietary carbon is "retained." The 300 g of daily carbon intake gives a half-time equilibrium value of (0.693 x 16,000 g C days)/ (300 g C) = 37 days which is in reasonable agreement with 40 days proposed by the ICRP (1982) for Reference Man. Equilibrium lags have been estimated from tissue samples and found to be from one to two years (Broecker et al., 1959; Libby et ai., 1964; Harkness et al., 1969; and Mydal e t al., 1971).
6.3 CARBON-14 UPTAKE AND RETENTION
/
51
6.3. Carbon-14 Uptake and Retention-Inhalation Ambient air contains about 330 ~LL/L of CO,. Exposure to elevated levels of 14C02 leads to a somewhat different pattern of uptake and retention than that resulting from ingestion of 14C in food. After an acute inhalation exposure, the 14C02equilibrates within a few minutes with air in the lung space and in the alveolar spaces. Alveolar air contains about 180 times the concentration of COz that is found in ambient air. Therefore, the specific activity of the 14C02is lowered to about 0.55 percent of its initial value (Buchanan and Nakao, 1952). As the alveolar COz equilibrates with venous blood and subsequently with the C02 and bicarbonate and tissue fluid of bone it is further diluted to perhaps 60 or 70 percent of its concentration in alveolar air. The incorporation or fixation of the bicarbonate into tissue fats, proteins, and carbohydrates takes place apparently only to a limited extent through carboxylation of a derivative of pyruvic acid to oxaloacetic acid (White et al., 1973). Subsequently oxaloacetic acid is rapidly oxidized by the citric acid cycle to C02. However, the oxaloacetate pool is in equilibrium with aspartic acid through transamination. Further, a-ketoglutamic acid is formed in the cycle, and it may similarly be converted into glutamic acid. Pyruvic acid derived from that amino acid and from other parts of the cycle may be converted into glucose or into alanine and thus enter proteins. Alternatively, it may be converted to acetic acid or other two-carbon moieties and enter into fat. In that manner, respiratory C 0 2 may enter into all organic components of body tissue except for its exclusion from certain essential metabolites (White et al., 1973). However, it enters through compartments that are turned over rapidly. Therefore, concentrations of 14C derived from atmospheric CO2 generally cannot reach as high a value in tissue as those derived from ingested carbon. Buchanan and Nakao (1953) have found that the fraction of tissue carbon in mice that is derived from inhaled C 0 2 is between 1.6 and 3.4 percent. Because about 99.45 percent of the alveolar Con is derived from recently ingested carbon compounds and tissue carbon, the total uptake from atmospheric CO, a t equilibrium must be between 5.5 x x 1.6 = 8.8 x x 3.4 = 1.9 X lo-' perceilt and 5.5 x percent. It is unlikely that this value is much different for any mammal although the time required to reach equilibrium probably varies with the size of the animal. For Reference Man, the total quasi-steady state level of atmospheric C 0 2 in fixed tissue carbon is, on this basis, between 16,000 g x 8.8 x = 1.4 g and 16,000 g x 1.9 x lop4 = 3.0
52
/
6. BEHAVIOR OF "C IN BIOLOGICAL SYSTEMS
g or less than the 4.1 g inspired in one day. The carbon derived from the 4.1 g d-I of inspired atmospheric COz "fixed" daily is approximated by: 16,000 g 16,000 g
X
X
8.8 X
x
0.693 days 37
-
0.693 1.9 X lo-'' x -days 37
=
2.6 x lo-' g/d and
=
5.7 x
lo-'
g/d or
0.63 t o 1.38 percent. Bones have about 20 times more organic carbon than carbonate (Henricks and Hill, 1950), but the specific activity of bone carbonate should reach a value as high as (0.7 percent x 0.7)/(1.9 x 10 percent) = 26 times that of the organic carbon; or, roughly, the bone dose will be about double that calculated from the total carbon as organic carbon. Following cessation of inhalation of 14C02, after the equilibrium state has been reached, the body pool of bicarbonate = C02, less that in bones, is eliminated at a rapid rate, almost all being washed out in one or two days (Hellman et al., 1953; and Baker et al., 1954). The bone carbonate pool, however, will be eliminated more slowly, the actual rate depending upon the amount of bone remodeling that took place during exposure and to some degree on the duration of the exposure. However, no major error is introduced in dose estimation by treating the bone carbonate as organic carbon despite its longer and more complex series of pools because the bone carbonate pool contains only about 0.75 percent of the total body carbon and about 20 percent of the total 14Cactivity under these circumstances. The above considerations may be contrasted with the reverse situation where the ingested carbon is of high, and inhaled C 0 2 is of low, 14Cspecific activity. In this case, under quasi-equilibrium conditions, 99 percent of the alveolar C 0 2 is derived from tissue and food. Consequently, all carbonate and organic carbon pools will have the same specific activity and the average bone dose from 14C02will be four percent of the bone dose from organic carbon and only 0.75 percent of the total body 14Cdose.
6.4
DNA Incorporation
Male sperm cells are produced from spermatogonia continuously in the adult male and consequently the 14Cspecific activity in the nuclear
6.5 DNA INCORPORATION
/
53
material must resemble that of food with a lag time of one or two years. On the other hand, female oocytes are laid down in the fetus before birth and thereafter remain dormant until they ripen just before being shed. It is uncertain how much of the adjacent tissue may be subject to turnover and renewal but, in any case, none of the deoxyribonucleic acid of the ovum, except the small fraction renewed by repair processes would contain 14Cof the current specific activity. There will, therefore, be a delay of 16 to 40 years in the human female for expression of the genetic effects of a given 14C/"C ratio. Calculation of the genetic effect to the current generation of an added increment of I4C to the atmosphere using a lag time of one or two years, therefore, is conservative.
6.5 Carbon-14 in Human Food Atmospheric C 0 2 is incorporated in cellular material by the photosynthetic action of green plants. Annual plants and grasses, from which most foodstuffs are derived, equilibrate with the 14C02of the air. Equilibration is subject to the slight differences in specific activity produced by the isotopic fractionation effects. The mass of 14C02 is slightly greater than 12C02and I3CO2.A small amount of atmospheric CO:, is also fixed in animal systems through phosphoenolpyruvate conversion to malate. That amount is negligible compared to that received through dietary absorption, however. Food materials produced by the growth of plants are consumed by animals and the protein, carbohydrate, and fat are broken down to simpler molecules, absorbed, and then reassembled to provide the protein, carbohydrate, fat and nucleic acids of the animals. During these processes there would be little change of the ratio of carbon isotopes. The isotope ratios do change slightly in going from the gaseous to the liquid phase and vice versa, but the changes are small compared to the uncertainties of estimating the consequences of radioactive decay in the plant or animal body. The decomposition on land of animal and plant materials returns C 0 2 to the atmosphere a t a rate approximately equal to the fixation rate of C 0 2 by the terrestrial biosphere. However, there is a delay time due to the fixation of C 0 2 in metabolic pools of materials with a low turnover rate such as forests and humus. In marine systems some metabolic C 0 2 reaches deep water which has a very slow turnover rate compared to shallow water. Some of the metabolic Con becomes fixed in marine sediments that effectively remove it from interchange with
54
/
6. BEHAVIOR OF
"C IN BIOLOGICAL SYSTEMS
CO2 available for metabolic cycling. These delays and removals result in reduction in specific activity of 14Cin C 0 2 estimated from the '"C production rate and size of C 0 2 reservoirs calculated without taking them into account. During the decades of the 1950s and '60s, nuclear explosions caused an addition of '"C to the atmosphere estimated at 9.6 MCi. This I4C has dispersed throughout the atmosphere and is slowly being absorbed into the marine environment with a half time of disappearance from the atmosphere of about six years. At the same time in accordance with Suess effect, the production of 12C02from the burning of fossil fuel is diluting both the naturally and artificially produced 14C(Suess, 1955). Consequently, the atmospheric 14C-level is declining in the northern hemisphere from a high of about twice the pre-test level to 8 pCi/g in 1983. Barring the resumption of atmospheric testing of nuclear explosives in the future, the decline will continue even with the addition of 14Cproduced by nuclear-power plants, until the ratio of newly produced 14C02to newly produced 12C02exceeds the ratio due to natural abundance of 14C. These considerations hold for the world population generally because atmospheric mixing time is rapid compared with the time required for equilibrium with terrestrial plants and with the marine environment. All of those processes are rapid compared with the nearly irreversible removal of 14Cby conversion to marine sediments and by radioactive decay. However, the considerations do not apply fully to populations living close to a source of I4C since some of their exposure may occur before much dilution takes place. It is, therefore, necessary to make estimates for populations living near nuclear-power plants and fuel-processing plants. It is instructive to examine the results of a sudden addition of I4CO2 to the atmosphere sufficient to raise the 14C/12Cratio significantly. Ignoring the time required to achieve relatively uniform distribution in the atmosphere of the hemisphere, the time required for uptake into growing plants to bring the whole plant to its maximum 14C/12C ratio is easily estimated. By the end of the second growing season, the harvested part of most food plants will be nearly a t equilibrium with the atmospheric CO,. Thus, the delay for most foodstuff of plant origin to reach maximum concentration of 14C is not more than one year. Tracer experiments with small animals show losses of '"C from various organs which follow several decay rates with biological half times less than one percent to greater than 100 percent of their life spans (Schubert and Armstrong, 1949; Buchanan and Nakao, 1952). It may be assumed that similar times are required t o gain full equilib-
6.6 CARBON-14 IN HUMAN FOOD
/
55
rium with food 14C. Animals that are conceived and brought to term during a period of elevated 14C, however, may have higher average (total body) activity concentrations in their tissues a t birth than their dams, because fetal tissues will more nearly reflect the current 14C composition of the food available to the dam. To account for the effect of changing ratios of 'TC/l2C in the calculation of lifetime radiation doses from 14C, it is necessary to estimate the "lagn time in the various steps, and, for great accuracy, to know the origin of foodstuffs and their corresponding lag times. Nydal et al. (1971) have reviewed the available data and published their own analysis. All of the data seem to be in agreement that the soft tissue 14C/12Cratio lags behind the tropospheric 14C/12Cby about one to two years. Since some tissues or body compartments have long turnover times, some approaching the life span, total body carbon in the adult subjected to a change in 14C/12C intake ratio may not approach equilibrium for decades.
6.6 Concentration Effects A problem in the estimation of the biological effects of any toxic material is the possibility of its passive or active accumulation in the biological material being studied. Depending upon its nature, any given material will follow one of at least three courses. As an example, poorly degradable organic chemicals that are new in the environment will, if continuously supplied, increase in concentration until they are degraded a t the same rate that they enter the environment. This assumes, of course, that the rate of supply remains constant, there being no non-toxic material of the same composition to dilute them, they may be passed from organism to organism without diminution of their potency. Radioactive isotopes of non-radioactive elements present in nature will, on the contrary, always be diluted. Their specific activity will be diminished as they encounter the stable isotope in the same chemical form comparable to the majority of organisms. In fact, in higher organisms, fatty tissue (-50 percent C, maximum) and fluid tissues like blood (-11 percent C) represent the extremes of carbon concentration likely to be encountered. Consequently, the maximum possible increase in concentration of carbon must be much less than five-fold. These considerations presuppose that the emitted or released 14Cis in the same chemical form that occurs in nature. However, a fraction
56
I
6. BEHAVIOR OF 14C IN BIOLOGICAL SYSTEMS
of 14C,especially from PWRs, is emitted as methane and ethane gases which are present only in limited amounts in the atmosphere. Because these gases are not rapidly metabolized by plants and animals only the immersion dose from these compounds needs to be considered a t the site of release. These gases are slowly converted by the action of sunlight to 14C02, and they mix with atmospheric 12C02. There is one other possibility of changes in concentration that requires discussion. If prey organisms are exposed temporarily to a relatively high 14C activity near its point of discharge, the fates of the high specific-activity carbon compounds passed on from prey to predator must be considered. For example, is there any possibility of some 14C-containingcomponent of nucleic acid, eventually reaching humans in undiluted form? Knowledge of metabolic pathways is quite sufficient to rule out any such possibility. Few organisms more complicated than viruses fail to synthesize their own purine and pyrimidine bases or the sugar backbone of the nucleic acids. Therefore, a t each predator-prey step in trophic level dilution must occur. Similar considerations also apply even to those amino acids which are not synthesized by higher organisms, provided only that a small fraction of the food supply is subjected to the higher specific-activity source.
6.7 Kinetics of Localized Releases on Vegetation Radioactive carbon dioxide from a "point" source diffuses into the atmosphere and becomes mixed by convection, diffusion, and atmospheric turbulence within a few kilometers of its origin. A full reduction of its concentration to complete uniformity, however, requires days and complete hemispheric mixing. Before extensive mixing, the effluent plume may reach ground level and the 14C02would then come in contact with the biosphere. Standard transport and diffusion theory may be applied to local meteorological data and to release of 14Cto calculate local concentrations. The concentrations are highly site- and time-specific depending upon the direction and speed of the transporting wind and upon the diffusion characteristics of the atmosphere. The solid lines of Fig. 6.1 are isolines of ground-level concentrations of 14C in air for a location in the north central U.S. predicting the annual average concentrations that might be typically expected on the basis of climatological winds and dispersion conditions to distances of about 80 kilometers from a source. The calculations of concentration assume a constant and continuous rate of emission of activity from the source. The dispersion
6.7 KINETICS OF LOCALIZED RELEASES
/
57
Annual Aver.(. h c . n t r a t l m / m i f o m _ ~ m t i n m ..ornee, (HC.
lumber. tn p r m t h e r i # c ~ r r * # p m d to: 1) aonrce 100 Cl/year 2 ) 03) e m e e n t r a t i m 330 v /L 3 ) a i r denmiti of 1.2 x 10 g m-3 and are given i n unit. of pCi/gC
5
)
wind data, 300 f t . l a w 1 for period 1/1/74-12/31/75
i N
DRESDEN IIUCUAR eonR Plant, Morri., I l l i n o i .
UX (27 l
Fig. 6.1 Annual average concentration for uniform continuous source and specific activity (in parentheses) for 100 Ci/year source.
estimates use standard theory with empirically determined parameters. The calculations assume no sinks of 14C. The numbers not in parentheses in Fig. 6.1 should be multiplied by the continuous source activity per second (e.g., Ci/s) to yield the predicted 14Cconcentration in Ci/m3. However, to estimate uptake by growing vegetation, the concentration of 14Cshould be given as a ratio to stable carbon. T o make the example more specific, a continuous emission rate of 100 Ci/y of I4C is used a s the source strength. With the additional assumptions listed on the figure, the isolines are also labelled, in parentheses, in units of specific activity (pCi14C/g12C).The
58
/
6. BEHAVIOR OF 14C IN BIOLOGICAL SYSTEMS
isolines can be scaled linearly to source strengths other than 100 Ci/ Y. In one study, 14C in natural materials was used to establish the average gaseous dspersion patterns of releases from nuclear installations (Otlet e t al., 1983). The problem of localized releases has been examined by Killough and Rohwer (1977) who have estimated that under unusual conditions local vegetation near a source may be found to have somewhat higher specific activity than is expected from the global average 14C02.This elevated 14C content, of course, cannot exist over a large area but, conceivably, might become a problem if a local vegetable source were the main supply of some specialty product (NCRP, 1984). Any local food supply would be mixed with products from other sources and thus be as effectively diluted by lower specific activity material as if the dilution had taken place in the atmosphere. The hemispheric mixing time for COP is relatively short compared to a growing season and, therefore, edible plant material tends to have almost the same 14Cspecific activity as the average atmospheric COz during the growth period. During a period of rapid change in ambient 14C02specific activity there should be only a short lag period, a maximum of a few months, between atmospheric 14C02 activity and that of edible plants.
7. Projected Radiation Doses From 14C 7.1 Environmental Models In order to assess the long-term behavioi of 14C added to the atmosphere as 14C02,it is necessary to consider four physical processes. These are: (1) transfer between the atmosphere and other reservoirs; (2) changes in the ability of the non-atmosphere reservoirs to take up and release 14C;(3) radioactive decay; and (4) since the ratio of 14Cto 12C is always desired, the future concentration of 12C as well. Uncertainties in the exchange of carbon between reservoirs and in the future inputs of fossil-fuel COz into the atmosphere suggest that simplified models may be acceptable for present purposes. 7.1.1 Compartment Model
The calculational model used (Machta, 1971) is a series of boxes each representing a reservoir of exchangeable carbon. It is assumed that no gain or loss of carbon occurs to the sea floor, by volcanic action, or by a natural phenomenon. Were there to be an exchange with the ocean silt, for example, there would be a net loss of 14Csince its long residence time in the silt would allow radioactive decay. Fig. 7.1 shows the various boxes in the model and the fractional removal between boxes per year as well as the 14C/12Cfractionation factor which has been used. The apparent ages of the oceanic reservoirs relative to that in the atmosphere also appear in the three 14C boxes. The biosphere is taken as part of the atmosphere for all practical purposes (because rapid exchange with the atmosphere is used) except for its lack of contact with the mixed layer of the-oceans. The buffering factor of the oceans, about 10 at present, increases as more C02 is absorbed by the oceans in accordance with estimates (Keeling, 1978). The box model has been tested (and adjusted) to predict certain known features. First, in a steady state, the.approximate cosmic-ray production of 1.8 atoms of 14C/cm2of the earth's surface per second yields an atmospheric specific activity of about 6 pCi 14C/gC which is 59
60
/
7. PROJECTED RADIATION DOSES
"c
I4C
Atmosphere Atmosphere
Mixed L a y e r Age Relative To Air:182y
Deep L a y e r
D e e p Layer
Relotive
Fig. 7.1 Parameters of box rnodela and their interchange of carbon. Exchange times shown by arrows between boxes refer to the time to transfer l/e of the carbon content between boxes by the indicated route. The correspondinghalf times to transfer material between boxea are given in parentheses.
almost exactly the commonly accepted 14C background before atmospheric nuclear testing. Second, the exchange coefficients have been adjusted such that the predicted growth in atmospheric 12C02between 1958 and 1980 closely matches that observed a t Mauna Loa, Hawaii. Third, the age of the deep ocean (-1000 years) has been chosen to lie in the range of accepted values. The close equality of the ages of the mixed and intermediate layers of the oceans suggests that we are dealing with a single very deep mixed layer.
7.1.2 Projected Environmental "C Specific Activity from NuclearPower Production Fossil-fuel COP has been added to the atmosphere in significant amounts since about 1860. A carbon cycle model yields future estimates
7.1 ENVIRONMENTAL MODELS
/
61
of C 0 2 atmospheric concentration (Keeling and Bacastow, 1977;j (Fig. 7.2) given the atmospheric inputs. These concentrations, in conjunction with estimated 14Cannual releases from nuclear-power production and the same carbon cycle model for the period 1950 to 2000 AD (Fig. 7.3), produce future 14Cspecific activities. Fig. 7.4 presents the model's prediction of the 14C specific activity from nuclear power generation as a function of time for the period 1950 to 2000. The 14C input to the model assumes that the annual release of 14Cwill plateau in the period 1990 to 2000 (Fig. 7.3). The model predicts a steady rise in 14C specific activity with a slower rate of increase after about 1980 due to the offsetting effects of a continuing rise in COP concentration and the leveling off of the 14C releases from nuclear-power production. Fig. 7.5 presents the total 14C specific activity in ground level air for the period 1955 to 2000. Included in this total specific activity are the contributions from 14Creleases from production of nuclear power (see
Fig. 7.2 Atmospheric concentration of CO, in rL/L or parts per million derived from the model calculation as a function of time for the period 1950-2000. Observed fossil fuel derived CO, emissions used until 1980 followed by an annual p w t h rate in emissions of 2% per year (Keeling and Bacastow, 1977).
-
1950
1960
1970
1980
1990
2000
Year
Fig. 7.3 Annual release of "C from nuclear-power production as a function of time for the period 1950-2000 as described in Section 3.4.
Fig. 7.4 Specific activity of 14C from nuclear power production as a function of time from 1950 to 2000 as predicted from a box model calculation utilizing estimates of "C release from nuclear power production (Section 3.4) and a growing emission of lPC from fossil-fuel combustion.
12
11
-2 "
.?
b
10 9
8
5
'
<
6
2 z s 2 f
5 4
3 2
1
----.
0 1950
-
1960
-----------1910
1980
1990
2000
Fig. 7.5 Total atmospheric I4C specific activity as a function of time. Included in the total are contributions from nuclear power production, from weapons testing, and natural atmospheric I T C .
7.2 ENVIRONMENTAL MODELS
/
63
Fig. 7.4), fallout from weapons testing (Sections 3.3 and 3.4), and natural 14Cin the atmosphere (about 6 pCi/gC).
7.2 Dosimetry The dosimetry of 14Cassumes that environmental I4C is in equilibrium with that in the human body so that the specific activity of the environmental 14Cis identical to that of biological 14C.Although there is a slighi isotopic fractionation in the photosynthetic process, the inaccuracies associated with neglecting that effect are small relative to the complicated biokinetics of I4C. There are two approaches to the dosimetry of 14C: the first approach assumes a static equilibrium between the specific activity of 14C in the environment and specific activity of I4C in the biological system; while the dynamic model is used for unit releases of I4C to a localized population.
7.2.1 Steady-State Specific-Activity Dosimetry Systems A static equilibrium system may be utilized for environmental conditions with relatively constant 14C specific activity or slowly changing specific activity. Moghissi and Carter (1977) have developed an equation which provides a simple method for such a calculation: where
Ha = the annual dose equivalent rate (mrem/y),
F = the fraction of carbon in the organ of interest, and C = the specific activity of I4C in (pCi/gC). Killough and Rohwer (1978) have developed a more detailed steadystate specific-activity model which can be used to estimate individualorgan and whole-body doses for both ingestion and inhalation pathways. The model takes into account physiological parameters such as carbon content of various organs which influence 14Cdose distribution within the body and is based on data for Reference Man (ICRP, 1975) (see Table 6.1). The dose equivalent rate Hd (rem/d) is calculated by the Eq. (7-2).
/
64
7. PROJECTED RADIATION DOSES
TABLE7.1-Carbon- 14 specific-activity dose equiualent-rate factors for various organs of Reference Man" Dose equivalent-rate factors (rern/d per pCi/gC)
Reference Organ
Ingestion
Total body Skeleton: Total endosteal cellsb Red marrowb Yellow marrowb Bonec Lungs Liver Kidneys Spleen Thyroid Testes G. I. tract Adipose fat
Inhalation
0.57
5.7 x lo-5
0.91 1O . 1.6 0.33 0.25 0.36 0.32 0.28 0.26 0.22 0.44d 1.7
9.1 x 1.0x 1.6 x 3.3 x 2.5 X 3.6 x 3.2 x 2.8 x 2.6 X 2.2 x 3.0 x 1.7 x
lo4 lo-' lo-' lo-' lo-" lo-' lo4 lo-'
lo'* lo-' lo-4
ICRP (1975) Factors based on information tabulated by Snyder et al. (1974). ' Defined a s skeleton minus marrow. Model of Dolphin and Eve (1966) employed in computation of factor. a
where the specific activity of all ingested carbon (&i/gC), and
A,
=
Fd
= the dose-rate factor from Table 7.1 (rem/d per ~ c i / g C ) .
The inhalation dose rate may be calculated in a similat manner by utilizing the inhalation column of Table 7.1 along with the specific activity of inhaled carbon (in &i/gC). The use of this system is most appropriate for exposure situations in which the major pathway of exposure is by ingestion and in which the environmental specific activity of 14Cvaries slowly with respect to the turnover rate of carbon in human tissue.
7.3 Dose to Man from 14C in the Environment 7.3.1 Natural 14C The average specific activity of 14Cin the atmosphere due to carbon14 production by cosmic rays is about 6 pCi/gC corresponding to a n atmospheric inventory of 3.8 MCi (UNSCEAR, 1977). The absorbed
7.3 DOSE TO MAN
1
65
dose rate in man is 1.25 mrad per annum which corresponds to about one percent of the average annual total-body natural background absorbed dose rate of 100 mrad.
7.3.2 Weapons Carbon-14 in the troposphere (present in the form of C02) reached a peak in 1965 as a result of the nuclear testing conducted in the late 1950s and early 1960s. Although there have been additional tests since that period, the addition of 14Cto the troposphere has since been offset by uptake by the oceans and biosphere. The dose equivalent rate in man from fallout I4C is estimated to have been about 0.96 mrem/y at maximum in 1965 and is currently about 0.37 mrem/y. These estimates are based on computed tropospheric 14Cspecific activities due to the injection of 9.6 MCi by weapons tests (Section 3.3).
7.3.3 Nuclear Power Evaluation of the dose resulting release of 14Cfrom a nuclear-power plant requires consideration of the local and regional dispersion. The concentrations and absorbed doses a short distance from the point of release could be variable depending on specific local conditions (Killough and Rohwer, 1978; NCRP, 1984). However, I4C in the form of 14 COz disperses rather rapidly so that the assumption of equilibrium between biological tissue and atmospheric specific activity remains valid. Projected absorbed dose rates in man from 14C released in the production of nuclear power have been based on I4C specific activity, as predicted from the box model distribution of I4C released from production of nuclear power for the period 1955 to 2000 (Section 7.2.2). The estimates of previous and projected releases of 14Care those that were given in Section 3.4. The results are shown in Fig. 7.6. Also shown are the contributions to absorbed-dose from natural I4C in the environment, fallout from weapons testing, and the total absorbeddose rate to man. The dose to human beings from 14C will be insignificant for the foreseeable future. The contribution from fallout will continue to decrease with time and that from nuclear power production will increase but will remain a t a level two orders of magnitude less than from that naturally produced I4C.
66
/
7. PROJECTED RADIATION DOSES
0.0001 1950
1960
1970
Year
1980
1990
Fig. 7.6 Projected Tissue Dose Rate in Man.
2000
8. Waste Management Though 14Cis widely used as a research tool in biological and medical research and is produced in nuclear power reactors, the waste-management history of this radionuclide appears to be limited. Management practices for 14Cwaste are different depending upon the origin of the waste. Nuclear power reactors generate I4C in specific waste streams and disposal follows practices set up for processing other radioactive wastes. As noted earlier, the best management practice for unwanted material is to reduce its formation. In a nuclear power reactor, the single most important step that can be taken t o reduce formation of 14Cis proper consideration of nitrogen as a precursor for 14Cformation. Reducing the quantity of 14Cformed in the fuel requires that the nitride-nitrogen impurity content of the fuel be reduced, and that air be removed from each fuel rod in a vacuum degassing step before the ends of the rod are closed by welding. In contrast t o reactor generated 14Cwaste, various medical and research institutions generate 14Cwaste in a variety of chemical and physical forms and these must follow a more diverse system of waste management. Consideration of any management practice should also include an evaluation of the potential dose reduction due to adopting that practice. Generally, as discussed in Section 7, the environmental dose is insignificant for other than naturally produced 14C.
8.1 Nuclear Power Reactors Extensive studies conducted a t various nuclear power reactors have provided data on 14Cconcentrations in various waste streams. Section 3.4 contains a brief review of available information. The exact distribution of 14C in various waste streams is unknown. However, it is known that a large fraction of generated 14C is released into the atmosphere (see Section 3.4 for details), an unknown, small fraction is retained in the off gas trapping system and is disposed of in shallow land burial, and a third fraction is retained in the liquid waste and is immobilized and disposed of in shallow land burial. A recent study by Snellman and Salonan (1982) provides evidence of the presence of 14C02in ion exchangers used for purification of water in light water 67
68
/
8. WASTE MANAGEMENT
reactors (less than 1% of 14Cis produced in the coolant and discharged compared to that in the gaseous effluents). The conclusions of Snellman and Salonan depend heavily on data for one Russian reactor. The technology for the removal of COP from various waste streams is well established and can be applied to 14C02if necessary. Because atmospheric releases from nuclear power reactors and from most research facilities are relatively small and because of the introduction of large quantities of 12C02originating from the combustion of fossil fuels into the atmosphere, there has been little need to collect I4CO2. However, provisions have been made to collect 14C02from fuel reprocessing plants.
8.1.1 Removal and Disposal
Suggested technologies for the removal of I4C from air effluents are summarized in ERDA (1975), Davis (1979), and Bray et al. (1977). These technologies generally require an initial catalytic oxidation step to convert hydrocarbons to COPand are followed either by a selective chemical or cryogenic absorption step for COz removal. These technologies, some of which have been carried to the pilot plant stage during development of processes for the removal of krypton from airborne effluents, appear to be readily developable if required for reduction of I4C releases to the environment. The fluorocarbon solvent-absorption process (Stephenson et al., 1976), developed for the recovery of krypton from the off-gas of LWR and LMFBR fuel-reprocessing plants, can also be used to collect COz. The C02 so collected could be discharged into a slurry of Ca(OH)2 (Croft, 1976) and converted to CaC03. Similarly, the KALC (Krypton Absorption in Liquid COP) process (Glass et al., 1976; Whatley, 1973), developed to recover and retain krypton from the COz gas stream of an HTGR fuel reprocessing plant, is also able for collection of 14C. Alternatively CaC03 and other solid 14C wastes can be immobilized using several agents for permanent disposal (Moghissi et al., 1978).
8.2 Institutional Waste Section 3.5 contains information on the quantities of waste generated in medical research and other facilities. Institutional wastes consist of a wide variety of organic materials, including liquid-scintillation fluids, biological wastes (such as animal carcasses, excreta, and
8.2 INSTITUTIONAL WASTE
/
69
bedding), and dry solid wastes. Eisenbud (1980) estimates that in the U.S. in 1978 a total of 221 Ci of 14Cwaste from 2390 institutions was shipped to burial sites for disposal.
8.2.1 Waste Treatment Most of the institutional waste is minimally treated and shipped to shallow-land-burial sites for disposal. Animal carcasses and other solid wastes are shipped in loose fillers such as diatomaceous earth or vermiculite. Some aqueous waste is disposed of in sanitary sewers and some is shipped to disposal sites. Gaseous institutional waste is generally not treated except for particle filtration. Incineration is a preferred method for disposal of low-level combustible waste. Waste generated in many biological and research facilities contains I4C a t specific activities dose to or only somewhat greater than that of atmospheric 14C.Because of the comparatively large volume of waste generated in many installations, the risks of packaging, transportation, and burial should be carefully compared to the risk from incineration and subsequent release of 14Cinto the atmosphere. In many cases that waste is incinerated in a facility which also combusts fossil carbon resulting in the release of 14Cwith a specific activity insignificantly above the atmosphere I4C (Eisenbud, 1980). Regulations of the U.S. Nuclear Regulatory Commission promulgated in 1981 permit incineration of certain low-level tritium- and 14C-wastessuch as scintillation fluids, animal carcasses, and other biomedical wastes (NRC, 1981). These regulations have led to a reduction of the volume of solid waste containing low levels of 14C.
8.3 Mobility of 14C Following Shallow-Land Burial
Carbon-14 in the immobilized high-level waste from nuclear power reactors remains isolated for the duration of the stability of containment. It is unlikely that I4C will be released from encapsulated waste unless the integrity of both the waste and the repository is destroyed. However, most of the waste in the commercial radioactive-waste burial sites in the U.S. are large volume, low activity concentration wastes such a s paper, rags, sorbed liquids, glassware, plastics, protective clothing, concrete, or animal carcasses. Both organic and inorganic decomposition products are found in certain radioactive waste burial trenches, and the radioactive leachate is chemically indistinguishable
70
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8. WASTE MANAGEMENT
from leachates obtained from ordinary sanitary landfills (Husain et al., 1979). Organic 14C is found in five to ten-fold greater concentrations than inorganic I4C(Matuszek et al., 1979) further indicating that leaching has taken place from the material buried at that site. The slow movement of water and the retention of much of the 14C in the solid immediately surrounding the trenches in a low-level waste disposal site, indicate that water transport will not contribute to 14C movement into the offsite environment (Matuszek, et al., 1979; Prudic, 1979). Biological decomposition of the buried wastes produces radioactive gases. Kunz (1982) provides data indicating the presence of 14C labelled methane and carbon dioxide, and other hydrocarbons in the vicinity of a low-level waste disposal site. The ratio of specific activities of these gases ranged widely. The mechanism for methane formation is elucidated by Frances et al. (1980) who showed the presence of methane forming bacteria in trench leachates collected from two lowlevel disposal sites. The respiration of radioactive gases from burial trenches is thus a potential pathway for the exposure of residents living in the vicinity of low-level waste disposal sites (Matuszek and Robinson, 1983). Hemming and Smith (1984) have concluded that present models of the transport of I4C from shallow land burial are crude and need to be improved, with many more data being required on the behavior of 14C and stable carbon inside and outside the burial facility.
8.4
Summary
The quantities of I4C produced in nuclear power reactors and used in medical and research facilities are small as compared to the natural 14C and bomb produced 14C and are diluted by the stable carbon resulting from the combustion of fossil fuels. If necessary, CO, can be collected from various waste streams of nuclear power reactors and fuel-reprocessing plants. Carbon-14 in the form of CaC03 or other appropriate forms can be immobilized and disposed of in a disposal site. Institutional wastes are generally of low activity concentration in large volumes. Appropriate volume-reduction techniques such as incineration may be used under certain conditions.
APPENDIX A
Glossary absorbed dose: The energy imparted to matter by ionizing radiation per unit mass of irradiated material at the place of interest. The special unit of absorbed dose in the rad. One rad equals 0.01 joules per kilogram. biological half-time: The time required for the body to eliminate one-half of an administered dosage of any substance by regular processes of elimination. biosphere: The life zone of the earth, including the lower part of the atmosphere, the hydrosphere, soil, and the lithosphere to a depth of about two kilometers. boiling w a t e r reactor (BWR):A nuclear reactor in which water used for coolant is allowed to boil. 2: Average concentration of radionuclide in the atmosphere at a downwind point (Ci m-"). k/Q': Ratio of average air concentration to release rate a t the source (s m-3). coolant: A substance, usually a liquid or gas, used for cooling any part of a reactor in which heat is generated. Such parts include not only the core but also the reflector, shield, and other elements that may be heated by absorption of radiation. computer model: The simulation of a physical system by use of a computer program (code) and a set of real world data. cross section, nuclear: The probability that a certain reaction between a nucleus and an incident particle or photon will occur. It is expressed as the effective "area" the nucleus presents for the reaction. "Macroscopic cross section" refers to the cross section per unit volume or per unit mass. "Microscopic cross section" is the cross section of one atom or molecule. dose equivalent (H): The product of the absorbed dose in rads, the quality factor, and any other modifying factors. Dose equivalent is expressed in reins and is considered to be related to the radiation risk. fast reactor: A nuclear reactor in which most of the fissions are 71
72 /
APPENDIX A
produced by fast neutrons, with little or no moderator to slow down the neutrons. fission product: Any radionuclide or stable nuclide resulting from nuclear fission, including both primary fission fragments and their radioactive decay products. g r a p h i t e moderated reactor (CMR): A nuclear reactor where graphite is used as a moderator, primarily gas cooled reactors. GWe: Gigawatts electrical. GWth: Gigawatts thermal. kiloton (KT): A unit of explosive energy equivalent to that released upon detonation of lo3 tons of TNT. heavy w a t e r reactor (HWR): A nuclear reactor in which heavy water serves as moderator and sometimes also as a coolant. high t e m p e r a t u r e g a s cooled reactor (HTGR): A reactor in which the coolant is pressurized helium gas; the fuel consists of fully enriched uranium and thorium. liquid metal fast b r e e d e r r e a c t o r (LMFBR): A type of fast reactor using highly enriched fuel in the core, fertile material in the blanket, and a liquid-metal coolant such as sodium; high energy neutrons fission the fuel in the compact core, and the excess neutrons convert fertile material to fissionable nuclides. maximum exposed individual (Maximum Individual): The individual whose locations and habits tend to maximize his radiation dose, resulting in a dose higher than that received by other individuals in the general population. megaton (MT): A unit of explosive energy equivalent to that released upon detonation of lo6 tons of TNT. MgU: Megagram of uranium, the SI equivalent of MTU. moderator: Material used to moderate or slow down neutrons from the high energies a t which they are released. MTU: Metric tons of uranium equivalent, as applied to mass of reactor fuel. MWe: Megawatts electrical. MWt: Megawatts thermal. nuclear reactor: An apparatus in which nuclear fission may be sustained as a self-sustaining chain reaction. power: The time rate of doing work; the unit of power is watt. pressurized w a t e r r e a c t o r (PWR): A nuclear reactor in which water is circulated under enough pressure to prevent it from boiling, while serving as moderator and coolant for the uranium fuel; the heated water is then used to produce steam for a power plant. quality f a c t o r (Q): A multiplying factor used with absorbed dose to express its effectiveness in causing detrimental biological effects.
GLOSSARY
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73
radioactive half-life: Time required for a radioactive nuclide to decrease to one-half its initial activity by radioactive decay. source strength (Q'): The number of curies of a radionuclide released to the atmosphere per unit time (Ci s-') (see XIQ'). specific activity: Total activity of a given nuclide per gram of a compound, element, or radioactive nuclide.
References ALBRITTON, E. C., Ed. (1954). Standard Values in Nutrition and Metabolism (W. B. Saunders Co., Philadelphia, Pennsylvania). ANBAR, M. (1978). "The limitations of mass spectrometric radiocarbon dating using CN-ions," page 152 in Proceedings of the First Conference of Radiocarbon Dating with Accelerators, Gove, H . E., Ed. (University of Rochester, Rochester, New York). ANDERSON, E. C. (1953). "The production and distribution of natural radiocarbon," Ann. Rev. Nucl. Sci. 2, 63. ANDERSON, R. L., COOLEY,L. R., BECK,T. J., A N D STRAUSS,C. S. (1978). Institutional Wastes, Report NUREG/CR-0028 (Nuclear Regulatory Commission, Washington, D.C.). T. A N D RUHLE,H. (1980). "Emission von ARNDT,J., GANS,I., LEHMANN, Kohlenstoff-14 und Tritium mit abwasser aus kernkraftwerken in der Bundesrepublik Deutschland," page 97 in Strahlenschutzproblerne i m Zusammenhang mit der Verwendung won Tritium und Kohlenstoff-14 und ihren Verbindungen, Strieve, F . and Kistner, G. Eds. (Institut h r Strahlenhygiene des Bundesgeshundheitsamts, Neuherberg). J. R. AND ANDERSON, E. C. (1957). "The distribution of carbon-14 ARNOLD, in nature," Tellus 9, 28. ASHENFELTER, J. E., GRAY,J., JR., SOWL,R. E., SVENDSEN, M., A N D TELEGADAS, K. (1972). "A lightweight molecular-sieve sampler for measuring stratospheric carbon-14," J. Geophys. Res. 77,412. BAES,C. F.,JR., GOLLER,H. E., OLSON,J. S., AND ROTTY,R. M. (1976). The Global Carbon Dioxide Problem, Report ORNL-5194 (Oak Ridge National Laboratory, Oak Ridge, Tennessee). BAKER,N., SHREVE, W. W., SHIPLEY,R. A,, INCEFY,G. E., A N D MILLER,M. (1954). "C14studies in carbohydrates metabolism: I. The oxidation of glucose in normal human subjects," J. Biol. Chem. 2 1 1,575. BECK,G. J. (1979). "Peach Bottom atomic power plant radwaste system backfit considerations," Trans. Chem. Nucl. Soc. 30, 560. BENNETT, C. L. (1979). "Radiocarbon dating with accelerators," Am. Sci. 67, 450. BENNETT,C. L., BEUKENS,R. P., CLOVER,M. R., GOVE,H. E., LIEBERT,R. B., LITHERLAND, A. E., PURSER,K. H., A N D SANDHEIM, W. E. (1977). "Radiocarbon dating using electrostatic accelerators: Negative ions provide the key," Science 198,508. D., GOVE,H. E., BENNETT,C. L., BEUKENS,R. P., CLOVER,M. R., ELMORE, KILIUS,L., LITHERLAND, A. E., AND PURSER,K. H. (1978). "Radiocarbon 74
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dating with electrostatic accelerators: Dating of milligram samples," Science 201,345. BERGER,R. AND SUESS,H. E., Ed. (1979). The Ninth International Radiocarbon Conference (University of California Press, Berkeley, California). BERLIN,N. I., AND TOLBERT,B. M.(1955). 'Metabolism of Glycine-2-C14in man: Further considerations of pulmonary excretion of C"Oz," Proc. Soc. Exp. Biol. Med. 88, 386. BERLIN,N. I., TOLBERT,B. M., AND LAWRENCE, J. H. (1951). "Studies in Glycine-2-C14metabolism in man: I. The pulmonary excretion of C1402,"J. Clin. Invest. 30, 73. BLANCHARD, R. L., BRINCK,W. L., KOLDE,H. E., KRIEGER,H. L., MONTGOMERY, D. M., GOLD,S., ARTIN, IN, A., AND KAHN,B. (1976). Radiological Surveillance Studies at the Oyster Creek B WR Nuclear Generating Station (U.S. Environmental Protection Agency, Office of Radiation Programs, EERF, RNEB, Cincinnati, Ohio). BOLIN,B. (1970). "The carbon cycle," Sci. Arner. 223, 124. BONKA,H. (1980). "Produktion und freisetzung von tritium und kohlenstoff 14 durch kernwaffenversuche, test-explosionen und kerntechnische anlagen, ein schlieblich wiederaufbereitungsanlagen," page 17 in Strahlenschutzprobleme im Zusarnmenhang mit der Verwendung oon Tritium und Kohlenstoff14 und Verbindungen, Strieve, F . and Kistner, G. Eds. (Institut fiir Strahlenhygiene des Bundesgesundheitsamtes, Neuherberg). BONKA, H., SCHWARZ, G., A N D WIBBLE,H. B. (1973). "Contamination of the environment by I4C produced in high temperature reactors," Kerntechnik 15, 297. R. E. AND YOUNG,R. K. (1972). "Preparation of gas samples for BOSSHART, liquid scintillation counting ofcarbon-14," Anal. Chem. 44. 1117. BRANSOME, E. D., JR.,ED. (1970). The Current Status of Liquid Scintilhtion Counting (Grune and Stratton, New York, New York). BRAY,G. R., MILLER,C. L., NGUYEN,T. D., AND RIEKE,J. W. (1977). Assessment of Carbon-14 Control Technology and Costs for the LWR Fuel Cycle, Report EPA 52014-77-013 (U.S. Environmental Protection Agency. Washington, D.C.). BROECKER, W.S., PENG,T. H. A N D ENGH,R. (1980). "Modeling the carbon system," Radiocarbon 22, No. 3,5. BROECKER, W. S., SCHULERT, A., A N D OLSON,E. A. (1959). "Bomb carbon14 in human beings," Science 130, 331. BROECKER, W. S. A N D LI, Y. H. (1970). "Interchange of water between the major oceans," J. Geophys. Res. 75, 3545. BROECKER, W. S. A N D WALTON, A. (1959). "Radiocarbon from nuclear tests," Science 1 3 0 , 309. B R O E ~ E W. R , S. (1973). "Factors controlling COzcontent in the oceans and at&sphere," page 32 in Carbon and the Biosphere, Proceedings of the 24th v o k h a v e n Symposium in Biology, Upton, N. Y., May 16-18,1972, CONF.'720510 (Technical Information Center, U.S. Atomic Energy Commission, Washington, D.C.).
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Carbon Monoxide (National Academy of Sciences, Washington, D. C.). NCRP (1976). National Council on Radiation Protection and Measurements, Tritium Measurement Techniques. NCRP Report No. 47 (National Council on Radiation Protection and Measurements, Bethesda, Maryland). NCRP (1977). National Council on Radiation Protection and Measurements, Enuironmental Radiation Measurements, NCRP Report No. 50 (National Council on Radiation Protection and Measurements, Bethesda, Maryland). NCRP (1978). National Council on Radiation Protection and Measurements, A Handbook of Radioactivity Measurements Procedures, NCRP Report No. 58 (National Council on Radiation Protection and Measurements, Bethesda, Maryland). NCRP (1984). National Council on Radiation Protection and Measurements, Radiological Assessment: Predicting the Transport, Bioaccumulation, and Uptake by Man of Radionuclides Released to the Enuironment, NCRP Report No. 76 (National Council -on Radiation Protection and Measurements, Bethesda, Maryland). NEA (1980). Nuclear Energy Agency- Organization for Economic Co-operation and Development, Radiobgical Significance and Management of Tritium, Carbon-14, Krypton-85, Iodine-129 Arising from the Nuclear Fuel Cycle (Nuclear Energy Agency-Organisation for Economic Co-operation and Development, Paris, France). NELSON,D. E., KORTELING,R. G., A N D STOTT, W. R. (1977). "Carbon-14: Direct detection a t natural concentrations," Science 198, 507. NOAKES,J. E. (1963). Natural Radiocarbon Measurements by Liquid Scintillation Counting, dissertation (University Microfilms, Ann Arbor, Michigan, NO. 63-5689). NOAKES,J. E., KIM,S. M., AND STIPP,J. J. (1965). "Chemical and counting advances in liquid scintillation dating," page 68 in Proceedings of the Sixth International Conference on Radiocarbon and Tritium Dating (Clearinghouse for Federal Scientific and Technical Information, National Bureau of Standards, U.S. Department of Commerce, Springfield, Virginia). M. G. (1946). "The half-life determination of NORRIS,L.D. AND INGHRAM, carbon (14) with a mass spectrometer and low absorption counter," Phys. Rev. 70,772. NORRIS,L. D. A N D INGHRAM, M. G. (1948). "Half-life of carbon 14," Phys. Rev. 73,350. NRC (1981). U.S. Nuclear Regulatory Commission, "Biomedical waste disposal," Fed. Reg. 46, 16230. Nuclear Engineering International (1984). "Nuclear power progress," Nuc. Eng. Int. 29, No. 360, 2. NYDAL, R., GULLIKSEN, S., L~VSETH, K. A N D SKOCSETH,F. H. (1984). "Bomb I4C in the ocean surface 1966-1981," Radiocarbon 26, No. 1,7. K.,, A N D SYRSTAD, 0.(1971). "Bomb "C in the human NYDAL,R., L ~ V S E T H population," Nature 232, 418. K., A N D GULLIKSEN, S. (1976). "A survey of radiocarbon NYDAL,R., L ~ V S E T H variations in nature since the Test Ban Treaty," in the 9th International Radiocarbon Conference, University of California, Los Angeles and San
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The NCRP The National Council on Radiation Protection and Measurements is a nonprofit corporation chartered by Congress in 1964 to: 1. Collect, analyze, develop, and disseminate in the public interest information and recommendations about (a) protection against radiation and (b) radiation measurements, quantities, and units, particularly those concerned with radiation protection; 2. Provide a means by which organizations concerned with the scientific and related aspects of radiation protection and of radiation quantities, units, and measurements may cooperate for effective utilization of their combined resources, and to stimulate the work of such organizations; 3. Develop basic concepts about radiation quantities, units, and measurements, about the application of those concepts, and about radiation protection; 4. Cooperate with the International Commission on Radiological Protection, the International Commission on Radiation Units and Measurements, and other national and international organizations, governmental and private, concerned with radiation quantities, units, and measurements and with radiation protection. The Council is the successor to the unincorporated association of scientists known as the National Committee on Radiation Protection and Measurements and was formed to carry on the work begun by the Committee. The Council is made up of the members and the participants who serve on the Scientific Committees of the Council. The Sciedtific Committees, composed of experts having detailed knowledge and competence in the particular area of the Committee's interest, draft proposed recommendations. These are then submitted to the full membership of the Council for careful review and approval before being published. The following comprise the current officers and membership of the Council: 92
THE NCRP Officers President Vice President Secretary and Treasurer Assistant Secretary Assistant Treasurer
Members
GEORGER. LEOPOLD RAYD. LLOYD ARTHURC. LUCAS CHARLESW. MAYS ROBER0.MCCLELLAN JAMES E. MCLAUGHLIN BARBARA J . MCNEIL THOMAS F. MEANEY CHARLESB. MEINHOLD MORTIMERL. MENDELSOHN WILLIAMA. MILLS DADEW. MOELLER A. ALANMOGHISSI ROBERT D. MOSELEY,JR. WESLEYNYBORG MARYE. O'CONNOR FRANKL. PARKER ANDREW K. POZNANSKI NORMANC. RASMUSSEN WILLIAMC. REINIC CHESTERR. RICHMOND JAMESS. ROBERTSON A. SAGAN LEONARD WILLIAMJ . SCHULL GLENNE. SHELINE ROYE. SHORE WARRENK. SINCLAIR LEWIS V. SPENCER J O H N B. STORER WILLIAM L. TEMPLETON ROYC. THOMPSON JOHN E. TILL ARTHURC. UPTON GEORGEL. VOELZ W. WEBSTER EDWARD GEORGEM. WILKENINC H. RODNEYWITHERS
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T H E NCRP Honorary Members LAURISTON S. TAYLOR, Honorary President
EDGARC. BARNES VICTORP. BOND REYNOLD F. BROWN AUSTIN M. BRUES FREDERICK P. COWAN JAMESF. CROW MERRILEISENBUD ROBLEYD. EVANS RICHARD F. FOSTER C u r r e n t l y , the following subgroups are a c t i v e l y e n g a g e d in f o r m u lating recommendations: Basic Radiation Protection Criteria Medical X-Ray, Electron Beam and Gamma-Ray Protection for Energies Up to 50 MeV (Equipment Performance and Use) X-Ray Protection in Dental Offices. Standards and Measurements of Radioactivity for Radiological Use Waste Disposal Task Group on Krypton-85 Task Group on Disposal of Accident Generated Waste Water Task Group on Disposal of Low-Level Waste Task Group on the Actinides Task Group on Xenon Task Group on Definitions of Radioactive Waste Biological Aspects of Radiation Protection Criteria Task Group on Atomic Bomb Survivor Dosimetry Subgroup on Biological Aspects of Dosimetry of Atomic Bomb Survivors Radiation Associated with Medical Examinations Radiation Received by Radiation Employees Operational Radiation Safety Task Group 1 on Warning and Personnel Security Systems Task Group 2 on Uranium Mining and Milling-Radiation Safety Program Task Group 3 on ALARA for Occupationally Exposed Individuals in Clinical Radiology Task Group 4 on Calibration of Survey Instrumentation Task Group 5 on Maintaining Personnel Exposure Records Task Group 6 on Radiation Protection for Allied Health Personnel Task Group 7 on Emergency Planning Instrumentation for the Determination of Dose Equivalent Apportionment of Radiation Exposure Conceptual Basis of Calculations of Dose Distributions Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Radiation Bioassay for Assessment of Control of Intake of Radionuclides Experimental Verification of Internal Dosimetry Calculations
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Internal Emitter Standards Task Group 2 on Respiratory Tract Model Task Group 3 on General Metabolic Models Task Group 6 on Bone Problems Task Group 8 on Leukemia Risk Task Group 9 on Lung Cancer Risk Task Group 10 on Liver Cancer Risk Task Group 11 on Genetic Risk Task Group 12 on Strontium Task Group 13 on Neptunium Task Group 14 on Placental Transfer SC-59: Human Radiation Exposure Experience SC-61: Radon Measurements SC-63: Control of Exposure to Ionizing Radiation from Accident or Attack SC-64: Radionuclides in the Environment Task Group 5 on Public Exposure to Nuclear Power Task Group 6 on Screening Models Task Group 7 on Soil Contamination as a Source of Radiation Exposure SC-65: Quality Assurance and Accuracy in Rediation Protection Measurements SC-67: Biological Effects of Magnetic Fields SC-68. Microprocessors in Dosimetry SC-69: Efficacy Studies in Diagnostic Radiology SC-70: Quality Assurance and Measurement in Diagnostic Radiology SC-71: Radiation Exposure and Potentially Related Injury SC-72: Radiation Protection in Mammography SC-74: Radiation Received in the Decontamination of Nuclear Facilities SC-75: Guidance on Radiation Received in Space Activities SC-76: Effects of Radiation on the Embryo-Fetus SC-77: Guidance on Occupational and Public Exposure Resulting from Diagnostic Nuclear Medicine Procedures SC-78: Practical Guidance on the Evaluation of Human Exposures to Radiofrequency Radiation SC-79: Extremely Low-Frequency Electric and Magnetic Fields SC-80: Radiation Biology of the Skin (Beta-Ray Dosimetry) SC-81: Aesessment of Exposure from Therapy Committee on Public Education Ad Hoc Committee on Policy in Regard to the International System of Units Ad Hoc Committee on Comparison of Radiation Exposures Study Grouv on Comparative Risk Task Group on Comparative Carcinogenicity of Pollutant Chemicals Task Force on Occupational E x ~ o s u r eLevels Ad Hoc Group on Model used f i r Assessing Transport of Low-Level Radioactive Waste Ad Hoc Group on Medical Evaluation of Radiation Workers
In recognition of its responsibility to facilitate and stimulate cooperation among organizations concerned with the scientific and related aspects of radiation protection and measurement, the Council has
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created a category of NCRP Collaborating Organizations. Organizations or groups of organizations that are national or international in scope and are concerned with scientific problems involving radiation quantities, units, measurements and effects, or radiation protection may be admitted to collaborating status by the Council. The present Collaborating Organizations with which the NCRP maintains liaison are as follows: American Academy of Dermatology American Association of Physicists in Medicine American College of Nuclear-Physicians American College of Radiology American Dental Association American Industrial Hygiene Association American Institute of Ultrasound in Medicine American Insurance Association American Medical Association American Nuclear Society American Occupational Medical Association American Podiatric Medical Association American Public Health Association American Radium Society American Roentgen Ray Society American Society of Radiologic Technologists American Society of Therapeutic Radiology and Oncology Association of University Radiologists Atomic Industrial Forum Bioelectromagnetics Society College of American Pathologists Federal Communications Commission Federal Emergency Management Agency Genetics Society of America Health Physics Society National Bureau of Standards National Electrical Manufacturers Association Radiation Research Soceity Radiological Society of North America Society of Nuclear Medicine United States Army United States Air Force United States Department of Energy United States Department of Housing and Urban Development United States Department of Labor United States Environmental Protection Agency United States Navy United States Nuclear Regulatory Commission United States Public Health Service
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The NCRP has found its relationships with these organizations to be extremely valuable to continue progress in its program. Another aspect of the cooperative efforts of the NCRP relates to the special liaison relationships established with various governmental organizations that have an interest in radiation protection and measurements. This liaison relationship provides: (1) an opportunity for participating organizations to designate an individual'to provide liaison between the organization and the NCRP; (2) that the individual designated will receive copies of draft NCRP reports (at the time that these are submitted to the members of the Council) with an invitation to comment, but not vote; and (3) that new NCRP efforts might be discussed with liaison individuals as appropriate, so that they might have an opportunity to make suggestions on new studies and related matters. The following organizations participate in the special liaison program: Defense Nuclear Agency Federal Emergency Management Agency National Bureau of Standards Office of Science and Technology Policy Office of Technology Assessment United States Air Force United States Army United States Coast Guard United States Department of Energy United States Department of Health and Human Services United States Department of Labor United States Department of Transportation United States Environmental Protection Agency United States Navy United States Nuclear Regulatory Commission
The NCRP values highly the participation of these organizations in the liaison program. The Council's activities are made possible by the voluntary contribution of time and effort by its members and participants and the generous support of the following organizations: Alfred P . Sloan Foundation Alliance of American Insurers American Academy of Dental Radiology American Academy of Dermatology American Association of Physicists in Medicine American College of Nuclear Physicians American College of Radiology
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American College of Radiology Foundation American Dental Association American Hospital Radiology Administrators American Industrial Hygiene Association American Insurance Association American Medical Association American Nuclear Society American Occupational Medical Association American Osteopathic College of Radiology American Podiatry Association American Public Health Association American Radium Society American Roentgen Ray Society American Society of Radiologic Technologists American Society for Therapeutic Radiology and Oncology American Veterinary Medical Association American Veterinary Radiology Society Association of University Radiologists Atomic Industrial Forum Battelle Memorial Institute Center for Devices and Radiological Health College of American Pathologists Commonwealth of Pennsylvania Defense Nuclear Agency Edison Electric Institute Edward Mallinckrodt, Jr. Foundation Electric Power Research Institute Federal Emergency Management Agency Florida Institute of Phosphate Research Genetics Society of America Health Physics Society James Picker Foundation National Aeronautics and Space Administration National Association of Photographic Manufacturers National Bureau of Standards National Cancer Institute National Electrical Manufacturers Association Radiation Research Society Radiological Society of North America Society of Nuclear Medicine united States Department of Energy United States Department of Labor United States ~nLironmentalProtection Agency United States Navy United States Nuclear Regulatory Commission
To all of these organizations the Council expresses its profound appreciation for their support. Initial funds for publication of NCRP reports were provided by a
THE NCRP
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grant from the James Picker Foundation and for this the Council wishes to express its deep appreciation. The NCRP seeks to promulgate information and recommendations based on leading scientific judgment on matters of radiation protection and measurement and to foster cooperation among organizations concerned with these matters. These efforts are intended to serve the public interest and the Council welcomes comments and suggestions on its reports or activites from those interested in its work.
NCRP Publications NCRP publications are distributed by the NCRP Publications' office. Information on prices and how to order may be obtained by directing an inquiry to: NCRP Publications 7910 Woodmont Ave, Suite 1016 Bethesda, Md. 20814 The currently available publications are listed below.
Proceedings of the Annual Meeting No. 1 2
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Title Perceptions of Risk, Proceedings of the Fifteenth Annual Meeting, Held on March 14-15, 1979 (Including Taylor Lecture No. 3) (1980) Quantitative Risk in Standards Setting, Proceedings of the Sixteenth Annual Meeting Held on April 2-3, 1980 (Including Taylor Lecture No. 4) (1981) Critical Issues in Setting Radiation Dose Limits, Proceedings of the Seventeenth Annual Meeting, Held on April 8-9, 1981 (Including Taylor Lecture No. 5) (1982) Radiation Protection and New Medical Diagnostic Procedures, Proceedings of the Eighteenth Annual Meeting, Held on April 6-7, 1982 (Including Taylor Lecture No. 6) (1983) Environmental Radioactivity, Proceedings of the Nineteenth Annual Meeting, Held on April 6-7, 1983 (Including Taylor Lecture No. 7) (1983) Some Issues Important in Developing Basic Radiation Protection Recommendations, Proceedings of the Twentieth Annual Meeting, Held on April 4-5, 1984 (Including Taylor Lecture No. 8) (1985). Symposium Proceedings
The Control of Exposure of the Public to Ionizing Radiation in the Event of Accident or Attack, Proceedings of a Symposium held April 27-29,1981 (1982) 100
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Lauriston S. Taylor Lectures No. 1
Title and Author T h e Squares of the Natural Numbers i n Radiation Protection by Herbert M . Parker (1977) W h y be Quantitative About Radiation Risk Estimates? by Sir Edward Pochin (1978) Radiation Protection-Concepts and Trade Offs by Hymer L. Friedell (1979) [Available also in Perceptions of Risk, see above] From "Quantity of Radiation" and "Dose" to "Exposure" and "Absorbed Dose''-An Historical Review by Harold 0. Wyckoff (1980) [Available also in Quantitative Risks in Standards Setting, see above] How Well Can W e Assess Genetic Risk?Not Very by James F. Crow (1981) [Available also in Critical Issues in Setting Radiation Dose Limits, see above] Ethics, Trade-offs and Medical Radiation by Eugene L. Saenger (1982) [Available also in Radiation Protection and New Medical Diagnostic Approaches, see above] The Human Environment-Past, Present and Future by Merril Eisenbud (1983) [Available also in EnuironmentrdRadioactivity, see above] Limitation and Assessment in RadiQtion Protection by Harald H . Rossi (1984) [Available also in 1985 in Some Issues Important in Developing Basic Radiation Protection Recommendations, see above]
NCRP Reports No.
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Control and Removal of Radioactive Contamination in Laboratories (1951) Recommendations for Waste Disposal of Phosphorus-32 and Iodine-131 for Medical Users (1951) Recommendations for the Disposal of Carbon-14 Wastes (1953) Radioactive Waste Disposal in the Ocean (1954) Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides i n Air and in Water for Occupational Exposure (1959) [Includes Addendum 1 issued in August 19631
NCRP PUBLICATIONS
Measurement of Neutron Flux and Spectra for Physical and Biological Applications (1960) Measurement of Absorbed Dose of Neutrons and Mixtures of Neutrons and Gamma Rays (1961) Stopping Powers for Use with Cavity Chambers (1961) Safe Handling of Radioactive Materials (1964) Radiation Protection in Educational Institutions (1966) Medical X-Ray and Gamma-Ray Protection for Energies Up to 10 Me V-Equipment Design and Use (1968) Dental X-Ray Protection (1970) Radiation Protection in Veterinary Medicine (1970) Precautions in the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides (1970) Protection against Neutron Radiation (1971) Basic Radiation Protection Criteria (1971) Protection Against Radiation from Brachytherapy Sources (1972) Specification of Gamma-Ray B r a c h y t h e r a ~Sources (1974) Radiological Factors Affecting Decision-Making in a N u clear Attack (1974) Review of the Current State of Radiation Protection Philosophy (1975) Krypton-85 in the Atmosphere-Accumulation, Biological Significance, and Control Technology (1975) Natural Background Radiation in the United States (1975) Alpha-Emitting Particles i n Lungs (1975) Tritium Measurement Techniques (1976) Radiation Protection for Medical and Allied Health Personnel (1976) Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV (1976) Environmental Radiation Measurements (1976) Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerator Facilities (1977) Cesium-137 From the Environment to Man: Metabolism and Dose (1977) Review of NCRP Radiation Dose Limit for Embryo and Fetus in Occuptionally Exposed Women (1977) Medical Radiation Exposure of Pregnant and Potentially Pregnant Women (1977)
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Protection of the Thyroid Gland in t h Event of Releases of Radioiodine (1977) Radiation Exposure From Consumer Products and Miscellaneous Sources (1977) Instrumentation and Monitoring Methods for Radiation Protection (1978) A Handbook of Radioactivity Measurements Procedures (1978)
Operational Radiation Safety Program (1978) Physical, Chemical, and Biological Properties of Radiocerium Relevant to Radiation Protection Guidelines (1978) Radiation Safety Training Criteria for Industrial Radiography (1978) Tritium in the Environment (1979) Tritium and Other Radionuclide Labeled Organic Compounds Incorporated i n Genetic Material (1979) Influence of Dose and Its Distribution in Time on DoseResponse Relationships for Low-LET Radiations (1980) Management of Persons Accidentally Contaminated with Radionuclides (1980) Mammography (1980) Radiofrequency Electromagnetic Fields-Properties, Quantities and Units, Biophysical Interaction, and Measurements (1981) Radiation Protection in Pediatric Radiology (1981) Dosimetry of X-Ray and Gamma-Ray Beams for Radiation Therapy i n the Energy Range 10 keV to 50 MeV (1981) Nuclear Medicine-Factors Influencing the Choice and Use of Radionuclides in Diagnosis and Therapy (1982) Operational Radiation Safety-Training (1983) Radiation Protection and Measurement for LAW Voltage Neutron Generators (1983) Protection in Nuclear Medicine and Ultrasound Diagnostic Procedures in Chddren (1983) Biological Effects of Ultrasound: Mechanisms and Clinical Implications (1983) Iodine-129: Evaluation of Releases from Nuclear Power Generation (1983) Radiological Assessment: Predicting the Transport, B i w cumulation, and Uptake by M a n of Radionuclides Released to the Environment (1984)
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Exposures from the Uranium Series with Emphasis on Radon and its Daughters (1984) 78 Evaluation of Occupational and Environmental Exposures to Radon and Radon Daughters i n the United States (1984) 79 Neutron Contamination from Medical Electron Accelerators (1984) 80 Induction of Thyrod Cancer by Ionizing Radiation (1985) 81 Carbon-14 i n the Environment (1985) 82 S I Units i n Radiation Protection and Measurements (1985) Binders for NCRP Reports are available. Two sizes make it possible to collect into small binders the "old series" of reports (NCRP Reports Nos. 8-30) and into large binders the more recent publications (NCRP Reports Nos. 32-82). Each binder will accommodate from five to seven reports. The binders carry the identification "NCRP Reports" and come with label holders which permit the user to attach labels showing the reports contained in each binder. The following bound sets of NCRP Reports are also available: Volume I. NCRP Reports Nos. 8, 9, 12, 16, 22 Volume 11. NCRP Reports Nos. 23, 25, 27, 30 Volume 111. NCRP Reports Nos. 32, 33, 35, 36, 37 Volume IV. NCRP Reports Nos. 38, 39,40, 41 Volume V. NCRP Reports Nos. 42,43,44,45, 46 Volume VI. NCRP Reports Nos. 47,48,49, 50, 51 Volume VII. NCRP Reports Nos. 52, 53,54, 55, 56, 57 Volume VIII. NCRP Report No. 58 Volume IX. NCRP Reports Nos. 59, 60,61, 62,63 Volume X. NCRP Reports Nos. 64,65,66, 67 Volume XI. NCRP Reports Nos. 68,69, 70,71,72 Volume XII. NCRP Reports Nos. 73, 74, 75, 76 (Titles of the individual reports contained in each volume are given above). The following NCRP Reports are now superseded and/or out of print: No. 1
Title X-Ray Protection (1931). [Superseded by NCRP Report No. 31
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Radium Protection (1934). [Superseded by NCRP Report No. 41 X-Ray Protectwn (1936). [Superseded by NCRP Report No. 61 Radium Protection (1938). [Superseded by NCRP Report No. 131 Safe Handling of Radioactive Luminous Compounds (1941). [Out of Print] Medical X-Ray Protection Up to Two Million Volts (1949). [Superseded by NCRP Report No. 181 Safe Handling of Radioactive Isotopes (1949). [Superseded by NCRP Report No. 301 Radiological Monitoring Methods and Instruments (1952). [Superseded by NCRP Report No. 571 Maximum Permissible Amounts of Radioisotopes in the Human Body and Maximum Permissible Concentrations in Air and Water (1953). [Superseded b y NCRP Report No. 221 Protection Against Radiations from Radium, Cobalt-60 and Cesium-137 (1954). [Superseded by NCRP Report No. 241
Protection Against Betatron-Synchrotron Radiations U p to 100 Million Electron Volts (1954). [Superseded by NCRP Report No. 5:l.l Safe Handling of Cadavers Containing Radioactive Isotopes (1953). [Superseded by NCRP Report No. 211 Permissible Dose from External Sources of Ionizing Radiation (1954) including Maximum Permissible Exposure to Man, Addendum to National Bureau of Standards Handbook 59 (1958). [Superseded by NCRP Report No. 391
X-Ray Protection (1955). [Superseded by NCRP Report No. 26 Regulation of Radiation Exposure by Legislative Means (1955). [Out of print] Protection Against Neutron Radiation Up to 30 Million Electron Volts (1957). [Superseded by NCRP Report No. 381 Safe Handling of Bodies Containing Radioactive Isotopes (1958). [Superseded by NCRP Report No. 371 Protection Against Radiations from Sealed Gamma Sources (1960). [Superseded by NCRP Report Nos. 33, 34, and 401
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NCRP PUBLICATIONS
Medical X-Ray Protection Up to Three Million Volts (1961). [Superseded by N C R P Report Nos. 33, 34, 35, a n d 361 A Manual of Radioactivity Procedures (1961). [Superseded b y N C R P Report No. 581 Exposure to Radiation i n a n Emergency (1962). [Superseded by N C R P Report No. 421 Shielding for High Energy Electron Accelerator Installations (1964). [Superseded by N C R P Report No. 51.1 Medical X-Ray and Gamma-Ray Protection for Energies U p to 10 MeV-Structural Shielding Design and Evaluation (1970). [Superseded by N C R P Report No. 491
Other Documents The following documents of the NCRP were published outside of the NCRP Reports series: "Blood Counts, Statement of the National Committee on Radiation Protection," Radiology 63, 428 (1954) "Statements on Maximum Permissible Dose from Television Receivers and Maximum Permissible Dose to the Skin of the Whole Body," Am. J. Roentgenol., Radium Ther. and Nucl. Med. 84, 152 (1960) and Radiology 75, 122 (1960) X-Ray Protection Standards for Home Television Receivers, Interim Statement of the National Council on Radiation Protectionand Measurements (National Council on Radiation Protection and Measurements, Washington, 1968) Specification of Units of Natural Uranium and Natural Thorium (National Council on Radiation Protection and Measurements, Washington, 1973) NCRP Statement on Dose Limit for Neutrons (National Council on Radiation Protection and Measurements, Washington, 1980) Krypton-85 in the Atmosphere- With Specific Reference to the Public Health Significance of the Proposed Controlled Release at Three Mile Island (National Council on Radiation Protection and Measurements, Washington, 1980) Preliminary Evaluation of Criteria For the Disposal of Trcnzsumnic Contaminated Waste (National Council on Radiation Protection and Measurements, Bethesda, Md, 1982) Control of Air Emissions of Radionuclides (National Council on Radiation Protection and Measurements, Bethesda, Md, 1984) Copies of t h e statements published in journals may be consulted in libraries. A limited number of copies of t h e remainingdocuments listed above a r e available for distribution b y NCRP Publications.
INDEX Biological Systems and "C, 47-58 adipose tissues, 49 alveolar C02, 51 amino acids, 48 biological half times, 49, 50, 54 bone organic carbon, 52 '4C/'2C ratio lagtime, 55 concentration effects, 55, 56 diffusion theory, 56 DNA incorporation, 52, 53 enzyme turnover rates, 49.50 excreted carbon, 48 fat and carbohydrate turnover rates, 49, 50 hemispheric mixing time, 58 human food, 53-55 ingestion, 47-50 inhaled C 0 2 retention, 51, 52 kinetics of localized releases, 56 liver glycogen, 49 localized releases, 58 metabolic pathways, 56 muscle glycogen, 49 nitrogen balance, 49 oxaloacetic acid, 51 plasma albumin, 49 proteins, 48, 49 reference man, 49, 50 uptake by growing vegetables, 57 '%Environmental Distribution, 24-35,54 anthropogenic COP,27 atmospheric concentration, 24 atmospheric reservoirs, 27 box models, 24 "C in the biosphere, 24-31 COI concentration in the atmosphere, 32 carbon reservoirs in the biosphere, 24 compartment model, 33 diffusion model, 24 fluxes, 31-33
inorganic carbon in oceans, 31 Living carbon, 28 non-woods carbon, 29 north woods carbon, 28 ocean sediment, 27 oceans, 30 organic shale, 27 primary production, 29, 30,32 reservoir estimates, 34, 35 solubility of CO2 in ocean water, 32 southern woods carbon, 28 suess effect, 33, 54 terrestrial biosphere, 27, 28 uptake capacity of ocean water, 33 weapons tests, 27 14CProperties, 3-5, 7, 24, 36, 45 discovery of "C, 3, 36 earth content, 3 exchangeable, 3,24 half-life, 4, 5 isotopic composition, 3 ocean, 3, 7 "C Radiation Doses, 59-70 atmospheric C 0 2model, 61 "C release from nuclear power production, 61 compartment models, 59 dose from environmental I4C,64 dose from natural "C, 64, 65 dose from nuclear power, 65,66 dose from weapon tests. 65 dosimetry, 63,64 environmental models, 59 organ dose, 64 projected "C specific activity from nuclear power, 60 "C sampling and analysis, 36-46 benzene synthesis, 41 biota and soil samples, 38
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"C sampling and analysis (Continued) COz absorption in NaOH, 36 C o n absorpt.ion, 37 cross contamination, 37 direct measurement of "CO,, 42 grab samples, 37 internal gas counting, 40, 41 isotope enrichment, 45 isotope fractionation, 37 laser absorption spectroscopy, 44 liquid scintillation counting, 41-43 mass spectrometry, 44 methoxyethylamine absorbent for COZ, 42 oxygen flash techniques, 39 particulate sampling, 38 presentation of "C data, 45 sample combustion, 39 sampling, 36 sodium carbonate absorbent, 43 solid source counting, 40 solubilized biological materials in liquid scintillation, 43 suspension of solids in liquid scintillation. 43 tube furnaces, 40 urine sample preservation, 38 water samples, 37 wet oxidation, 39
"c Sources, 6-23 atmospheric fluctuation, 8 BWR, 16 cosmogenic, 7 effective cross sections, 13
environmental release, 17, 19-22 fast reactors, 17, 18 fractional isotopic production, 13 fractionation, 7 graphite moderated reactors, 18.19 heavy water reactors, 19 HTCR, 13 labeled compounds, 23 LMFBR, 13 LWR, 13, 14-16 methane, 9 natural occurrence, 6 natural production, 7 nitrogen reaction, 13, 14 nuclear power, 1&22 production of "C in nuclear reactors, 14-16 PWR, 16 specific activity of "C, 9 transfer from stratosphere, 9 tree rings, 7 weapons production, 9
"C Waste Management, 67-70 14Cmobility a t the disposal site, 69, 70 "C waste immobilization, 68 fluorocarbon solvent-absorption process, 68, 69 incineration, 69 institutional waste, 68 methane formation at the disposal site; 70 power reactor waste, 67.68 waste disposal, 68 waste treatment, 69