Uncertainty in Ncrp Screening Models Relating to Atmospheric Transport, Deposition and Uptake by Humans (Ncrp Commentary)

NCRP COMMENTARY No. 8

UNCERTAINTY IN NCRP SCREENING MODELS RELATING TO ATMOSPHERIC TRANSPORT, DEPOSITION AND UPTAKE BY HUMANS

Issued September 1, 1993

National Council on Radiation Protection and Measurements 7910 Woodmont Avenue I Bethesda, Maryland 20814-3095

LEGAL NOTICE This Commentary was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its documents. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this Commentary, nor any person acting on the behalf of any of these parties: (a) makes any warranty or representation, express or implied, with respect to the accuracy, completeness or usefulness of the information contained in this Commentary, or that the use of any information, method or process disclosed in this Commentary may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or process disclosed in this Commentary, under the Civil Rights Act of 1964, Section 701 et seq. as amended 42 U.S.C.Section 2000e et seq. (Title VII) or ony other statutory or common law theory governing liability.

Libmy of Congress Catalogiog-i~PublicationData

National Council on Radiation Protection and Measurements. Uncertainty in NCRP screening models relating to atmospheric transport, deposition and uptake by humans p. cm. -- (NCRP commentary ; no. 8) An evaluation of the Council's screening techniques for determining compliance with environmental standards. "Prepared by Scientific Committee 64- 16 on Uncertainty in NCRP Screening Modelsu--Ref. lncludes bibliographical references. ISBN 0-929600-28-2 1. Radiation--Environmental aspects. 2. Radiation dosimetry. 3. Air--Pollution. 4. Radiation--Toxicology. I. National Council on Radiation Protection and Measurements. Scientific Committee 64- 16 on Uncertainty in NCRP Screening Models. II. National Council on Radiation Protection and Measurements. Screening techniques for determining compliance with environmental standards. LII. Title. IV. Series. RA569.N353 1992a 92-36388 616.9'89707 1--dc20 CIP

Copyright O National Council on Radiation Protection and Measurements 1993 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 I n 1986, the National Council on Radiation Protection a n d Measurements (NCRP) published NCRP Commentary No. 3, Screening Techniques for Determining Compliance with Environmental Standurds (a revision was issued i n 1989). The screening models in Commentary No. 3 a r e for releases of radionuclides to the atmosphere. The Environmental Protection Agency (EPA) subsequently used the screening models i n Commentary No. 3 a s the basis for a computer code (COMPLY) to allow the radionuclide user to show compliance with t h e Clean &r Act for Radionuclides (40 CFR P a r t 61). The explicitly stated basis for the use of screening models in the absence of site specific data is t h a t sufficient conservatism is incorporated into the screening calculation so t h a t the actual environmental dose will always be less t h a n or, a t worst, equal to the calculated screening dose. The major problem with the environmental screening calculation is t h a t it can be a combination of various models and parameters with varying degrees of uncertainty in each part of the calculation. I n the absence of site specific data, i t is not possible to calculate the exact uncertainty i n screening calculations. The EPA asked the NCRP to assess the uncertainty in the Commentary No. 3 models and parameters and this commentary is the result of that request. This document reviews the assumptions of Commentary No. 3 and indicates areas of conservatism a s well a s assumptions which could lead to underestimates of actual dose. Situations a r e identified where the use of Commentary No. 3 should be restricted or modification made prior to application. The primary assumptions affecting bias are also reviewed. The screening models i n NCRP Commentary No. 3 a r e similar to those presented i n the full report currently under preparation by the NCRP on screening models for releases to the atmosphere, ground water and surface water. I n general, the conclusions of this Commentary apply to t h e relevant models of t h a t report. The Commentary was prepared by Scientific Committee 64-16 on Uncertainties in Application of Screening Models. Serving on the Committee were: F. Owen Hoffman, Chairman Oak Ridge National Laboratory Oak Ridge, Tennessee

Members Andr6 Bouville National Cancer Institute New York, New York

Charles W. Miller Centers for Disease Control Atlanta, Georgia

Steven R. Hanna Sigma Research Corporation Westford, Massachusetts

F. Ward Whicker Savannah River Ecology Laboratory hen, South Carolina

Consultant B. Gordon Blaylock Oak Ridge National Laboratory Oak Ridge, Tennessee

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PREFACE

NCRP Secretariat

E. Ivan White Serving on Scientific Committee 64 on Radionuclides i n the Environment were: Melvin W. Carter, Chairman Atlanta, Georgia Meln bers

Edward L. Albenesius Savannah River Ecology Laboratory &en, South Carolina

William A. Mills Oak Ridge Associated Universities, Inc. Washington, D.C.

Wayne R. H a n s e n Los Alamos National Laboratory Los Alamos, New Mexico

William L. Templeton Battelle Pacific Northwest Laboratories Richland, Washington

Bernd K a h n Georgia Institute of Technology Atlanta, Georgia William E. Kreger Bainbridge Island, Washington

J o h n E. Till Radiological Assessment Corporation Neeses. South Carolina David A. Waite Ebasco Environmental Bellevue, Washngton

F. W a r d Whicker Savannah River Ecology Laboratory Aiken, South Carolina This Commentary was reviewed by Scientific Committee 64 a n d the NCRP Board of Directors. The Council wishes to express its appreciation to t h e Committee members for t h e time and effort devoted to the preparation of t h s Commentary.

Charles B. Meinhold President, NCRP Bethesda, Maryland September 1, 1993

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Atmospheric Transport and Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Isolated Point Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Atmospheric Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Dispersion Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Release Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 Wind Direction Duration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5 W i n d s p e e d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.6 Mixing Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.7 Terrain Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Sources in the Presence of Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Area Source Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Dry a n d Wet Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Food Chain Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Plant Contamination Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Transfer to Animals and People . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Validation of Food Chain Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Transfer of 1 3 7 ~from s Air to Pasture Vegetation . . . . . . . . . . . . . . . . . . 3.3.2 Transfer of 1311 from Air to Pasture Vegetation . . . . . . . . . . . . . . . . . . . 3.3.3 Transfer of 1 3 7 ~from s Air to Milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Transfer of 13'1 from h r to Milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5 Transfer of 1 3 7 ~from s Air to Meat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Human Dietary Habits and Usage Factors

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External and Internal Dose Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Inhalation and External Dose Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Effect of Indoor Occupancy and of Source Geometry . . . . . . . . . . . . . . . . 5.1.1.1 Inhalation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1.2 External Irradiation from Cloud Immersion . . . . . . . . . . . . . . . . 5.1.1.3 External Irradiation from Ground-Deposited Activities . . . . . . . . 5.1.2 Effect of Age a t Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.1 Inhalation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2.2 External Irradiation from Cloud Immersion . . . . . . . . . . . . . . . . 5.1.2.3 External Irradiation from Ground-Deposited Activities . . . . . . . . 5.2 Ingestion Dose Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Atmospheric Transport and Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Terrestrial Exposure Pathways to Humans . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Usage Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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References

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TheNCRP

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NCRPCommentaries

1. Introduction The assessment of the potential impact of environmental releases of radionuclides is largely dependent on the use of mathematical models. The evaluation of these models has been a continuing effort of the National Council on Radiation Protection and Measurements (NCRP). I n 1984, NCRP published Report No. 76, titled Radiological Assessment: Predicting the Transport, Bioaccumulation and Uptake by Man of Radionuclides Released to the Environment (NCRP, 1984). For users of small quantities of radionuclides, NCRP Report No. 76 recognized t h a t simple screening calculations should suffice in most cases to demonstrate compliance with environmental dose limits. NCRP Report No. 76 a n d other publications (Morgan and Henrion, 1990) recognized t h a t increased model complexity does not always bring about increased accuracy. I n 1986, NCRP Commentary No. 3, Screening Techniques for Determining Compliance with Environmental Standards was issued. This Commentary presented screening models and parameter values for assessing potential releases of small quantities of radionuclides to the atmosphere. An essential feature of these screening models is that compliance with dose limits should be assured provided that calculated screening estimates of dose to individuals did not exceed one-tenth of relevant dose limits (identified a s the limiting value in Commentary No. 3). However, when screening estimates exceed one-tenth of a dose limit, expert assistance is advised to enable more realistic calculations to be made, including a n analysis of uncertainty. I t is the objective of this Commentary to evaluate the reliability of the screening models in NCRP Commentary No. 3 and to identify conditions when the models a n d parameter values may not be applicable. Of particular interest are situations where the extent of underestimation may exceed a factor of ten. Also of interest are conditions in which the assumptions used for screening calculations lead to very large overestimates of actual doses. For either of these two situations, recommendations will be made to either improve the model, parameter values and assumptions made in NCRP Commentary No. 3 or to define the conditions under which the screening models should not be applied. The screening models i n NCRP Commentary No. 3 (NCRP, 198611989) a r e similar to those presented in the full report currently being prepared by the NCRP on screening models for releases to the atmosphere, ground water and surface water. I n general, the conclusions of this Commentary apply to the relevant models of t h a t report.

2. Atmospheric Transport and Deposition NCRP Commentary No. 3 (NCRP, 198611989) describes screening procedures to be used to determine compliance with environmental radiation standards. These procedures begin with a n assumed release of radioactive material to the atmosphere. Models are then used to estimate the subsequent atmospheric transport and deposition of the released material. The purpose of this Section is to review the uncertainty associated with the use of these models.

2.1 Isolated Point Source Appendix A of NCRP Commentary No. 3 (NCRP, 198611989) contains the specific equations that are used in the atmospheric transport and dispersion portion of the screening procedures recommended in the Commentary. The basic calculational model upon which Screening Levels I1 and 111' are based is the Gaussian plume atmospheric dispersion model for calculating the ground-level concentration on the plume centerline (Gifford, 1968). If the point of release for the radionuclide effluent under consideration is greater than 2.5 times the height of the nearest, most influential building structure and the downwind distance between the release point and the receptor is greater than 100 m, the release is considered to be a n isolated point source. This is because the source is well above the perturbed flow around the neighboring buildings (Wilson and Britter, 1982). Under these conditions, the 22.5O sectoraveraged form of the Gaussian plume model is used for the screening calculations. The NCRP Commentary No. 3 (NCRP, 198611989) screening model for a n isolated point source has not been specifically subjected to extensive validation studies. However, a review of the literature provides information on the general validity of such a modeling approach. Beck (1988) provides a good general overview of the uncertainties in environmental models. The International Atomic Energy Agency Safety Series 100 (IAEA, 1989) discusses the components of model uncertainties and provides suggestions of methods for conducting Monte Carlo sensitivity studies. The four components of model uncertainty are: (1) input data errors, (2) stochastic or turbulent fluctuations, (3) model physics errors and (4) lack of knowledge and lack of data. Hanna (1993) has synthesized the results of the evaluation of a wide variety of atmospheric dispersion models using field data, with the result that the better models consistently exhibit a relative root-mean-square-error (rmse) of about 60' or 70 percent. The averaging time for these model evaluation exercises was about one hour. The typical 95 percent confidence limits on model uncertainty are about two times the relative rmse, or plus or minus a factor of 1.2 or 1.4. These results are valid in the absence of buildings. Miller and Hively (1987) reviewed a number of experimental studies in which measured air concentrations have been compared with predictions made by the Gaussian plume model. The results of their analysis are summarized in Table 2.1. NCRP Commentary No. 3

' ~ e v e Il is the simplest approach and incorporates the most conservatism. Levels 11 and 111are more detailed and correspondingly less conservative.

2.1 ISOLATED POINT SOURCE

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(NCRP, 198611989) clearly specifies that the methods presented "...are designed to be used for long-term releases, either intermittent or continuous, from point sources only. They should not be used to calculate radionuclide air concentrations resulting from short-term accidental releases." From Table 2.1, it appears that, in general, the Gaussian plume model is capable of predicting the desired concentrations within a factor of four for flat terrain and within a factor of ten for sites with complex terrain or meteorology. I t should be noted, however, that these results are for a limited number of field validation studies. Under complex terrain conditions, for example, model validation results are highly dependent on the exact site involved, the meteorological conditions during the release and the exact algorithm used in the calculation (Bendel and Cresswell, 1977). TABLE 2.1 - Estimate of the ratw of predicted-to-observed air concentratwns associated with predict wns made by the Gaussian atmospheric dispersion moclel uncter various release conditions (Miller and Hively, 1987).

Conditions Highly instrumented site: ground-level centerline concentration within 10 km of a continuous point source Ground-level releases Elevated releases Maximum air concentration for elevated releases Annual average for a specific point, flat terrain, within 10 km downwind of the release point Annual average for a specific point, flat terrain, 10 to 150 km downwind of the release point

Range

0.8 to 1.2 0.65 to 1.35 0.51 to 1.5

Specific hour and receptor point, flat terrain, steady meteorological conditions Elevated releases without building wake effects Elevated releases with building wake effects Short-term, surface-level releases with building wake effects using temperature gradient method of estimating atmospheric stability Wind speeds >2 m s" Wind speeds <2 m s-' Short-term, surface-level releases without building wake effects using temperature gradient method of estimating atmospheric stability Wind speeds >2 m s-' Wind speeds <2 m s-' Complex terrain or meteorology (e.g., sea breeze regimes) Annual average concentrations Short-term releases Urban releases Annual average concentrations S24 h concentrations

0.1 to 10 0.01 to 100 0.25 to 4 0.1 to 10

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1 2. ATMOSPHERIC TRANSPORT AND DEPOSITION

Dispersion along the shore of a large body of water, such a s a n ocean or one of t h e Great Lakes, obviously represents a complex meteorological condition. But what distinguishes flat terrain from complex terrain? Unfortunately, there is no easy answer to this question. Jones (1986) presents some criteria which may be used to determine if terrain effects can be neglected in calculating air concentrations (Table 2.2). Obviously, the uncertainty associated with using the Gaussian plume model is potentially much larger for sites t h a t fail to meet t h e criteria in Table 2.2 t h a n i t is for those sites t h a t do meet these criteria.

TABLE 2.2 - Criteria for neglecting terrain effects (Jones, 1986). Conditions Neutral and unstable

Criteria

- the gradient of the surrounding terrain should be less than about 1 in 10

- for a ridge upwind of the source either or

H > 1.5 h x > 20 h in neutral conditions x > 10 h in very unstable conditions

- for an isolated hill upwind of the source

H > 1.5 h either or x >7h - for a hill or ridge downwind of the source either H>h+o,(x) or 0, ( 4 h Stable

- the gradient of the surrounding terrain should be less than about 1 in 100

- for a n obstacle upwind of the source

H >h x > 40 h in slightly stable conditions x > 100 h in very stable conditions - for an obstacle downwind of the source H>h+o,(x) either or 0,( 4 > h either or

Note: The criteria in Table 2.2 are based on a change of 30 percent in the wind speed between flat and complex terrain. The wind speed a t a height of 10 m must be a t least 1 m 9.'. H is the release height in m and h is the obstacle height in m. x is the distance between obstacle and source in m and o, (x) is the vertical plume standard deviation in m.

I t should also be noted t h a t there is always uncertainty i n any air concentration calculation. The very nature of the turbulent flow t h a t is largely responsible for dispersing a plume prevents u s from predicting all of the details of this flow (Venkatram, 1983). Increasing the state of our knowledge, and t h u s the number of variables i n models, can reduce the uncertainty i n the resulting output to some finite minimum (inherent) uncertainty. It is unlikely, however, t h a t modeling efforts have yet reached this minimum uncertainty (Venkatram, 1983).

2.1 ISOLATED POINT SOURCE

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The accuracy of any atmospheric dispersion model is greatly dependent on the values chosen for the variables in the model. The screening model used in NCRP Commentary No. 3 (NCRP, 198611989) includes a number of input parameters t h a t are either explicitly included in the methodology or a r e strongly recommended a s default parameter values. The uncertainty associated with these choices is discussed i n the sections which follow.

2.1.1 Atmospheric Stability The stability of the atmosphere refers to the tendency of a parcel of air to continue moving once i t is set in motion. If a parcel of air, e.g., a portion of a radioactive plume, tends to remain in motion once the initial driving force is removed, the atmosphere is said to be unstable. If the parcel tends to return to its original location, the atmosphere is stable. If the parcel tends to stay where it is placed, neither moving on nor returning to its original position, the atmosphere has neutral stability. Ideally, stability should be continuously measurable like many other atmospheric variables, e.g., temperature a n d wind velocity. If stability is based on the Richardson number or the Monin-Obukhov length, stability can be calculated from measurable variables such a s the vertical temperature gradient, the vertical wind speed gradient and the vertical heat flux (Brenk et al., 1983). While some modelers a r e beginning to incorporate continuous stability variables, for most dispersion purposes, atmospheric stability is still divided into a number of finite classes ranging from A, most unstable, to F or G, most stable. Neutral stability is Class D. The stability class of the atmosphere is inferred from one or more readily measurable quantities, e.g., solar insolation, wind speed, standard deviation of the wind direction, etc. A number of different methods have been proposed for specifying atmospheric stability class, but none has gained uniform acceptance. Furthermore, these methods often differ in the stability class each specifies for a given meteorological condition (Miller, 1977). The atmospheric stability class is used in the Gaussian plume model i n a n indirect manner. The dispersion parameters t h a t are a n integral part of the model a r e chosen, in part, on the basis of stability. For screening purposes, the authors of Commentary No. 3 followed the lead of Gifford a n d Hanna (1975) and assumed that, for long-term averages, the atmosphere is neutral, i.e., stability category D. Furthermore, Jones (1986) presents statistics for the United Kingdom t h a t show t h a t D is the most likely stability category to persist for six hours or longer. The NCRP screening model for a n isolated point source depends on the vertical dispersion parameter. The value of this parameter is taken from Briggs (1974). If the atmosphere is, on the average, more unstable than D, the value used will tend to overpredict the air concentration due to emissions from ground-level sources while underpredicting the concentration from elevated sources. The Briggs vertical dispersion parameter values may be over a n order of magnitude higher for Class A than they are for Class D. If the atmosphere is, on the average, more stable than D, the value used will tend to underpredict the air concentration due to emissions from ground-level sources a n d overpredict the concentration from elevated sources. The difference between Briggs Class D and Class F is up to one order of magnitude. Also, the differences in vertical dispersion parameter values tend to be smaller the closer the receptor is to the source.

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2. ATMOSPHERIC TRANSPORT AND DEPOSITION

I t should also be noted t h a t vertical dispersion usually decreases with increasing height. This is because the impact of turbulence caused by obstacles on the ground decreases with a n increase of height above the ground (Brenk et al., 1983). Because elevated plumes take some time to reach ground level, a decrease in vertical dispersion may actually result in a decrease i n ground-level air concentration for releases above approximately 50 m. However, the Briggs dispersion parameters used in Commentary No. 3 are based, a t least i n part, on elevated release data; therefore, thls effect should be a t least partially accounted for i n the above analysis.

2.1.2

Dispersion Parameters

The Gaussian plume model has a number of theoretical prerequisites, assumptions and boundary conditions t h a t are rarely completely fulfilled in the atmosphere (Brenk et al., 1983). I n practice, however, many of these limitations can be compensated for through the judicious selection of the dispersion parameters used in the model. Changes in the dispersion factors strongly affect the resulting air concentration calculated by the Gaussian plume model (Pasquill, 1974). A number of empirical representations of the Gaussian dispersion factors have been constructed a s a function of downwind distance a n d atmospheric stability (Gifford, 1976). These empirical functions are based on measurements a t different locations and, in some cases, different interpolations of the same data sets. Vogt (1977) calculated annual average diffusion factors for each of six different sets of dispersion parameters and annual average meteorological statistics for Julich, Federal Republic of Germany. He found t h a t the maxima differed by over a n order of magnitude and the location of the maxima varied by a factor of five, depending on which set of dispersion parameters was used. The vertical dispersion parameters used in NCRP Commentary No. 3 are those developed by Briggs (1974) for diffusion over open country a t downwind distances between 100 m and 10,000 m. These parameters have subsequently been used in regulatory models, e.g., Hanna et al., 1985. The Briggs values are based on interpolations between a number of different d a t a sets, many of them for elevated releases (Gifford, 1976). As a result, the screening methods presented i n Commentary No. 3 for a n isolated point source a r e most applicable to terrain t h a t is not marked by large surface roughness elements. Since such roughness elements tend to increase the amount of diffusion i n the atmosphere (Briggs, 1974), use of the Briggs dispersion parameters may tend to overpredict the actual air concentration due to emissions at ground level. I n Commentary No. 3, values of the diffusion factor for a n isolated point source are calculated a s a function of downwind distance for various heights of release a n d presented in graphical form. However, for screening purposes, it is assumed for release heights greater t h a n zero t h a t the diffusion factor is constant a t downwind distances between 100 m (the starting downwind distance for the calculation) a n d the distance where the maximum value of the diffusion factor for t h a t particular release height is calculated to occur. As a result, the model will clearly tend t o overpredict the air concentration near the source for elevated releases. Also, this assumption makes it inappropriate to use this screening model to reconstruct a release based o n long-term air concentration or deposition data measured near the point of release.

2.1 ISOLATED POINT SOURCE

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2.1.3 Release Height The height of the release is another important parameter in the Gaussian plume model. In general, the higher the release point, the lower the resulting maximum downwind ground-level air concentration. The value of the release height includes not only the physical height of the stack but also any additional height due to the rise of the plume as a result of its thermal buoyancy or momentum (Hanna, 1975). The NCRP screening methodology for an isolated point source requires the user to determine the release height used in the calculations without accounting for the vertical rise of the plume. No default values can be recommended for this parameter as it is wholly site-specific. By excluding the effect ofplume rise, errors in this parameter should translate into a significant overestimation of the calculated air concentration at distances within 1 km of the release point.

2.1.4 Wind Direction Duration The NCRP screening methodology includes a default assumption that the wind blows toward the receptor of interest 25 percent of the year. This assumption may lead to a n overestimation of the annual average downwind air concentration in flat terrain. Although it may appear that a theoretical worst-case underprediction is a factor of four, it is very unlikely that the wind blows 100 percent of the time from a given direction. There are release points located in deep valleys that are dominated by up or down valley flow, e.g., Figure 2.1. Under extreme conditions it is possible that any one wind direction could persist about 25 percent of the time.

2.1.5 Wind Speed In the Gaussian plume model, the air concentration is inversely proportional to the wind speed. The user of the screening methodology must choose an annual average wind speed for the location of concern. Figures 2.2 and 2.3 (Holzworth, 1972) present climatological information on annual-average wind speeds within the mixed layer (see below) across the United States. The average wind speed at the height of a release will likely be lower than that given by Holzworth. This information or more site-specific information, if available, may be used to specify the wind speed used in the screening calculation. In the absence of additional information, the recommended default annual average wind speed of 2 ms-' appears to be a reasonably conservative value for screening purposes.

2.1.6 Mixing Height The rate of vertical dispersion in the atmosphere is strongest within the mixing layer. The height of the mixing layer is on the order of 1km, but the mixing height at any specific time and location will vary depending on such factors a s terrain, solar insolation, time of year, synoptic meteorological conditions and others. In general, the mixing height represents the vertical extent of surface-induced effects on diffusion processes (Lyons and Scott, 1990).

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1 2. ATMOSPHERIC TRANSPORT AND DEPOSITION

Often atmospheric dispersion models treat the top of the mixing layer a s a n impenetrable barrier. Material released above the mixing height is not allowed to diffuse towards the ground, and material released below the mixing height is trapped within the mixing layer. This assumption is a n oversimplification of true atmospheric processes. I n reality, some material diffuses through the mixing height "barrier." Furthermore, the mixing height may change during the day. For example, material released above the mixing layer may be brought down into the mixing layer if the mixing height rises during the day. I n the NCRP screening model, the effects of the mixing height are ignored and t h e plume is assumed to continue diffusing indefinitely i n the vertical direction. However, in the Gaussian plume model, the plume is generally assumed to reflect off both the surface of the e a r t h and the top of the mixing layer (Figure 2.4). Eventually, the plume will become uniformly mixed in the vertical, and the Gaussian distribution will no longer apply (Turner, 1970). The downwind distance at which it can be assumed that the plume is uniformly mixed will depend o n the mixing height and the height of the release. For annual average conditions, this will likely occur within a few ten's of kilometers of the release point. It is recommended that the NCRP screening models not be used beyond 10 km, since the effects of neglecting the mixing height would begin to influence the solution at that distance. Beyond that distance, the NCRP solution would become progressively less conservative.

Fig. 2.1. Annual (1977) wind rose for valley and plateau meteorological stations (61-m level) at Widows Creek Steam Plant, Alabama (Hanna et al., 1982). The radial lines represent wind hrections and the concentric circles represent fractional occurrence of that wind direction.

2.1 ISOLATED POINT SOURCE

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Fig. 2.2.

Fig 2.3. Isopleths (m s.') of mean annual wind speed averaged through the afternoon mixing layer (Holzworth, 1972).

9

10 / 2. ATMOSPHERIC TRANSPORT AND DEPOSITION

Fig. 2.4. Virtual source model to represent the reflections from the ground and the top of the mixing layer (Clarke, 1979). (A = height of mixing layer above ground; h = height of release point above ground.)

2.1.7 Terrain Effects As noted above, the presence of complex terrain can increase the uncertainty associated with Gaussian plume model calculations. One of the primary reasons for this fact is that, in most applications of the Gaussian model, it is assumed that the earth is flat. This assumption can lead to large uncertainties in complex terrain, even for annual average conditions. For example, consider a release that occurs below the height of terrain surrounding a valley, with the nearest receptor located high on the side of the valley, directly across from the release point (Point A, Figure 2.5). Procedures are available to adjust plume evaluation a s a result of terrain effects (Hanna et al., 1982). The NCRP screening models, however, will calculate a ground-level concentration for a point on the valley floor (Point B, Figure 2.5). Depending on the release height and the downwind distance, this could result in a significant

2.2

SOURCES IN THE PRESENCE O F BUILDINGS /

11

underprediction of the exposure to the nearest receptor. As a result, it is recommended that if the NCRP screening models are used in complex terrain, it should be assumed that the release point and the receptor are located at the same elevation, i-e., assume a ground-level

release even for an elevated stack. 2.2 Sources in the Presence of Buildings

As can be seen in Table 2.1, uncertainties will be larger in the presence of buildings than for isolated point sources. This is because of the wide variety of flow distortions and vortices that are generated by buildings (Hosker, 1982; 1984; Huber, 1984; 1988; Meroney, 1982). Working Groups of the National Radiological Protection Board (NRPB) of the United Kingdom have developed a set of models [see Jones (1983) for a report on models for building wakes] and prepared a report on the uncertainties in their dispersion estimates (Jones, 1986). When the emissions have a low exit velocity and are from vents on a building, the NRPB recommends a near-wake model for the region between one and five building heights. downwind, and a virtual source model a t farther distances downwind. They refer to laboratory and field studies by Barker (1982), Fackrell(1984) and Foster and Robins (1984), showing that their models are generally conservative with uncertainties of about a factor of two for simple building shapes (rectangular shapes with length to width ratios between 0.3 and 3). The NRPB does not recommend its models for all conditions, and proposes the use of physical models (i.e., laboratory studies) for circumstances: (1) not covered by the mathematical models, (2) in which their use is open to question or (3) when greater accuracy is required. Snyder's (1981) guidelines for physical modeling should be followed. Fackrell (1984) summarizes the NRPB's philosophy towards physical modeling: "Very near to a building, a real plume, especially from a low source, can be greatly distorted and displaced by the complicated flow field and by small local site features. These simple models should not be expected to predict the concentration field in this region. A wind tunnel study would seem best a t the present time, especially for complex sites, and is probably also required if the source is not passive (i.e., with significant momentum or buoyancy)." Physical modeling is especially needed for scenarios where simple models are not conservative, such as confinement situations where the wind speed is reduced and plume spread is inhibited by vertical walls. The most extreme example of a confinement scenario is a gas emission in a courtyard with no exits, where the only avenue of escape is by means of entrainment by air passing over the top of the courtyard. In this situation, the simple models could be in error by two or three orders of magnitude (underestimation), and physical modeling should be employed. In the last section of Appendix A of Commentary No. 3, the NCRP also recommends the use of physical models if the doses are calculated to be too large by this screening procedure. I t should be mentioned, however, that physical modeling is . relatively expensive (about $50,000 to $100,000 for a comprehensive study).

12 / 2. ATMOSPHERIC TRANSPORT AND DEPOSITION

Fig. 2.5. Location for calculation of screening doses for mountain-valley sites.

While NCRP Commentary No. 3 recommends physical modeling for those cases where the screening technique identifies a large dose, the Commentary, in contrast to the NRPB reports (Jones, 1983; 1986) makes no detailed, specific recommendations regarding the need for physical modeling i n certain scenarios. Consequently, the recommended Level I1 a n d I11 screening models may underpredict concentrations by several orders of magnitude for confinement scenarios. For simple street canyon scenarios, the amount of underprediction would be less (a factor of five to ten) because of the fact t h a t ventilation along the direction of the street canyon is possible. I n most cases, the NCRP models are to be applied to small sources where it is inefficient and impractical to conduct a physical modeling study. Consequently, if confinement appears to be important, it is recommended that the conservative assumption be made that the source release point and the receptor are located on the same building surface. NCRP Commentary No. 3 recommends the use of a scaling formula suggested by Wilson and Britter (1982) for situations where the source and receptor are on the same building surface. The "constant," B,, in their formula is taken to be 30, which is the most conservative value recommended in their paper for a variety of source-receptor positions. Figure 2.6 contains one of Wilson and Britter's (1982) plots of observed wind tunnel d a t a t h a t were used to derive B,, including a line which represents B, = 9. The laboratory experiments employed simple rectangular building shapes. I t is seen t h a t a value of B, = 30 was chosen to provide a conservative bound to the hundreds of data points. Thus, this formula is conservative for the simple scenarios and building geometries studied by Wilson a n d Britter (1982).

2.2 SOURCES IN THE PRESENCE OF BUILDINGS

0.1

0.2

0.5

1.0

2.0

5.0

1 13

10

Distance from Vent ( r / J f l )

Fig. 2.6. Observed dilution factors for &ont vents and receptors on all building faces &om wind tunnel studies (Wilson and Britter, 1982). The equation, B, = c M u H r 2=/ ~9.0, is drawn, where c or cMis observed concentration, c, is initial concentration, UHis wind speed a t building height, Q is emission rate, r is distance from source to receptor, A is building frontal area, Ve is the exit velocity from the vent, Qe is the volume flow &om the source, and Ke is the normalized concentration a t the source.

14

1

2 . ATMOSPHERIC TRANSPORT AND DEPOSITION

Nevertheless, the caution a t the end of their paper should be remembered, "Considerable work is still required to define the limits of where such general rules can be put to some use, and where specific (physical) model studies will still be required." For sources and receptors not on the same building surface, and where the distance between them is less t h a n about 2.5 times the square root of the building frontal area, Commentary No. 3 recommends the use of a screening equation from Miller and Yildiran (1987). A box model is assumed, with a vertical dimension equal to the building height and a lateral dimension equal to one meter. This latter assumption is quite arbitrary, b u t yields concentration predictions t h a t are conservative when compared with about 40 sets of field d a t a from tracer experiments around reactor structures. The median value of the ratio of predicted to observed air concentrations is about four, and the ratio is less than unity (i.e., a n underprediction) during only 10 percent of the field experiments. For sources a n d receptors not on the same building surface, and a t distances larger t h a n about 2.5 times the square root of the building frontal area, a simple correction to the Gaussian equation suggested by Gifford (1968) is recommended i n Commentary No. 3. Gifford suggested that the "constant", C, is i n the range between 0.5 a n d 2.0 a n d the NCRP chose a value of 1.0. This assumption may provide a reasonable fit to field data, but it is not conservative. It has a n uncertainty of about a factor of two for simple building shapes and flat terrain. Also, Ramsdell (1990) demonstrates t h a t this model is not conservative for all wind speeds. However, the model does not begin to underpredict until the wind speed exceeds 5 m sml. All of the NCRP Commentary No. 3 modeling approaches i n the presence of buildings assume t h a t t h e building from which the release is occurring is the most influential on the dispersion process. This may not always be the case, however. For example, if a release point is on a building a n d there is a much larger building in the immediate vicinity, the larger one may be the most influential on the resulting dispersion. As a result, the cross-sectional area of the most influential building should be used i n all NCRP calculations, even if that building is not the source of the release. I t is possible to invent specific scenarios where the uncertainties due to neglecting the presence of a large nearby building would be on t h e order of a factor of 10 or 100. 2 . 3 Area Source Estimates The screening methods presented i n NCRP Commentary No. 3 (NCRP, 198611989) a r e designed for point releases, not area or volume sources such as a uranium mill tailings pile. If the user wishes to apply the NCRP screening methods to a n area source, it is recommended t h a t the following assumptions be made: (1) calculate a pseudo point source release rate by inte g rating over the entire area of the source, (2) take the release height to be zero and (3) locate t h e release point a t the edge of the area source nearest the location of the closest receptor. Use of these assumptions should result i n a conservative estimate of air concentration.

2.4 DRY AND WET DEPOSITION

1 15

2.4 Dry and Wet Deposition Except for nonreactive gases such a s Kr, material released into the atmosphere may be deposited on the surface of the earth, and thus removed from the plume, through dry and wet processes. The latter include both incorporation of the material directly into the precipitation (rainout or snowout) and removal of the material by precipitation falling through the plume (washout). Dry deposition is commonly characterized with physical parameters in terms of a dry deposition velocity, Vd (cm s-I). This parameter is defined a s the ratio between the rate of deposition per unit area (Bq m"s-') and the air concentration (Bq m-3). Table 2.3 presents a summary of measured values of Vd for both particulates and reactive gases. I t should be noted t h a t most of the measurements considered in Table 2.3 were made over short-time periods. I t is expected that long-term measurements would exhibit a smaller range and geometric standard deviation (Hoffman et al., 1984).

TABLE 2.3 - Asummary of measured values for dry and wet &position parameters (after Hoffman et al., 1984). Geometric Standard ~eviation~

Number of Observationsa

Geometric Mean

Dry deposition velocity, Vd(cm 9.') Particulates C Reactive gases

98 99

0.33 0.73

7.7 3.2

0.024 to 20 0.069 to 7.8

Washout ratio ~(Lrain~A)~

52

3.3 X lo6

3.8

4.3 x lo4 to 2.5 x lo6

Parameter (units)

Range

aMany observations made during short time periods; a smaller geometric standard deviation and range are expected for long-term averages. bunitless, the geometric standard deviation is the exponential of the standard deviation of log-transformed data. 'Particle size was not supplied for most measurements reported. d~oncentration.

Wet deposition may be characterized in terms of a wet deposition velocity, Vw. The unitless washout ratio, w, is t h e ratio between the concentration of material i n precipitation a n d the concentration of t h a t material in the air. The value of w may differ by a factor of about 800 depending on whether one uses concentrations per unit volume or per unit mass. The wet deposition velocity is the product of the washout ratio and the precipitation rate. Table 2.3 presents a summary of 52 measurements of w generally made over short-time periods. The total deposition of material can be parameterized in terms of a total deposition velocity, VT, where VT = Vd + Vw. NCRP Commentary No. 3 (NCRP, 198611989) uses VT = 1,000 m d" (1.2 cm s-l). If we assume a rainfall rate of 1 m y-l, which is typical for the

16 / 2. ATMOSPHERIC TRANSPORT AND DEPOSITION

southeastern United States, and a median value of w from Table 2.3, 3.3 x lo5, then V, = 1 cm s-l. This implies that Vd = 0.2 cm s-l in the NCRP screening methodology. I t is clear from Table 2.3 that there may be large uncertainties associated with parameterizing deposition processes. The value of VT associated with Commentary No. 3, however, appears to be consistent with values reported for a number of locations using data for 1311 and 1 3 7 ~ fallout s from the Chernobyl nuclear power station accident (Kohler et al., 1991). It should be noted, however, that these aerosols were well mixed in the atmosphere by the time the measurements were taken. Depositing materials near the point of release would likely be less well mixed. For this reason, a conservative bias is suspected in the screening model; however, the magnitude of this bias is indeterminate due to insufficient data. In the presence of precipitation, wet deposition usually dominates over dry for submicron aerosols. For more arid climates, VT remains constant so the result is to raise the effective value of V,.This may result in a tendency for the screening model to overpredict VT in extremely arid climates. Another area of concern is the use of the screening model under conditions where snowfall dominates the deposition process. Snow on the ground can be blown into drifts. This can result in a n uneven spatial distribution of incorporated material as snow melts. Theoretically, this might result in the NCRP screening model underpredicting the ground deposition rate in localized areas that are sites of snow accumulation. The NCRP recommends that the screening model be tested under conditions where snowfall dominates the deposition process and significant drifting of fallen snow can occur to ascertain the magnitude of potential underestimation of localized deposition. The uncertainties associated with the atmospheric transport and deposition models used in NCRP Commentary No. 3 (NCRP, 198611989) have been discussed in this Section. The radiological assessment process set forth in NCRP Commentary No. 3, however, also involves food chain transport, human exposure and dosimetry. The uncertainties associated with these elements of an assessment using screening models are discussed in subsequent sections of this Commentary.

3. Food Chain Transport The food chain transport models used in NCRP Commentary No. 3 are quasi-equilibrium solutions for a continuous deposition over a period of 30 y. The processes considered by the models are direct deposition onto agricultural land and uptake to vegetation from radionuclides t h a t have accumulated in the top 15 cm of surface soil. Exposure is estimated for three types of food products: (1) milk, (2) meat and (3) vegetables. No distinction is made between the animal sources of milk or meat nor are distinctions made between leafy and nonleafy vegetables or root crops. Examination of the NCRP screening models and parameters lends general confidence that the predictions tend to be conservative under normal circumstances. Perhaps the strongest evidence of this is the comparison of screening model predictions to real data sets (see Section 3.3). Another line of evidence is the comparison of radionuclide concentrations in vegetables, meat and milk per unit deposition rate (e.g., Table 5, NCRP Commentary 3), with comparative values estimated by a model intended to provide more realism and which has been extensively tested against real data (Whicker and Kirchner, 1987; Whicker et al., 1990). I n all cases compared, the NCRP screening model predicted higher radionuclide concentrations in foods than the more "realistic" model (Table 3.1).

TABLE 3.1 - Equilibrium concentrations of selected radionuclides in foods per unit chronic deposition rate as estimated by the NCRP Commentary No. 3 screening model and the PATHWAY food chain model. Units are Bq kg-' per Bq m-adl. Food Type

Radionuclide

NCRP Screening Modela

PATHWAY^

Fresh wet vegetables Meat Milk

'Data from Table 5, NCRP Commentary 3 (NCRP, 198611989). b ~ a tfrom a Tables 3 , 4 and 5, Whicker and Kirchner (1987). Maximum seasonal values are given for PATHWAY, which gives seasonally-dependent output.

On the other hand, one can think of special ecosystems and circumstances where it appears quite possible that the screening model could underpredict concentrations of radionuclides in foods and hence produce dose estimates that may be too low. Ecological systems t h a t may

18

3. FOOD CHAIN TRANSPORT

depart significantly from typical agricultural systems, in terms of radionuclide transport, include arctic a n d alpine areas, regions with sandy or rocky, nutrient-depleted soils and regions of fish-producing oligotrophic lakes. Because of morphologic a n d physiologic adaptations of plants, the lack of nutrient analogue elements for important radionuclides, and certain characteristics of soils and watersheds, organisms in such environments may exhibit unusually high tendencies to take up radionuclides such a s 1 3 7 ~and s ''~r. Some of these organisms may be consumed by people (Porter et al., 1967; Whicker and Schultz, 1982). One circumstance which may be encountered t h a t is not dealt with by the NCRP screening model is the resuspension of contaminated soil a n d subsequent deposition on vegetation, or subsequent inhalation by people and animals. Another circumstance is the ingestion of surface soil by food-producing animals and people, especially children. These pathways a r e often unimportant in many agricultural and urban systems, although experience shows t h a t there a r e cases where these mechanisms should not be ignored. The following discussion provides a few examples t h a t illustrate the potential of the NCRP screening model to fail i n its objective of providing adequate conservatism. However, these examples show t h a t the possible lack of conservatism i n the screening model due to neglect of the soil ingestion pathways is not very significant.

3.1 Plant Contamination Processes Equation 3.1 provides the basis for the calculation of radionuclide concentrations in vegetation i n NCRP Commentary No. 3.

c, where

Cv Vd ca,

=

v*c,.

I).

f, 1-exp(-REtJ

-

E '

B, 1 -exp(- A,tJ

+P

43

I

(3.1)

concentration of radionuclide in (and on) vegetation (Bq kg"); deposition velocity for wet and dry deposition (m d-'1; concentration of radionuclide in air a t the nearest potential location of pastures and/or growing vegetables (Bq m-3); interception fraction, the fraction of deposited activity intercepted and retained by edible portion of crop (dimensionless); effective decay constant for removal of the radionuclide deposited on vegetation (d"), where ,IE = ,Ii + (0.693/tw); radioactive decay of constant (d-l) = 0.693/TR, where TR is the radioactive half life i n days; weathering half life, the time required for one-half of the originally deposited material to be lost from vegetation (d); time period of above-ground crop exposure to contamination during the growing season (d); standing crop biomass of edible portion of crop a t harvest (kg m-2); concentration ratio for the transfer of the element to the edible portion of a crop from dry soil (Bq kg-' plant per Bq kg-') soil;

3.1 PLANT CONTAMINATION PROCESSES

/Ig

RHL tb

P

1 19

=

effective rate constant for removal of radionuclides from soil (d-l),where RB = R, + AH,; = rate constant for removal of radioelement from soil by harvesting and leaching (d"); = period of long-term deposition and build up in soil (d); and = areal density for the effective root zone in soil (kg m'2 dry mass).

These concentrations, in turn, drive the resultant concentrations in animal products such as milk, meat, etc. Equation 3.1 includes a term for direct deposition from the atmosphere to the foliage and one for uptake from contamination which accumulates in the soil. The parameters for the deposition velocity, interception fraction, plant biomass, weathering rate, removal rate IYom soil and root zone depth and soil density are considered radionuclide-independent. The parameters describing physical decay rate, uptake from the soil and transfer to milk and meat are radionuclide-dependent. The model and parameters used in the screening calculations provide values for vegetation concentrations that are conservative for most agricultural ecosystems. However, the paragraphs that follow will illustrate the potential for severe underprediction in certain circumstances. One example is the case of lichens, mosses and other forms of low, dense, long-lived -vegetation adapted to harsh, often nutrient-deficient environments. Such plants may exhibit interception fractions approaching 1.0, low biomass values of the order of 0.05 kg m-2, and very long weathering halftimes, e.g., 10 y for 1 3 7 ~ inslichens (Lidkn and Gustafsson, 1967; Hanson and Eberhardt, 1969). If these parameters are used in the NCRP screening model, the concentrations predicted for 1 3 7 ~ins vegetation exceeds the normal prediction by over three orders of magnitude (Table 3.2). This would similarly affect animals consuming this vegetation. TABLE 3.2 - Comparison of equilibrium concentrations of radionuclides in vegetation and milk per unit deposition rate between those given by the default parameters of the NCRP Commentary No. 3 screening model and by parameters and pathways made possible under special circumstances. Units are Bq kg-' per Bq m-* d l . Case

Radionuclide

Dry Vegetation (Bq kg-' per Bq m-2 d-')

Ratio special case default NCRP

NCRP default parameters

137~s 239~u

Lichenslmosses

137~s

Nutrient poor soils

137~9

740

21

Resuspension

137~s

2 10

5.8

Soil ingestion

137~s 239pu

36

9.2 x

lo4

2.6 x

lo3

Milk (Bq L" per Bq m2 d-')

Ratio special case default NCRP

20 1 3. FOOD CHAINTRANSPORT

Another special situation concerns nutrient-poor soils t h a t exhibit low cation exchange capacity and thus bind radionuclides such a s 1 3 7 ~ very s poorly. I n such soils, the contaminants remain vulnerable to rapid uptake by plants and, because nutrient elements (e.g., Kf and Ca'3 are subject to leaching losses, they provide less competition for ion transport across root membranes. Soils so characterized include those of the sandy lower coastal plain i n the southeastern United States, rocky highlands i n the British Isles, etc. If the parameters i n the screening model are modified to account for this situation, enhanced vegetation concentrations can result. For example, if a Bv of 10, a growing season of 120 d, and a n areal soil density of 100 kg m-2 a r e substituted, the concentration of 1 3 7 ~i ns vegetation becomes larger t h a n the screening value by a factor of 2 1 (Table 3.2). This s deer meat from t h e lower phenomenon has resulted in enhanced concentrations of 1 3 7 ~in coastal plain (Jenkins and Fendley, 1971; Whicker, 1983) a n d in beef (Roessler e t al., 1969) from the same region. A potential shortcoming of the NCRP screening model structure is the lack of treatment of t h e resuspension process. Resuspension of contaminants from the soil surface by wind or other disturbance can result i n the subsequent deposition on foliage (Smith et al., 1982; Sehmel, 1980). The relative importance of this process depends on many factors, b u t in arid or semi-arid environments, a t least, the process deserves evaluation. Even in agroecosystems i n the humid southeastern United States, the resuspension process may be important (Pinder a n d McLeod, 1988; 1989; Pinder e t al., 1990). A similar phenomenon, rainsplash, may cause transport from the soil surface t o vegetation a s well (Dreicer et al., 1984) but this will not be evaluated here. A simplified method of treating resuspension in the context of the conditions simulated i n the NCRP screening model is to assume t h a t the soil surface continuously ,receives deposition from t h e atmosphere a n d loses contamination by the first order processes of radioactive decay and percolation:

QSS

where

QSS Cai, Vd A

*pert

=

CaiI v* [1 Ai + Ap

-

e -(A,

+

Lp)

x

tl

soil surface density (Bq m'2); a i r concentration (Bq ~ n - ~ ) ; = deposition velocity (m d-l); = physical decay constant of radionuclide (d-l); a n d = r a t e constant for percolation of radionuclide downward below the layer subject to resuspension (d-I).

= =

This Equation implicitly assumes t h a t 100 percent of the material deposited reaches the soil ultimately. The value of A,,,, has been estimated from the work of Anspaugh et al. (1975) to be of the order of 1.98 x d" (Whicker and Kirchner, 1987). Since the parameter specified i n t h e NCRP screening model is dose from the final year of 30 y of continuous deposition, the exponential term in Equation 3.1 may be dropped (equilibrium will be reached i n about 175 d). The concentration in vegetation a t equilibrium may be estimated from:

3.2 TRANSFER TO ANIMALS AND PEOPLE

QSS

CVJ

where

C,, RF fR

Y AE

=

/

21

RF V, f, y A,

= concentration in vegetation from resuspension (Bq kg7'); = resuspension factor (m-'); = interception fraction (unitless); = standing crop biomass (kg m-2);and = effective loss rate constant from the vegetation via physical decay, weathering (d-l).

If one uses an extreme value of RF = 1 0 'm" ~ (Anspaugh et al., 1975), which is possible only for very fresh deposits in desert areas, a QSS value of 50 Bq me2(from Equation 3.2) for a deposition rate of 1 Bq m-2 d-l, and other values a s specified in NCRP Commentary No. 3 [i.e., Vd = 1,000 m 6'; fR= 0.25; Y = 0.12 kg m-2; AE = ln 2 (14 d)-'1, the C , , will exceed the value of Cv, for direct deposition and root uptake by a factor of six (Table 3.2). A slower percolation rate might be plausible under some conditions, so that a ten-fold underprediction by the screening model is possible if resuspension is ignored under arid conditions.

3.2 Transfer to Animals and People I n the case of ' 3 7 ~ s for , a deposition rate of one Bq m-2 d-l, the NCRP screening model predicts an equilibrium concentration on pasture vegetation (C,) of 36 Bq kg-'. The insmilk (Cmilk)is estimated to be 4.6 Bq L.' which is the product of the concentration of 1 3 7 ~ d L ' ~ and ) C,. The forage ingestion rate (16 kg d-l), the transfer coefficient to milk (8.0 x screening model formulation ignores soil ingestion athways to both animals and people. If this pathway is added to the calculation above for l3 Cs in milk, a different value may result. For example, in the case,

5:

where

QSS

=

equilibrium soil surface density as defined in Equation 3.2 (Bq m-2);

Q,

= soil intake rate (g d-l);

X

= depth of soil profile subject to ingestion (cm); = soil surface density (g ~ r n - ~and ); = transfer coefficient to milk (d L - ~ ) .

P F,

22 1 3. FOOD CHAINTRANSPORT

Using reasonable values of QSS = 5.0 x 10' Bq m-2 and Q, = 500 g d-', (Whicker d L-', the value of Cmilk and Kirchner, 1987), X = 0.2 cm, P = 1.2 g cm-3 and F, = 8.0 x does not change from t h a t estimated using the NCRP screening value. However, if one considers a more extreme situation, the value of QSS a t equilibrium could increase to perhaps 530 Bq m-' if the percolation half time were to increase from 35 d to 1 y. If also the cow were to ingest 1,000 g soil per d and X were to be 0.1 cm, the estimated value of Cmilk could increase from 4.6 to 8.1 Bq L-' (Table 3.2). The effect of ignoring soil ingestion by foodproducing animals for less soluble radionuclides could be slightly greater. I t is possible t h a t the appropriate value for Fm applied to soil ingestion is less than the value applied to forage ingestion, however, very few data are available to support such a change. It does not appear t h a t ignoring soil ingestion is Likely to lead to large (order of magnitude) underestimation by the NCRP screening model.

3.3 V a l i d a t i o n of Food C h a i n M o d e l s The evaluation of uncertainty in model predictions is best achieved by comparing predicted results against actual data obtained through direct measurements (Ng and Hoffman, 1988). This process is often referred to as model validation (IAEA, 1989). I n this Commentary, data will be employed t h a t are primarily useful for testing the food chain transfer of 13'1 a n d 1 3 7 ~ s in agricultural systems. Most of these data have become available since the Chernobyl accident of 1986. Since these data represent the results of a relatively short-term event, vegetation-to-air and milk-to-air concentration ratios obtained with the NCRP screening model can only be compared against ratios of time-integrated concentrations of Chernobyl fallout in air, vegetation, milk and meat. In these comparisons, it is assumed t h a t the infinite time-integrated concentration of 1 3 7 ~ insvegetation, milk and meat is dominated by the timeintegrated concentration obtained during the 180 d post-deposition (Kijhler et al., 1991).

3.3.1 Transfer of 1 3 7 ~from s Air to Pasture Vegetation The food chain models employed i n NCRP Commentary No. 3 assume that deposition of radionuclides from the atmosphere occurs from both wet and dry processes. At the locations where data have been obtained on Chernobyl fallout, radionuclides have been well mixed in the atmosphere. Under these conditions, in-cloud scavenging by rain (rainout) may be more important t h a n when a plume of radionuclides is below t h e cloud-forming layer of the atmosphere and uncontaminated air enters the plume from above (washout). Thus, higher values of wet deposition with respect to a given ground-level air concentration are expected for globally dispersed Chernobyl fallout data than for data collected near a release source. For locations t h a t received precipitation during the passing of the Chernobyl plume, the estimate of air-to-forage transfer of 1 3 7 ~iss nearly the same estimate a s t h a t made with the NCRP screening model (Table 3.3). For locations where dry deposition tended to dominate, the use of the NCRP screening model would have overestimated the air-to-vegetation transfer s a factor of three t o seven. Overestimation by the NCRP screening model occurred of ' 3 7 ~ by a t Tokai, Japan where very heavy rainfall (40 mm in 1 d) likely caused a rapid wash-off of 137~ from s the vegetation surface.

3.3 VALIDATION OF FOOD CHAIN MODELS

/

23

TABLE 3.3 - Predicted to observed ratios for the transfer of 13'cs from air-to-pasture vegetation. Location

CR:eg/air

Observation Prediction

Reference

(m3 kg-') NCRP screening result

37,300

Chernobyl fallout data Loviisa, Finland Budapest, Hungary Roslulde, Denmark Neuherberg, Germany Tranvik, Sweden Petten, Netherlands Geel, Belgium Tokai, Japan Berlin, Germany Data on cosmogenic ' ~ e Oak Ridge, Tennessee, USA

Kohler et al. (1991) II (9

,I

,I t, 9 9,

44,000

Bondietti et al. (1984)

'Concentration ratio.

In Table 3.3, data are also given for air-to-vegetation ratios for cosmogenic 7 ~ determined e a t Oak Ridge, Tennessee. Like Chernobyl radionuclides sampled a t distances far from the reactor, cosmogenic 7 ~ ise well mixed in the troposphere, where it becomes attached to ambient aerosols, and is readily deposited with rain. The observed median value of the air-tovegetation transfer of 7 ~ isenearly identical to the value predicted by the NCRP screening model for 1 3 7 ~ a ss well a s for other long-lived radionuclides having relatively low values of soil to plant uptake. 3.3.2 Transfer of 13'1 from Air-to-Pasture Vegetation

The transfer of from air-to-pasture vegetation is highly dependent on the chemical and physical form of 1 3 1 ~in the air. The NCRP screening model assumes that all forms of iodine will be deposited from the atmosphere a t a rate of 1,000 m d-'. Data on time-integrated concentrations of 13'1 in air and pasture vegetation have been obtained for 11 sites in the Northern Hemisphere where Chernobyl fallout was monitored a n d from systematic measurements of reactor releases made in the vicinity of the Quad Cities Nuclear Power Plant in the United States (Table 3.4). Under both sets of conditions, 13'1 in air was present primarily a s a nonreactive gas (most probably CH31) with the remaining fraction composed of elemental iodine vapor (I2) and small particles (Kohler et al., 1991; Voillequk et al., 1981). For these sets of data, the NCRP model overestimated the air-to-vegetation transfer of 1311 by factors ranging from 2.1 to 9.0.

24 1 3. FOOD CHAINTRANSPORT

TABLE 3.4 - Predicted to observed ratios for the transfer o f 13'1 from air-to-pasture vegetation. Location

":eg/air 3

(m NCRP screening result

Observation Prediction

kg-')

15,300

Chernobyl fallout data Roskilde, Denmark Neuherberg, Germany Budapest, Hungary Geel, Belgium Portland, Oregon, USA Tranvik, Sweden Petten, Netherlands Oak Ridge, Tennessee, USA Tokai, Japan Berlin, Germany Loviisa, Finland BWR releases Quad Cities, USA

Reference

Kijhler et al. (1991) t I,

,, (1

, 9

4,500

3.4

Voillequh et al. (1981)

aConcentration ratio.

3.3.3 Transfer of 1 3 7 ~ from s Air-to-Milk Unlike the transfer to forage vegetation, the transfer from air-to-forage-to-milk for ' 3 7 ~ s is overestimated substantially by the NCRP screening model (Table 3.5). This overestimation could be due to the relatively short time-periods over which milk data were collected (i.e., four to six months). For 1 3 7 ~ sa, significant fraction of the total time-integrated concentration in milk may result from the cow's ingestion of surface soil after the initial 1 3 7 ~deposit s has weathered off of vegetation surfaces. For example, long-term ' 3 7 ~ stime-integrated concentrations obtained for weapons fallout (WNSCEAR, 1977) indicate t h a t the ratio of the integrated milk concentrations (Bq d L - ~ )to the total amount deposited (Bq m-2) was approximately 2 to 16. By comparison, the equivalent relationship for the NCRP screening ~ Bq m-2. model is about 5 Bq d L -per 3.3.4 Transfer of

1311 from

Air-to-Milk

Substantial overestimations a r e produced with the NCRP screening model for 13'1 transfer to milk (Table 3.6). Because of the short half-life of 13'1, additional processes like soil-plant uptake and soil ingestion are not so important a s is the grazing on pasture vegetation contaminated from direct atmospheric deposition. The only exceptions to the consistent trend toward overestimations are with data for Chernobyl fallout i n goat milk, the highest value obtained from 6 1 regions t h a t received Chernobyl fallout surveyed by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1988), and Site No. 4

3.3 VALIDATION OF FOOD CHAIN MODELS

/

25

TABLE 3.5 -Predicted to observed ratios for the transfer of 13'cs fmm air-to-pasture-to-dk. Observation Prediction

Location NCRP screening result Chernobyl fallout data AnguiUara, Italy Tokai, Japan Neuherberg, Germany Budapest, Hungary Geel, Belgium Loviisa, Finland Tranvik, Sweden Roskilde, Denmark Berlin, Germany Petten, Netherlands UNSCEAR [high value from 59 regions (Chernobyl fallout)]

Reference

4,700 1,010 733 711 400 350 338 278 255 230 187 8,400

4.7 6.5 6.7 11.9 13.6 14.1 17.2 18.7 20.7 25.5 0.57

Kohler et al. (1991) I*

fl

, UNSCEAR (1988)

'Concentration ratio.

TABLE 3.6 - Preo!icted to observed ratios for the transfer of '"I from cdr-to-pasture-to-mi&. Location NCRP screening result

CRb* im3 I,-')

aConcentration ratio.

Reference

2,450

Chernobyl fallout data Anguillara, Italy Oak Ridge, Tennessee, USA Tokai, Japan Portland, Oregon Neuherberg, Germany Geel, Belgium Grenoble, France Ispra, Italy Cadarache, France (goat milk) Aachen, Germany UNSCEAR (high value from 61 regions) USA (high value from 19 regions)

BWR releases Quad Cities, USA (Location #2) Quad Cities, USA (Location #4)

Observation Prediction

Kohler et al. (1991)

H o f i a n et al. (1988)

UNSCEAR (1988) Richmond et al. (1988)

390 1,600

Voillequk et al. (1981)

26

1 3. FOOD CHAINTRANSPORT

near Quad Cities Nuclear Plant, USA. The large overestimates of the NCRP screening model in Table 3.6 reflect both the overestimation of the air-to-vegetation transfer and the transfer from forage to milk. Data on the transfer of 13'1 to milk following deposition of Chernobyl fallout indicate a milk transfer coefficient (F,) that is approximately a factor of three to six times less than the value 0.01 d L-' that has been commonly assumed in most radiological assessment models and which has also been adopted in the NCRP screening model. 3.3.5 Transfer of z 3 7 ~ from s Air-to-Meat

As with the data for 1 3 7 ~ concentration s in milk, the data for 1 3 7 ~ insbeef (Table 3.7) may not have been collected over a sufficient length of time for secondary processes like resuspension and direct soil ingestion by cattle to contribute to the time-integrated concentration. For the data collected, however, the NCRP screening model produces an overestimation ranging from a factor of 2.6 to about 15. Substantial underestimation, however, can occur for animals grazing on vegetation growing on nutrient-poor soils (see Sections 3.1 and 3.2).

TABLE 3.7 - Predicted to observed ratios for the transfer of 13'cs from air-to-pasture-to-beef. Location

CRieatlair (m3

Observation Prediction

Reference

2.6

Kohler et al.(1991)

L")

NCRP screening result

13,400

Chernobyl fallout data) Budapest, Hungary Anguillara, Italy Roskilde, Denmark Neuherberg, Germany Loviisa, Finland

5,100 3,200 1,500 1,100 900

aConcentration ratio.

4.2 8.9 12.2

14.9

Il

I1

11 I,

4. Human Dietary Habits and Usage Factors Another potential shortcoming of the NCRP screening model is the rather standard dietary patterns a n d usage factors assumed. It was the intent, within NCRP Commentary No. 3, to produce generic values of consumption rates and occupancy times t h a t would be reasonable estiinates for a potentially maximally exposed individual. No credit is given for processed food or for the consumption of food harvested outside the region nor for distinguishing between time spent indoors versus outdoors. These assumptions will likely lead to substantial overestimation of the exposure to average individuals residing in the general region of the assessment. To some extent, the conservatism inherent with the assumed generic values of dietary a n d usage factors i n NCRP Commentary No. 3 is intended to compensate for the absence of conservatism i n other sections of the overall screening model. O n the other hand, in certain regions, there a r e individuals who consume unusually large quantities of certain foods, or foods not considered in the screening models. For example, heavy reliance on fish and wild game meat could lead to intakes considerably higher than those predicted from the screening models. This is particularly a n important consideration for 1 3 7 ~ swhich , accumulates readily in meat a n d fish flesh. Work i n Sweden following the Chernobyl accident, for example, showed that some of the highest 1 3 7 ~ concentrations s occurred in fish from oligotrophic lakes, despite the fact t h a t deposition over the region occurred from atmospheric releases (Mascanzoni, 1987). Research in many parts of Europe following the Chernobyl accident showed extreme variations i n plants a n d animals, hence the potential for certain critical groups to ingest unusually large quantities of 1 3 7 ~ s (Desmet et al., 1990). People with "nonconventional" dietary habits might include certain ethnic groups t h a t rely on hunting, fishing a n d gathering, self-sufficient farmers or ranchers, economically disadvantaged groups having access to hunting or fishing areas, and sportsmen. The wellknown lichen-reindeerlcaribou-LaplanderEskimo food chain provides one important example (Whicker, 1983). As mentioned previously, the NCRP screening model does not account for direct ingestion of soil by humans. Research, however, has documented soil ingestion rates by people of s that for a deposition rate different ages (LaGoy, 1987). A calculation for 1 3 7 ~ shows of 1 Bq m-2d-l, the human intake rate from vegetables, milk and meat is 3,180 Bq y-l, based strictly on the screening model. If the person were to ingest 1 g soil d'l, a fairly extreme but possible value (LaGoy, 1987), and QSSwere 50 Bq m-2 (calculated from Equation 3.2), only 7.6 additional Bq would be ingested i n a year. Even in more extreme cases, such a s the intake of less-soluble radionuclides, longer percolation half-times (up to 1 y) and a soil depth ( x ) (Equation 3.4) of only 0.1 cm, the additional intake is less t h a n the sum from ingestion of vegetables, milk and meat. Therefore, the absence of the soil ingestion pathway i n the NCRP screening model of Commentary No. 3 is very unlikely to lead to order-of-magnitude underpredictions.

5. External and Internal Dose Factors Appendix C of NCRP Commentary No. 3 (NCRP, 198611989) contains t h e specific equations t h a t are used i n the dose calculations and includes the dose factors t h a t are recommended for inhalation, ingestion, a i r immersion and ground surface exposure. The purpose of this Section is to discuss the degree of conservatism a s well a s the uncertainty associated with t h e dose factors.

5.1 Inhalation and External Dose Factors I n NCRP Commentary No. 3, the doses from inhalation a n d from external irradiation from both immersion a n d deposited radionuclides a r e inferred from the concentrations in ground-level air. The doses per unit a i r concentration resulting from inhalation of airborne activity, immersion in the contaminated air and exposure to material deposited on the ground surface were calculated as follows:

INEX =IM M + (BR)(INH)

where INEX

IMM INH BR GND

+

(V,)(GND)(1 - exp[-(A,

+

A,.)

TI/(',

+

A,)}

( 5 -1)

the annual dose per unit air concentration (Sv per Bq mm3); the dose factor for the immersion pathway (Sv per Bq m-3); the inhalation dose factor (Sv B ~ - ' ) ; the volume of air inhaled annually (8,000 m3); the dose rate factor for exposure to radionuclide deposited on t h e ground surface (Sv y'l per Bq m-'1; the radioactive decay constant (y-l); the environmental removal constant for t h e soil surface (0.01 y-l); the period of deposition (30 y); and the deposition velocity for both wet and dry deposition (m y").

The external dose from cloud immersion is calculated using the assumption of a semi-infinite cloud geometry. I n the calculation of the external dose from ground deposition, the geometry assumed is t h a t of a n infinite plane source a t the air-ground interface. No consideration of ground surface roughness effects or of the vertical distribution of the radionuclide in the soil is included i n the calculation of the external dose from ground deposition. Other assumptions t h a t a r e implicit but not stated in NCRP Commentary No. 3 (NCRP, 198611989) a r e that: (1) the individuals exposed spend all of their time outdoors i n rural areas and (2) the individuals exposed a r e all adults. The influence of these assumptions on the dose estimates, a s well a s the uncertainties associated with the dose factors, will be discussed.

5.1 INHALATION AND EXTERNAL DOSE FACTORS

/

29

5.1.1 Effect of Indoor Occupancy a n d of Source Geometry In temperate climates, people spend most of their time indoors. I n the most recent UNSCEAR report (UNSCEAR, 1988), it is assumed that, on average, people a r e outdoors only 20 percent of the time. Indoor occupancy affects both the doses from inhalation and the doses from external irradiation. Source geometry has only a n impact on the doses from external irradiation.

5.1.1.1 Inhalation. People inside buildings, are to some extent, protected against airborne pollution originating outside. Firstly, some of the airborne activity is removed by filtration i n cracks, crevices a n d pores through which air diffuses and, secondly, some of the activity t h a t enters the building deposits on floors, walls, ceilings and furniture, thereby reducing the indoor a i r concentration (Roed, 1990). The indoor-to-outdoor air concentration ratio and, therefore, the dose reduction factor, is highly variable a s it depends, among other factors, on the physical a n d chemical nature of the radionuclide considered, on the integrity of t h e construction, on t h e indoor-outdoor temperature difference, on the wind speed and on the deposition velocity of the contaminant (DOE, 1990). A number of experiments have been conducted to evaluate the importance of the filtration effect of buildings. I n England, during the Windscale accident, radioactive iodine was released a t a distance of approximately 20 m from a newly-erected wooden h u t with tight-fitting windows and reasonably tight-fitting doors (Megaw, 1962). From the results, it was concluded t h a t the iodine inhaled by people inside the building may be 20 to 80 percent of t h a t outside, depending on wind velocity and direction. Biersteker et al. (1965) measured SO2 levels inside a n d outside 60 houses in Rotterdam during the winter a n d found indoor concentrations lower t h a n outdoors by a factor varying from 3 to 15, depending on the year of construction. Similar results were found by other researchers (Alzona et al., 1979; Biersteker et al., 1965; Cohen and Cohen, 1979; 1980; Christensen a n d Mustonen, 1987; Megaw, 1962; Roed, 1986; 1987; Yocom et al., 1971). A representative value of the indoor-to-outdoor air concentration ratio seems to be 0.3 for t h e depositing radionuclides, but values less t h a n 0.1 have been observed. Assuming people spend 80 percent of their time indoors a n d t h a t the indoor concentration of a depositing radionuclide is 30 percent of the outdoor value, the time-averaged a i r concentration to which people a r e exposed, C,,, can be related to the outdoor air concentration, CaiP,as:

C,,

=

(0.8

x

0.3

x

C,)

+

(0.2

x

1

x

C,)

=

0.44

x

C,

.

The a i r concentrations calculated in NCRP Commentary No. 3 to estimate dose will probably overestimate the actual a i r concentration to which people are exposed by a factor of about two for depositing radionuclides, assuming that the outdoor a i r concentrations a r e predicted accurately.

30 / 5. EXTERNAL AND INTERNAL DOSE FACTORS

5.1.1.2 External Irradiation from Cloud Immersion. Buildings provide shielding against external irradiation. The indoor-to-outdoor dose ratios, called shielding factors, vary according to the gamma-energy spectrum of the radionuclide considered, the activity distribution in the radioactive cloud, and the characteristics of the building. Examples of variation of shielding factors a s a function of gamma energy are presented in Figure 5.1 for a semi-detached house and in Figure 5.2 for a block of houses (Le Grand et al., 1990). From these figures, the shielding factor is a t most equal to 0.5 (on the first floor of a semi-detached house) and can be less than 0.001 (in the basement of a multistory building).

I --

-

-

First Floor

-

---

-

-

m-•

Outside

-

1 -

-

-

/*

Ground Floor

---

-

--

-

-

-

=-

5

0.1

,

Basement, 0.2

/

0.3 0.5

1 .O

2.0

3.0 5.0

10

Source Energy (MeV)

Fig. 5.1. Shielding factors for different locations in a semi-detached house as a function of source energy (Le Grand et al., 1990).

5.1 INHGLATZON AND EXTERNAL DOSE FACTORS /

I

,Outside

I

31

Outside

*

a------.-------,,,,-----

------o---------------0

-

-

-

-

-

--

-

Second Floor

----

_- - - - - ----- - - - -

-00-----

---

-

-

-

-

-

--

L

a

- Basement o - * * 5

-

-

--

0.1

0.2

0.3 0.5

1 .O

2.0

3.0 5.0

Source Energy (MeV)

Fit3 5.2. Shielding factors for different locations in a house block as a function of source enerm -- for two different environments. Facing neighboring buildings (open circles). Facing an open area (filled-in circles) (LeGrand et al., 1990).

The variability of the shielding factors is illustrated in Figure 5.3, which shows calculated results corresponding to the housing characteristics in three French apartments for several radionuclides typically encountered in nuclear reactor releases (LeGrand et al., 1990). In this

32 / 5. EXTERNAL AND INTERNAL DOSE FACTORS

example, the minimum values are less than 0.05 for most of the radionuclides considered; the average values range from 0.05 to 0.2, depending upon the energy spectrum of the radionuclide; the maximum values are calculated to be about 0.35 for 8 8 ~ in r the worst-shielded apartment.

I

-

I

W o r s t Shielded Apartment

-

Seine Maritime Gard A Pas de Calais 0

(-Average

-

-

tl

-

f\

+-

Best Shielded Apartment

Fig. 5.3. Shielding factors for three "apartments" (Le Grand et al., 1990).

It is also worth mentioning that doses received outdoors in an urban area are expected to be smaIler than those received in an open area because of the presence of building materials between the individual considered and some part of the radioactive cloud (see Figure 5.4 taken from Le Grand et al., 1990). As shown in Figure 5.2, the doses received in a street are about half those received in an open area.

5.1 INHALATION AND EXTERNAL DOSE FACTORS

/

33

In addition, the assumption of a semi-infinite geometry in the calculation of the cloud immersion dose is conservative in many cases. Figure 5.5 illustrates how the ratio of the dose calculated using a realistic cloud geometry and of the dose calculated using a semi-infinite geometry varies according to the distance from the release point for different release heights, for average atmospheric dispersion conditions and for a 0.7 MeV gamma emitter (Le Grand et al., 1980). Under those conditions, the assumption of a semi-infinite geometry leads to an over-estimation of the dose of about 20 percent for distances from the site of release that are greater than 2 km. Outdoor Shielding Factor

Cross Section of Street

Fig. 5.4. Geometry for outdoor exposure from a plume in an urban area with multistoried buildings at the sides of the street (Le Grand et al., 1990).

Finally, the value of the dose factor may depend, to some extent, on the radiation transport equations or model used in the calculations. The differences, however, are small in comparison with the other effects discussed above. As an example, Table 5.1 compares, for

34

1 5. EXTERNAL AND INTERNAL DOSE FACTORS

several radionuclides, the dose factors recommended in NCRP Commentary No. 3 with those recently published by Jacob et al. (1990). Assuming that people spend 80 percent of their time indoors, in buildings providing an average shielding factor of 0.2, and 20 percent of their time outdoors, where the dose rate is on average equal to 70 percent of the dose rate calculated with the assumption of a semi-infinite geometry, the average dose factor due to external irradiation from cloud immersion, I%,, can be compared with the dose factor calculated as in NCRP Commentary No. 3, IMM, as:

IMM,,

=

[(0.8 x 0.2)

I

1

1

+

(0.2 x 0.7)] IMM

I

I

=

0.3 IMM.

(5.3)

I

...

-

-

Diffusion normale ii = 5ms-' E = 0.7 MeV

0

2

4

I

I

I

I

I

0

6

8

10

krn

Fig. 5.5. Ratio D/D, of dose equivalent rates resulting &om cloud immersion calculated using two different methods. I n this example, dose equivalent rates are calculated according to downwind distance from the site of release for 0.7 MeV photons present in a radioactive cloud emitted horn a stack of height H under average atmospheric dispersion conditions and a wind speed, ii,of 5 m s-'. The calculation of the dose equivalent rate D assumes a realistic cloud geometry while the calculation of the dose equivalent rate Dmassumes a semi-infinite cloud geometry (Le Grand et al., 1980).

5.1

INHALATION AND EXTERNAL DOSE FACTORS / 35

The actual dose resulting from cloud immersion is therefore expected, on average, to be about three times smaller than that calculated in NCRP Commentary No. 3, assuming the outdoor air concentration is predicted accurately.

6.1.1.3 External Irradiation from Ground-Deposited Activities. The degree of protection provided by a building depends, here again, on the distribution of the outdoor activity a s well as on factors such as thickness and composition of walls. Shielding effects were reviewed by Burson and Profio(1975;1977). Their recommended shielding factors (indoor-to-outdoor dose ratios) are highest for wood-frame houses without basements (average of 0.4 with a representative range from 0.2 to 0.5)and lowest for basements of multistory stony structures (average of 0.005 with a representative range from 0.001 to 0.015). Similar values were obtained more recently in studies conducted in Europe (Jacob and Meckbach, 1987;Jensen, 1984;Le Grand et al., 1987). An average shielding factor of 0.2 is used by UNSCEAR (1988).

TABLE 5.1 - Dose equivalent rates (mSv t 1 p e r Bq m'3) per unit air activity for an adult anthropomorphic phantom, standing on a smooth air-ground interface. The values listed in NCRP Commentary No. 3 (NCRP. 1986L989) are commred with those calculated bv Jacob et al. (1990). Radionuclide 88~r

Female Breasts

Gonadsa

Lungs

Red Marrow

Effective Dose

Jamb e t aL

NCRP

Jamb et aL

NCRP

Jamb et a].

NCRP

NCRP

Jawbetil

NCRP

(1990)

(198611989)

(1990)

(1986/1989)

(1990)

(198611989)

(1990)

(198611989)

(1990)

(198611989)

0.39

0.38

0.32

0.34

0.36

0.31

0.35

0.32

0.35

0.36

Jamb et m

l

aAverage of doses in ovaries and testes.

Another degree of conservatism built into NCRP Commentary No. 3 is that shielding from the vertical migration of radionuclides in soil and removal of radionuclides from surfaces by erosion and cleaning are neglected. These effects are dependent on the chemical properties and radioactive half-life of the radionuclides. Most of the data available are for 13'cs.After the reactor accident a t Chernobyl, it was found that cesium was better retained on urban surfaces than ruthenium or iodine (Jacob et al., 1987). Even so, the exposure rate from

36 /

5. EXTERNAL AND INTERNAL DOSE FACTORS

radiocesium deposited over lawns and meadows is about eight times higher than that over pavement and asphalt 3 y after the accident (Figure 5.6) (Jacob and Meckbach, 1990). In rural areas, the external dose rate fiom ground-deposited radionuclides decreases with time as a result of the downward migration of the radionuclides in soil. This effect is more important for long-lived radionuclides such a s 1 3 7 ~ than s for the short-lived ones, which do not have enough time to migrate significantly in the soil before their radioactive decay. Miller et a2.(1990) inferred from measurements of 1 3 7 ~ins soil from nuclear weapons tests that the dose rates in air a t 1 m above ground per unit inventory of 1 3 7 averaged ~s a factor of 1.8 higher in the forest as compared to the field areas and a factor of four as compared to deeply plowed land. Beck (1980) calculated exposure rates a t 1 m above ground for all radionuclides of interest and for various assumptions of distribution in the soil. For most radionuclides, the results corresponding to a range of distributions in soil are found to lie within a factor of about five.

I

I

I

-

I

I

I

I

I

I

I

I

I

I

-

Lawns and Meadows

-

-

C

Predominant Permeable

-

-

-

-

-

-

Pavement and Asphalt I

I

I

I

1

I

I

I

I

2

I

I

3

Time After Deposition (years)

Fig. 5.6. Ratio of external exposures from wet deposited 13'csin different urban environments to the exposure over an infinite lawn (Location Factors), as derived from measurements (Jacob and Meckbach, 1990).

I t should be noted that differences between the dose factors obtained with various models are relatively small. For a given distribution in soil, Chen (1991) found that the models are generally in excellent agreement. Jacob and Meckbach (1990) measured the gamma dose rates due to 1 3 7 ~ deposited s as a result of the Chernobyl accident and compared them with

5.1 INHALATION AND EXTERNAL DOSE FACTORS

/

37

several model predictions. The results, presented in Figure 5.7, also show a good agreement between experimental and predicted values. It is therefore clear that indoor occupancy and radionuclide removal or migration result in doses of external irradiation from ground-deposited activities that are much lower than those expected from NCRP Commentary No. 3, especially in urban areas. As an example, if it is assumed that people spend 80 percent of their time indoors, with a shielding factor of 0.2, and 20 percent of their time outdoors in urban areas, where the exposure rate is taken, on average, to be 50 percent of the value recommended in NCRP Commentary No. 3, the estimated dose factor for external irradiation from ground deposition, GND,,, is:

GND," = 0.5 x [(0.8 x 0.2)

+

(0.2 x 1.0)] GND

=

0.18 GND.

(5.4)

In other words, in this example, the actual dose resulting from external irradiation from ground deposition is expected to be about five times smaller than that calculated using NCRP Commentary No. 3, assuming that ground &position is accurately predicted.

1 .o

10

Time after Deposition (y)

Fig. 5.7. Reduction of the gamma dose rate from 13'cs over a lawn due to the migration into the soil. The numbers indicate different measuring sites (GA 6 4 = Gale e t al., 1964; Ja 88 = Jamb and Paretzke, 1988; UNSC 88 = UNSCEAR,1988; Mi 90 = Miller et al.,1990; Ja 90 = Jacob e t al., 1990).

38

1 5. EXTERNAL AND INTERNAL DOSE FACTORS

5.1.2 Effect of Age at Exposure The age a t exposure has some effect on the dose factors for inhalation and for external irradiation. However, this effect is not to be considered if, for regulatory compliance, doses are requested for "reference man" a s opposed to "any individual." The endpoint "reference man" is defined. It has no uncertainty or variability. The choice of endpoint to apply when using the NCRP screening model is dependent on the definition of dose limit adopted by the responsible regulatory authorities and the following applies only if doses are requested for "any individual."

5.1.2.1 Inhalation. Because of differences in anatomical, physiological and biokinetic data a s a function of age, the dose factors used for adults may not apply to infants or to children. Age-dependent dose factors have been published by the International Commission on Radiological Protection (ICRP) in its Publication 56 (ICRP, 1989) for some of the most radiologically significant radionuclides that might be released to the environment due to various human activities ( 3 ~14c, , 9 0 ~ r9,5 ~ r9,5 ~ bl ,o 3 ~ ul ,o 6 ~ ulZ91, , 1311, 132~, 1 3 4 ~ s1, 3 7 ~ ~ , 144ce, 238Pu, 2 3 9 ~ u2, 4 1 ~2~4 1 , ~ m2,3 7 ~ p2,3 9 ~ p )The . dose factors for inhalation that are presented in ICRP Publication 56 are considered to be temporary as they are based on an "old" lung model. They will be revised when the "new" lung model that is being prepared by the ICRP becomes available. In Table 5.2, the dose factors currently recommended by ICRP are compared with those provided in NCRP Commentary No. 3. The dose factors for inhalation that are recommended by ICRP for adults are very similar to those given in NCRP Commentary No. 3. However, for most radionuclides, the dose factors presented in ICRP Publication 56 decrease with age and are a factor of two to five higher for 1-y old infants than for adults. An extreme case is that of 2 3 9 ~ pfor , which the dose factor for infants is about ten times higher than that for adults. However, when the dose factors are multiplied by the breathing rates, BR, which increase as a function of age, the compensatory effect is such that there is little variation with age in the inhalation doses per unit air concentration. It should also be noted that, because of biological variability, individuals of the same age may have very different h s e factors. For example, Dunning and Schwarz (1981) found frequency distributions that resemble lognormal with ranges over one to two orders of magnitude for thyroid doses from intakes of 1311a t specific ages. 5.1.2.2 External Irradiation from Cloud Immersion. For a given air concentration, the organ and effective doses received by individuals of various sizes (or ages) vary to some extent. As an example, the effective doses per air kerma at 1 m above ground resulting from a semi-infinite source in air are shown in Figure 5.8 as a function of photon energy for three anthropomorphic phantoms (Saito et al., 1990). Within the energy range considered, the dose factors for babies are estimated to be higher than those for children, which are in turn higher than those for adults. The differences, however, are not very large; the baby-to-adult dose ratios amount, for most energies, to a factor of two or less.

5.2 INGESTION DOSE FACTORS

/

39

Table 5.2 - Comparison of effective dose equivalent factors for inhalation of a unit activity of a radionuclide. Radionuclide

Inhalation Class

Effective dose factor (Sv B ~ " )

ICRP

NCRP Commentary No. 3 Appendix A Adult value

Publication 56 Part 1 Adult value

Maximum value

Age

'Tritiated water. b~rganically bound tritium.

5.1.2.3 External Irradiation from Ground-Deposited Activities. For a given activity deposited per unit area of ground, the organ and effective doses received by individuals of various sizes also vary to some extent. Calculations made by Jacob et al. (1990) and by Saito et al.(1990) using four anthropomorphic phantoms representing an adult male, an adult female, a child and a baby showed that doses received by the baby were estimated to be, in general, higher than those received by the adult by about 20percent (Figure 5.9). 5 . 2 Ingestion Dose Factors As in the cases of inhalation and external exposure, because of differences in anatomical, physiological and biokinetic data as a function of age, the dose factors used for ingestion by adults and employed in the NCRP Screening Model may not apply to infants or to children. In Table 5.3 the dose factors for ingestion that are recommended by ICRP in its Publication 56 (ICRP, 1989) are compared with those provided in NCRP Commentary No. 3. There is a good agreement in the values for adults: however, for most radionuclides, the dose factors for

40 / 5. EXTERNAL AND INTERNAL DOSE FACTORS

0

ICRP Adult Child (7 years) Baby (8 weeks)

Photon Energy (MeV) Fig. 5.8. Effective dose equivalent per unit air kerma a t 1 m above ground resulting from a semi-illfinite source in air for various anthropometric phantoms (Saito et al., 1990).

-

0

0.01

0.1

ICRP Adult Child (7 years) Baby (8 weeks)

1

Photon Energy (MeV)

Fig.6.9. Effective dose equivalent per unit air kerma a t 1m above ground resulting from a plane source in depth of 0.5 g of soil for various anthropomorphic phantoms (Saito et al.,1990;Jacob et al., 1990).

5.3 CONCLUSION

/ 41

ingestion presented in ICRP Publication 56 decrease with age and are a factor of two to five times higher for 1-y old infants than for adults. An extreme case is that of 1311, for which the dose factor for infants is eight times higher than that for adults. Because milk and leafy

vegetables are foodstuffs that are consumed by both children and adults i n approximately the same amounts, the NCRP model does not seem to be conservative with respect to the ingestion pathway, at least as far as the dose factors are concerned. 5.3 Conclusion The degrees of conservatism and of uncertainty associated with the dose factors used in NCRP Commentary No. 3 have been discussed. Important assumptions implicit in NCRP's calculations are that: (1) the individuals exposed spend all of their time outdoors in unbuilt areas and (2) the individuals exposed are all adults. The first assumption results in a built-in conservatism in the calculated doses. The second assumption has a contrary effect a s the dose factors, either for external or for internal irradiation, generally decrease with age. Regarding inhalation, the doses per unit activity intake are higher for babies than for adults, by a factor that is about five on average. This effect is compensated by the increase of the breathing rate with age. Even though there is a large individual variability i n the

doses per unit intake, it seems reasonable to conclude that the inhalation doses calculated in NCRP Commentary No. 3 will not be exceeded by any individual by a factor greater than ten. With respect to external irradiation, the doses calculated i n NCRP Commentary No. 3 are clearly overestimates a s they do not take into account the shielding provided by buildings. A more refined dose estimation would result in higher doses for babies than for adults, but still lower than those calculated in NCRP Commentary No. 3. The ingestion pathway presents a different situation. Here again, the doses per unit activity intake are higher for babies than for adults, by a factor that is about five on average, but can be equal or greater than ten for some radionuclides. However, this effect is only compensated to a small extent by a n increase in the food consumption rate with age. Cows' and goats' milk are usually among the foodstuffs that contribute most of the activity intake through ingestion, but their consumption rate, on average, is about the same for babies and for adults. Given the large individual variability i n both the foodstuffs consumption rates and

i n the doses per unit activity intake, it cannot be conclucEed that the dose factors used for ingestion in NCRP Commentary No. 3 are conservative for all individuals and for all radwnuclides.

42

1

5.

EXTERNAL AND INTERNAL DOSE FACTORS

TABLE 5.3 - Comparison of effectiue dose equivalent factors for ingestion of a unit activity of a radionuclide. Radionuclide NCRP Commentary No. 3 Appendix A Adult value

Tritiated water. b~rganicallybound tritium.

Effective dose factor (Sv ~ q . ' ) ICRP Publication 56 Part 1 Adult value

Maximum value

(Age)

6. Summary This review of NCRP Commentary No. 3 indicates numerous assumptions t h a t are, by necessity, conservative and other assumptions which could, under special circumstances, lead to underestimates of actual dose. These assumptions and the direction of bias associated with their effect on screening calculations a r e listed in Table 6.1. I n addition, there a r e several situations in which the use of NCRP Commentary No. 3 should be restricted or modifications made prior to application. This summary Section reviews the primary assumptions affecting bias i n the screening calculations a n d other issues of concern i n the areas of atmospheric dispersion a n d deposition, exposure pathways assessment, dietary and usage factors, and internal a n d external dose factors. TABLE 6.1 - Bias in NCRP screening model assum~tions. Assum~tion Wind blows 25 percent of t h e time toward receptor Wind speed is 2 m s-' Class D stability is p evalent 100 percent of t h e time No plume depletion; n o plume rise 1,000 m d" deposition velocity for a l l reactive g a s e s a n d particles Continuous discharge over 30 y Loss r a t e from aoil is 0.01 y" No resuspension from soil surface 100 percent grazing period Default p a r a m e t e r values for food chain transfer No shielding from g a m m a radiation No loss from food preparation No soil ingestion No transfer from watershed to aquatic food products Non-conventional food chain pathways (i.e., wild game, mushrooms) not considered Models a r e n o t specific for regions w i t h n u t r i e n t poor soils Default p a r a m e t e r s for dietary and usage factors A d u l t dose factors only

Direction of bias

a

+ + +d ++ ++ + - - -

- - -e

-, + +I

- - 13

"Direction of bias: + = suspected conservatism; + + = moderate conservatism (may approach one order of magnitude in special circumstances); + + + = strong conservatism (overestimation may exceed one order of magnitude in special cases; 0 = bias unknown; - = suspected tendency towards underestimation; - - = moderate tendency to underestimate (may approach one order of magnitude in special circumstances), - - - =strong tendency to underestimate (may exceed one order of magnitude in special cases). For mobile radionuclides this effect may be extreme, but the end result will be less important because the process of direct deposition on to vegetation from the atmosphere will dominate the contamination of vegetation. 'Bias is only of minor importance as other processes dominate the magnitude of the calculated dose. d lpercent ~ ~grazing period is mainly restricted to small farms and warm climates. Large commercial dairies rely primarily on silage and concentrates. ePrimaril~of importance for 13'cs in southeast coastal plains and in the Arctic. f ~ l i g hunderestimation t may occur for individuals of special population subgroups; for typical individuals, foods are generally obtained from a region that is substantially larger than the local area considered by the NCRP screening model. gOf importance only for assessments applicable to "any individual" as opposed to "reference man."

r

44

/

6. SUMMARY

6.1 Atmospheric Transport and Deposition

The assumption t h a t the wind persists on a n annual average basis 25 percent of the time in any direction is conservative, even for the wind direction with the highest wind frequency. The value of 25 percent is only likely to be approached under extreme conditions. The screening models are intended for use i n the evaluation of emissions occurring over a prolonged period of time (on the order of 1 y). Because of the highly variable nature of meteorological conditions that can prevail during a n acute release, the screening model a t Levels I1 and I11 should not be employed for the assessment of accidental releases. The Screening Level I, which permits minimal dilution of the air concentration a t the point of release, contains sufficient conservatism to make this level of screening useful for both accidental and routine release assessment. The assumption of stability category D for all conditions may over or underestimate air concentrations, depending on site-specific conditions, but the magnitude of this uncertainty should not be large for annual average conditions. A source of conservatism is inherent in the assumption t h a t the effect of plume rise due to high exhaust velocities and temperatures is not considered. The height of the release is, therefore, the physical rather than the effective stack height. This assumption wilI produce a bias towards overestimating air concentrations near a source of release. Very close to a n elevated release source (a tall stack or building vent), the predicted air concentrations a t ground level will be overestimated. This apparent conservative bias is offset by the fact t h a t any potential receptor will not be located a t a fixed location and may well move about to locations where the plume has approached the ground surface. The models in Screening Levels I, I1 and I11 should not be employed to reconstruct, or back calculate, releases using measurements of radionuclides in air, soil or vegetation. Because there is a tendency to overestimate actual doses from a given release rate when these models are used to screen against dose limits, substantial underestimation of the actual release rates may occur when these same models are used for the purpose of source term reconstruction from environmental samples. The underlying assumptions of the screening models do not appropriately handle the effects of limited vertical mixing a t distances far downwind from the location of release. I t is therefore recommended that the screeningmodels not be applied for the calculation of air concentrations a t distances much greater t h a n a few tens of kilometers. For such calculations to be made, modifications will be needed in the models described i n NCRP Commentary No. 3. For most circumstances, however, the location of the nearest potential receptor will be much closer to the point of release t h a n a few tens of kilometers. Caution must be exercised when calculating air concentrations in narrow valley situations. Under these conditions, no credit should be given for the effect of stack height. I t is recommended that, beyond Screening Level I, the models not be applied for the calculation of air concentrations for releases to courtyards or street canyons formed

6.2 TERRESTRIAL EXPOSURE PATHWAYS TO H U M ' S

(10)

(11)

(12) (13)

/

45

by avenues of tall buildings, due to potential reconcentration of the radionuclides i n the plume that can occur from the development of pockets of a i r stagnation. I n their present form, the screening models are not applicable to atmospheric releases of radionuclides from large areas. The models a r e currently structured for point source emissions. Modifications to the current screening methodology, however, can be made to account for releases from area sources. The use of a total (wet and dry) deposition velocity of 1,000 m d-' appears to be reasonably conservative for most climatic situations. For very arid climates, deposition of submicron particles may be substantially overestimated. It may underestimate the deposition of radioiodine in elemental form by about a factor of two. For very large (>20 micron) particles, underestimates of deposition may be more substantial. The value of deposition velocity does not affect concentrations in the a i r calculated with t h e screening model. This is a source of conservative bias for air concentrations calculated further than a few kilometers from the source. Caution is warranted when the screening model is used in high snowfall areas. This is especially applicable for locations a t high elevations a n d far northern latitudes. Snow can be a n effective scavenger of particulate radionuclides i n the atmosphere. Redistribution of initially deposited material a n d accumulation can occur i n snow drifts. The extent to which the assumptions i n other p a r t s of NCRP Commentary No. 3 a r e sufficiently conservative to compensate for these effects is unknown a t the present time.

6.2 Terrestrial E x p o s u r e P a t h w a y s to H u m a n s For most cases, the assumptions used for estimating the transfer of radionuclides through terrestrial pathways of exposure should lead to overestimates of actual exposure. This conclusion is supported by numerous attempts (for a limited set of radionuclides) to validate model predictions with independent field data. I n a few special cases, however, underestimation may occur. (1) For releases of radiocesium to ecosystems of poor soils a n d low growth vegetation (e.g., lichens, mosses, heath), such a s exist in the Arctic a n d southeastern United States coastal plain, food chain bioaccumulation may exceed the generic screening assumptions employed i n NCRP Commentary No. 3. The extent to which conservatism in other areas of the screening model offset t h e potential lack of conservatism in the food chain models for releases of radiocesium under these conditions remains to be investigated. (2) For arid a n d semi-arid environments (or conditions), resuspension a n d rainsplash may enhance the inhalation and ingestion of long-lived, insoluble radionuclides to a n extent t h a t the generic assumptions employed in NCRP Commentary No. 3 may not be sufficiently conservative. The extent to which conservatism elsewhere i n the screening models offsets the absence of resuspension and rainsplash remains to be investigated. (3) The screening model could underestimate exposure when extreme circumstances lead to large amounts of soil ingested by either grazing animals or humans.

46 1 6 . SUMMARY

(4)

(5)

However, the magnitude of this underestimation is very unlikely to exceed a factor of two, even in extreme cases. The screening models i n NCRP Commentary No. 3 could easily be revised to take the direct soil ingestion pathway into account to help offset other potential underconservatisms. The exposure pathways considered in NCRP Commentary No. 3 a r e external exposure to the ground surface, inhalation, and ingestion of vegetables, milk and meat. Since the advent of the Chernobyl accident, other exposure pathways have been identified t h a t may produce higher individual exposures to radiocesium t h a n those assumed for generic screening. These pathways include the harvesting of forest mushrooms, the dependence on wild game a s a major dietary source and ingestion of fish from oligotrophic lakes in watersheds receiving atmospheric deposition. The extent to which conservatism in other areas of the screening model of NCRP Commentary No. 3 will offset the potential lack of conservatism in the generic exposure pathways assumed for radiocesium remains to be investigated. A source of conservative bias is associated with the assumption that no losses will occur from food processing and t h a t no foods will be imported from uncontaminated locations beyond the nearest location where milk, meat and vegetables are produced.

6.3 Usage Factors The usage factors specified in NCRP Commentary No. 3 are not absolute maximum values. Furthermore, these factors are specific to the generic exposure pathways involving inhalation of air, ingestion of vegetables, milk and meat, and external exposure to radionuclides deposited on the ground surface. Maximum values for individuals are probably well within a factor of two of the generic values assumed in Commentary No. 3. The usage factors in Commentary No. 3 are conservative in t h a t no credit is given for indoor shielding, migration to and from the region of contamination, or consumption of foods grown outside the region of contamination. All vegetables consumed are assumed to have the same concentration a s would "leafy vegetables." To some extent the conservatism inherent with these assumptions will compensate for the potential absence of conservatism in other sections of the NCRP screening model.

6.4 Dosimetry

The use of ICRP effective dose equivalent conversion factors for screening calculations is warranted if the target of the dose limit is "reference man." This target may not be valid for regulations t h a t set dose limits for "any individual" in the population. As these dose limits refer to the maximum dose received i n 1 y, consideration must be given to age groups other than reference adults. (1) Given the large individual variability i n consumption rates and in the doses per unit activity intake, i t cannot be concluded t h a t the dose factors used for ingestion in NCRP Commentary No. 3 a r e conservative for all individuals and for all radionuclides. This problem is especially acute for those ingestion pathways where children may consume a s much or more of a given food type (e.g.,fresh milk) t h a n

6.4 DOSIMETRY

(2)

(3)

/

47

adults. For some radionuclides and organs (e.g., dose to the thyroid from l3'1), the extent of underestimation in the use of "reference man" dose conversion factors may exceed a factor of ten for the exposure of small children and infants. Numerous sources of conservative bias elsewhere in the screening model may compensate for this potential to underestimate, but the combined effect of all sources of bias toward over and underestimation remains to be examined in more detail. For inhalation, compensation occurs because younger ages, although associated with higher dose conversion factors than adults, have lower volumetric inhalation rates. Even though there is large individual variability in the doses per unit intake, it seems ,reasonable to conclude that the inhalation doses calculated for screening using NCRP Commentary No. 3 will not result in an underestimate exceeding a factor of ten. Additional compensation occurs because lower air concentrations occur in the indoor environment and all exposures are conservatively assumed to be outdoors. The extent of this compensation is, on the average, about a factor of three. The uncertainty due to the lack of age dependency in the external dose calculations is small. Overestimates on the order of a factor of five may occur because of the failure to account for indoor shielding.

7. Final Remarks From the outset of this study it has been desired to employ a formal uncertainty analysis to investigate the magnitude of the overall bias in the screening calculations. Such formal uncertainty analyses are mainly feasible for specific release scenarios involving specific radionuclides, locations and receptors. Detailed analysis of a range of release scenarios was beyond the scope of the present review; however, efforts focused on identifying specific circumstances in which the bias associated with the use of the screening model would be extreme. Further review and investigation are indeed warranted. Additional efforts to validate the NCRP screening models against field data may quantify the effect of compensatory bias and result in future recommendations for modification of the screening approach. Recommendations for revisions, however, should strive to preserve the original intent behind the screening approach; which is, calculational simplicity to achieve accuracy within a n acceptable margin for error, allowing for greater degrees of assessment sophistication when calculated doses approach or exceed applicable dose limits.

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Krypton-85 i n the Atmosphere- With Specific Reference to the Public Health Significance of the Proposed Controlled Release at Three Mile Island (1980)

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3

Screening Techniques for Determining Compliance with Environmental Standards- Releases of Radionuclides to the Atmosphere (1986), Rev. (1989)

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Guidelines for the Release of Waste Water from Nuclear Facilities with Special Reference to the Public Health Significance of the Proposed Release of R e a t e d Waste Waters at Three Mile Island (1987)

5

Review of the Publication, Living Without Landfills (1989)

6

Radon Exposure of the U.S. Population-Status of the Problem (1991)

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Misadministration of Radioactive Material in Medicine-Scientific Background (1991)

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Uncertainty in NCRP Screening Models Relating to Atmospheric Transport, Deposition and Uptake by Humans (1993)

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