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