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Radiological RiskAssessment andEnvironmental Analysis
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Radiological RiskAssessment and
Environmental Analysis
Edited by
j o h n e . t i l l
h e l e n a . g r o g a n
12008
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3Oxford University Press, Inc., publishes works that furtherOxford Universitys objective of excellencein research, scholarship, and education.
Oxford New YorkAuckland Cape Town Dar es Salaam Hong Kong KarachiKuala Lumpur Madrid Melbourne Mexico City NairobiNew Delhi Shanghai Taipei Toronto
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Copyright 2008 by Oxford University Press, Inc.
Published by Oxford University Press, Inc.198 Madison Avenue, New York, New York 10016www.oup.com
Oxford is a registered trademark of Oxford University Press
All rights reserved. No part of this publication may be reproduced,stored in a retrieval system, or transmitted, in any form or by any means,electronic, mechanical, photocopying, recording, or otherwise,without the prior permission of Oxford University Press.
Library of Congress Cataloging-in-Publication DataRadiological risk assessment and environmental analysis / edited by John E. Tilland Helen A. Grogan.
p. ; cm.Includes bibliographical references and index.ISBN 9780 1951272701. Radiation dosimetry. 2. RadiationSafety measures. 3. Health riskassessment. I. Till, John E. II. Grogan, Helen A.[DNLM: 1. Radioactive Pollutantsadverse effects. 2. Accidents, Radiationprevention & control. 3. Environmental Exposureprevention & control.4. Environmental Monitoringmethods. 5. Radiation Injuriesprevention & control.6. Risk Assessment. WN 615 R1292 2008]RA569.R328 2008363.1799dc22 2007036918
9 8 7 6 5 4 3 2 1
Printed in the United States of America
on acid-free paper
http://www.oup.com/http://www.oup.com/ -
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For
Susan S. Till, PhD,
and
R. Scott Yount,
Our spouses, who have been so supportive and patient aswe worked together on this book.
In Memoriam
Todd V. Crawford
A dear friend and professional colleague who contributedto chapter 3 and who passed away before the bookspublication.
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Preface
This textbook is an update and major revision of Radiological Assessment:
A Textbook on Environmental Dose Analysis, published by the U.S. Nuclear
Regulatory Commission in 1983. The earlier book was widely used as a graduate-
level text and as a reference book at universities, in special courses, and by
individuals who perform radiological assessment. Although the previous book made
a unique contribution in bringing together different elements of radiological assess-
ment as a science, a number of deficiencies were difficult to resolve at the time it
was written. For example, there was considerable disparity in the level of detailamong the chapters and in the information that each provided. In this new book, we
have tried to address some of these deficiencies. It is written more specifically as a
textbook, and it includes examples and sample problems throughout.
We have worked hard to improve the editing so there is better consistency among
the chapters and greater cohesiveness in the different subjects presented. Neverthe-
less, we recognize some differences still exist in the way material is presented. These
are due in large part to the multiple authors who contributed to this work. We do
not believe, however, that an individual author could have adequately captured the
state-of-the-art science and effectively conveyed the in-depth concepts required in
such a textbook. That is why, as with the 1983 edition, we asked other scientists to
contribute to the text. It is an honor and privilege to have the participation of these
respected scientists, and we are indebted to their efforts and expertise. This book
would not have been possible without them.
There have been many significant changes in radiological assessment over the
past 20 years, and we have tried to capture them. Some changes were caused
by the natural evolution and improvements of the underlying sciences that make
up radiological assessment, and other changes resulted from events such as the
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viii Preface
Chernobyl accident, phenomenal advances in computer technology, and a vastly
different interest in understanding the health implications of radioactive materials
that are or may be released to the environment by nuclear facilities.
It is apparent that interest in radiological assessment will continue to grow. Thereis a renewed emphasis on nuclear power as an energy source. Many nuclear power
plants around the world have matured and will, at some point, require significant
upgrades to their design or need to be decommissioned altogether. Government
authorities in many countries are working to find the right balance between eco-
logical destruction and remediation of environmental sites that were contaminated
with radioactive materials during decades of deliberate disposal of residues from
the production of nuclear weapons. We hope that this textbook will provide a reli-
able reference document for teaching and will set a standard for how radiologicalassessment should be performed.
Unfortunately, there are some elements of radiological assessment that we could
not include. One example, briefly discussed in chapter 1, is communication of radi-
ological assessment results and the participation of stakeholders. Another example
is how to screen sources, materials, and pathways of exposure in order to focus the
assessment on those elements that are the most important. These subjects had to be
omitted to keep to a reasonable length.
We did not want the book to be simply a compilation of papers by authors; rather,
we worked hard with the contributors to have the chapters fit together as cohesiveelements to cover the entire science of radiological assessment. Undertaking this
effort to employ the talents of a diverse group of individuals who are recognized as
experts in their own right is a considerable challenge. We hope we have achieved suc-
cess at merging the materials so that this textbook is useful, readable, and applicable
to a wide variety of users.
John E. Till
Helen A. Grogan
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Acknowledgments
We are very grateful to the contributors to this book who provided their chapters
while diligently and thoroughly responding to our editing suggestions and ideas.
We especially thank them for their cooperation and persistence because this effort
has taken such a long time to complete. Some chapters were revised significantly
over the time we have been working together, and the patience that our contributors
showed during this time is especially appreciated. We have been truly fortunate to
have some of the best scientists in our profession to assist us.
Editing and proofreading this book has been the primary responsibility ofMs. Cindy Galvin and Ms. Julie Wose. Cindy works as the editor for our research
team. In addition to her routine duties trying to keep our technical reports and pub-
lications in top quality, the book has been a responsibility she willingly took on for
almost two years. She has had to work with the different contributors and accommo-
date their individual styles and writing mannerisms. She has accomplished this task
in a pleasant and professional way. Julie Wose assisted with proofreading and check-
ing text, references, tables, and figures. Julie has a superb ability to pour through
hundreds of numbers in tables and figures looking for errors or items that seem
incongruous with other information being presented. She has been of immense help
throughout the course of this effort.
Ms. Shawn Mohler assisted us with graphics in some chapters. Shawn has a
superb talent for creating new graphic art or revising old or outdated artwork.
We especially acknowledge our entire research team, Risk Assessment Corpora-
tion (RAC), who agreed to contribute to the book or who helped us in other ways.
RAC team members Art Rood, Jim Rocco, Lisa Stetar, Lesley Hay Wilson, and Paul
Voillequ contributed chapters to the book. We fully understand and appreciate the
extra effort required to contribute to this book while they were meeting deadlines
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x Acknowledgments
for other projects. A considerable amount of the work presented in our chapters
was taken from work performed by the RAC team as a whole. Therefore, everyone
on our team deserves credit and special recognition for their contribution to this
high-quality and unique volume.A special thanks and recognition to Dr. Bob Meyer, who co-edited the first book,
published in 1983. Bob was instrumental in keeping the idea of an updated version
alive over the years. Bobs new work responsibilities, intense commitments, and
busy schedule prevented him from continuing this collaboration. Nevertheless, his
contributions and involvement in radiological assessment over the past 30 years
have helped shape the science.
Finally, we acknowledge the patience of Peter Prescott of Oxford University
Press, who has exemplified an extraordinary patience with us in delivering themanuscript. Peter has worked with us from the beginning. Since we began assem-
bling and editing this book, our lives have undertaken a number of turns, both
positive and negative. Our research always had to come first because of commitments
to customers, which meant that we often lost our focus on the book. Nevertheless,
Peter always maintained his confidence in the book and its importance. We appreci-
ate this dedication to our effort by both Peter Prescott and his entire staff at Oxford
University Press.
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Contents
Contributors xxv
1 The Radiological Assessment Process 1John E. Till
Radiological Assessment Process 2
Source Term 3
Environmental Transport 5
Environmental Transport of Plutonium in Air During
the 1957 Fire at Rocky Flats 6
Exposure Factors 8
Rocky Flats Representative Exposure Scenarios 9
Hanford Site Scenarios for Native Americans 10
Conversion to Dose 12
Uncertainty in Dose Coefficients 12
Appropriate Use of Dose Coefficients as a Function of Age 13
Conversion of Dose to Risk 13
Why Risk? 14
Risk Coefficients 14Uncertainty Analysis 15
Use of Uncertainty for Determining Compliance with Standards 17
Validation 18
Communication of Dose and Risk and Stakeholder Participation 21
Communication of Results from Radiological Assessment 21
Stakeholder Participation 24
Conclusion 28
References 28
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xii Contents
2 Radionuclide Source Terms 31Paul G. Voillequ
Radionuclides of Interest and Their Properties 32
Situations That Do Not Require Source Terms 37Human Activities Producing Releases of Radionuclides 38
Uranium Mining 39
Uranium Milling 39
Uranium Conversion 40
Uranium Enrichment 40
Weapon Component and Fuel Fabrication 41
Reactors 42
Source Terms for Normal Operations 44Source Terms for Accidents 56
Fuel Processing Plants 58
Solid Waste Disposal 61
Source Term Development for Facilities 62
Source Terms for Prospective Analyses 62
Source Terms for Retrospective Analyses 64
Problems 66
References 69
3 Atmospheric Transport of Radionuclides 79Todd V. Crawford, Charles W. Miller, and Allen H. Weber
The Atmosphere 80
Composition 80
Vertical Extent Important for Atmospheric Releases 80
Scales of Motion 81
Macroscale 84
Mesoscale 84Microscale 87
Input Data for Atmospheric Transport
and Diffusion Calculations 89
Source 89
Winds 90
Turbulence and Stability 93
Atmospheric Stability Categories 94
Pasquill-Gifford Stability Categories 95
Richardson Number 99
Mixing Height 106
Meteorological Data Quality 108
Modeling of Transport and Diffusion 108
Gaussian Diffusion Models 109
Instantaneous Point Source 109
Continuous Point Source 110
Continuous Line Source 110
Continuous Point Source Release from a Stack 111
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Contents xiii
Sector Averaging 112
Modifications Based on Source Characteristics 113
Special Considerations 116
Summary of Gaussian Plume Model Limitations 119Puff-Transport and Diffusion Models 120
Puff Transport 120
Puff Diffusion 121
Sequential Puff-Trajectory Model 122
Multibox Models 125
Calculation Grid 125
Calculation Methods and Limitations 126
Particle-in-Cell Models 126Screening Models 127
Atmospheric Removal Processes 128
Fallout 129
Dry Deposition 129
Wet Deposition 130
Model Validation 132
Model Uncertainty 134
Guidelines for Selecting Models 136
Regulatory Models 137 AERMOD Model 137
CALPUFF Model 138
CAP88 Model 138
Conclusions 139
Problems 139
References 140
4 Surface Water Transportof Radionuclides 147Yasuo Onishi
Basic Transport and Fate Mechanisms 149
Transport 149
Water Movement 149
Sediment Movement 150
Bioturbation 151
Intermedia Transfer 151
Adsorption and Desorption 151
Precipitation and Dissolution 152
Volatilization 153
Physical Breakup 153
Degradation/Decay 153
Radionuclide Decay 153
Transformation 153
Yield of Daughter Products 153
Radionuclide Contributions from Other Environmental Media 154
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xiv Contents
Radionuclide Transport Models 154
Accidental Radionuclide Releases 155
Routine Long-Term Radionuclide Releases 156
Rivers 159Basic River Characteristics 159
Screening River Model 163
Estuaries 167
Basic Estuarine Characteristics 167
Screening Estuary Methodology 171
Coastal Waters and Oceans 176
Basic Coastal Water and Ocean Characteristics 176
Coastal Water Screening Model 179Lakes 181
Basic Lake Water Characteristics 181
Small Lake Screening Model 183
Large Lake Screening Model 185
Sediment Effects 187
Numerical Modeling 190
Governing Equations 190
Some Representative Models 191
Chernobyl Nuclear Accident Aquatic Assessment 192
Radionuclide Transport in Rivers 194
Aquatic Pathways and Their Radiation Dose Contributions 197
New Chernobyl Development 200
Problems 200
References 203
5 Transport of Radionuclides
in Groundwater 208
Richard B. Codell and James O. Duguid
Applications of Groundwater Models for Radionuclide
Migration 209
Geologic Isolation of High-Level Waste 209
Shallow Land Burial 210
Uranium Mining and Milling 210
Nuclear Power Plant Accidents 211
Types of Groundwater Models 211
Groundwater Models for High-Level Waste Repositories 211
Near-Field Performance 212
Far-Field Performance 213
Groundwater Models for Shallow Land Burial of Low-Level Waste 215
Groundwater Models for Mill Tailings Waste Migration 215
Equations for Groundwater Flow and Radioactivity Transport 216
Groundwater Flow 216
Saturated Flow 218
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Contents xv
Unsaturated Flow 219
Mass Transport 219
Chain Decay of Radionuclides 220
Percolation of Water into the Ground 221Parameters for Transport and Flow Equations 222
Diffusion and Dispersion in Porous Media 222
Molecular Diffusion 222
Dispersion 222
Macrodispersion 223
Determination of Dispersion 224
Porosity and Effective Porosity 224
Hydraulic Conductivity for Saturated Flow 226Sorption, Retardation, and Colloids 228
Transport Based on Assumption of Equilibrium
(Retardation Factor) 228
Transport Based on Geochemical Models 231
Colloid Migration 232
Methods of Solution for Groundwater Flow and Transport 233
Numerical Methods 233
Finite Difference 233
Finite Element 234Method of Characteristics 234
Random Walk Method 234
Flow Network Models 235
Advection Models 235
Analytic Elements 235
Analytical Solutions of the Convective-Dispersive Equations 236
Point Concentration Model 237
Flux Model 240Generalization of Instantaneous Models 243
Superposition of Solutions 243
Simplified Analytical Methods for Minimum Dilution 243
Models for Population Doses 246
Source Term Models for Low-Level Waste 250
Model Validation and Calibration 251
Misuse of Models 253
Problems 254
References 254
6 Terrestrial Food Chain Pathways: Concepts and Models 260F. Ward Whicker and Arthur S. Rood
Conceptual Model of the Terrestrial Environment 262
Strategies for Evaluating Food Chain Transport 268
Predictive Approaches 268
Direct Measurements 268
Statistical Models 269
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xvi Contents
Mechanistic Models 270
Choosing a Predictive Approach 271
Model Attributes 271
Mechanistic Models: The Mathematical Foundationsfor Single Compartments 273
Concepts and Terminology of Tracer Kinetics 273
Single-Compartment, First-Order Loss Systems 275
Source and Sink Compartments 275
Single Compartments with Constant Input Rates 277
Single Compartments with Time-Dependent Input Rates 280
Single-Compartment, NonFirst-Order Loss Systems 284
The Convolution Integral 284Borels Theorem 285
Derivation of Rate Constants Involving Fluid Flow
Compartments 286
Numeric Solutions 287
Individual Transport Processes: Concepts and Mathematical
Formulations 287
Types of Processes 288
Continuous Processes 288
Discrete Processes 288Stochastic Processes 288
Deposition from Air to Soil and Vegetation 289
Gravitational Settling 289
Dry Deposition 290
Wet Deposition 292
SoilVegetation Partitioning of Deposition 294
Transport from Soil to Vegetation 295
Suspension and Resuspension 296Root Uptake 303
Transport from Vegetation to Soil 307
Weathering 307
Senescence 308
Transport within the Soil Column 309
Percolation 310
Leaching 310
Other Natural Processes Producing Vertical Migration in Soil 317
Tillage 318
Transport from Vegetation to Animals 318
Transport from Soil to Animals 320
Ingestion 320
Inhalation 321
Transfers to Animal-Derived Human Food Products 321
Ingestion Pathways to Humans 323
Dynamic Multicompartment Models: Putting It All Together 324
Conclusions 331
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Contents xvii
Problems 333
References 334
7 Aquatic Food Chain Pathways 340Steven M. Bartell and Ying Feng
Aquatic Ecosystem Classification 341
Conceptual Model for an Aquatic Environment 342
Physicochemical Processes 343
Radionuclide Uptake and Concentration Factors 344
Examples of Bioconcentration Factors 348
Bioconcentration Factors in Screening-Level Risk Estimations 351
Bioaccumulation Factors in Estimating Exposure 352Bioaccumulation under Nonequilibrium Conditions:
The Chernobyl Cooling Pond Example 354
Initial137Cs Contamination in the Chernobyl Cooling
Pond Water 355
The Chernobyl Cooling Pond Ecosystem 356
Chernobyl Cooling Pond Model Structure 357
Food Web Structure 360
Population Dynamics and Biomass Distributions 360
Spatial and Temporal Radionuclide Ingestion Rates 362Radionuclide Transport and Distribution 363
Case Studies in Exposure and Bioaccumulation 364
Case 1: Homogeneous and Steady-State Exposures 364
Case 2: Homogeneous and Dynamic Radioactive Environment 364
Case 3: Homogeneous and Dynamic Radioactive Environment
with Dynamic Population Biomass 366
Case 4: Heterogeneous and Dynamic Radioactive Environment 366
Case 5: Heterogeneous and Dynamic Radioactive Environmentwith Varying Biomass 367
Case 6: Dynamic Exposures and Variations in Feeding Rates 368
Case 7: Dynamic Exposures and Multiple Prey 368
Discussion of the Chernobyl Modeling Results 368
Temporally and Spatially Dependent Ecological Factors 370
Problems 371
References 372
8 Site Conceptual Exposure Models 376James R. Rocco, Elisabeth A. Stetar, and Lesley Hay Wilson
Evaluation Area 377
Interested Party Input 377
Exposure Pathways 378
Sources and Source Areas 379
Radionuclides 380
Exposure Areas 381
Potentially Exposed Persons 382
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xviii Contents
Behaviors and Activities 383
Exposure Media 383
Exposure Routes 383
Transport Mechanisms 384Transfer Mechanisms 385
Exposure Scenarios 385
Exposure Factors 386
Problems 387
References 388
9 Internal Dosimetry 389
John W. Poston, Sr., and John R. FordExternal versus Internal Exposure 389
Internal Dose Control 392
Regulatory Requirements 394
ICRP Publication 26 Techniques 394
Tissues at Risk 397
ICRP Publication 30 Techniques 399
Determination of the Tissue Weighting Factors 399
Secondary and Derived Limits 400Other Definitions 402
Calculation of the Committed Dose Equivalent 402
Dosimetric Models Used in the ICRP 30 Calculations 409
Model of the Respiratory System 409
Model of the Gastrointestinal Tract 417
Dosimetric Model for Bone 419
Submersion in a Radioactive Cloud 421
Recent Recommendations 422ICRP Publication 60 422
Dosimetric Quantities 423
Dose Limits 429
Age-Dependent Doses to the Public (ICRP Publications
56, 67, 69, 71, and 72) 430
ICRP Publication 56 432
ICRP Publication 67 434
ICRP Publication 69 437 ICRP Publication 71 437
ICRP Publication 72 442
ICRP Publication 89 443
ICRP Publications 88 and 95 444
Summary 444
Problems 445
References 445
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Contents xix
10 External Dosimetry 447David. C. Kocher
Dose Coefficients for External Exposure 448
Definition of External Dose Coefficient 449Compilation of External Dose Coefficients 449
Description of Dose Coefficients in Current Federal Guidance 450
Applicability of Dose Coefficients 452
Effective Dose Coefficients 453
Dose Coefficients for Other Age Groups 454
Corrections to Dose Coefficients for Photons 454
Shielding during Indoor Residence 455
Effects of Ground Roughness 455Exposure during Boating Activities 456
Exposure to Contaminated Shorelines 456
Point-Kernel Method 457
Description of the Point-Kernel Method 457
Point-Kernel Method for Photons 459
Applications of the Point-Kernel Method for Photons 459
Point-Kernel Method for Electrons 461
Problems 461
References 462
11 Estimating and Applying Uncertainty in Assessment Models 465Thomas B. Kirchner
Why Perform an Uncertainty Analysis? 468
Describing Uncertainty 469
Probability Distributions 471
Descriptive Statistics 471
Statistical Intervals 476Confidence Intervals 476
Tolerance Intervals 479
Typical Distributions 483
Correlations and Multivariate Distributions 485
Assigning Distributions 487
Deriving Distributions from Data 490
Estimating Parameters of a Distribution 491
Using Limited Data 491
Using Expert Elicitation 493
Methods of Propagation 497
Analytical Methods 497
Sum and Difference of Random Variables 498
Product of Random Variables 499
Quotient of Random Variables 500
Formulas for Normal and Lognormal Distributions 501
Linear Operations 501
Geometric Means and Standard Deviations 501
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xx Contents
Mathematical Approximation Techniques 502
Mean 502
Variance 502
Propagation Using Interval Estimates 504Sum and Difference 504
Products and Quotients 504
Other Functions 505
Covariance and the Order of Operations 505
Monte Carlo Methods 506
Generating Random Numbers 508
Potential Problems with Monte Carlo Methods 508
Sampling Designs 509
Simple Random Sampling 509
Latin Hypercube Sampling 509
Importance Sampling 510
Sampling Designs to Partition Variability and True
Uncertainty 511
Number of Simulations 511
Interpretation of the Output Distributions 513
Sensitivity Analysis 517Local Sensitivity Analysis 518
Global Sensitivity Analysis 520
Statistics for Ranking Parameters 521
Uncertainty and Model Validation 522
Summary 524
Problems 525
References 526
12 The Risks from Exposure to Ionizing Radiation 531
Roger H. Clarke
Radiobiological Effects after Low Doses of
Radiation 533
Biophysical Aspects of Radiation Action on Cells 533
Chromosomal DNA as the Principal Target for Radiation 535
Epigenetic Responses to Radiation 535
Effects at Low Doses of Radiation 537 Dose and Dose-Rate Effectiveness Factor 538
Genetic Susceptibility to Cancer 538
Heritable Diseases 539
Cancer Epidemiology 540
Japanese A-Bomb Survivors 541
Other Cohorts 542
In Utero Exposures 543
Uncertainties in Risk Estimates Based on Mortality Data 544
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Contents xxi
Risk Coefficients for Cancer and Hereditary Effects 545
Cancer Risk Coefficients 545
Hereditary Risk 546
Overall Conclusions on Biological Effects at Low Doses 547Problems 549
References 549
13 The Role of Epidemiology in Estimating Radiation Risk:
Basic Methods and Applications 551
Owen J. Devine and Paul L. Garbe
Measures of Disease Burden in Populations 552
Estimating Disease Risk 552Estimating Disease Rate 554
Estimating Disease Prevalence 555
Measures of Association between Disease Risk and
Suspected Causative Factors 557
Risk Ratio 557
Risk Odds Ratio 559
Exposure Odds Ratio 559
Study Designs Commonly Used in EpidemiologicInvestigations 563
Cohort Designs 563
CaseControl Designs 565
Nested Designs 566
Assessing the Observed Level of Association between
Disease and Exposure 566
Interpreting Estimates of Disease Exposure Association 567
Confidence Intervals 570Issues in Radiation Epidemiology 580
Conclusion 584
Problems 584
References 587
14 Model Validation 589
Helen A. Grogan
Validation Process 590Model Composition 591
Model Performance 593
Calibration 594
Tests of Model Performance 596
Testing for Bias 596
Measures of Scatter 598
Correlation and Regression 599
Visual Display of Information 599
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xxii Contents
Reasons for Poor Model Performance 603
User Error 604
The Model 605
The Assessment Question 607 Conclusions 608
Problems 608
References 609
15 Regulations for Radionuclides
in the Environment 613
David C. Kocher
Principal Laws for Regulating Exposures to Radionuclidesand Hazardous Chemicals in the Environment 614
Institutional Responsibilities for Radiation Protection of the Public 617
Responsibilities of U.S. Governmental Institutions 617
U.S. Environmental Protection Agency 617
U.S. Nuclear Regulatory Commission 617
U.S. Department of Energy 618
State Governments 618
Role of Advisory Organizations 619Standards for Controlling Routine Radiation Exposures
of the Public 619
Basic Approaches to Regulating Exposure to Radionuclides
in the Environment 619
Radiation Paradigm for Risk Management 620
Chemical Paradigm for Risk Management 622
Linear, Nonthreshold DoseResponse Hypothesis 622
Radiation Protection Standards for the Public 624
Guidance of the U.S. Environmental Protection Agency 624
Radiation Protection Standards of the U.S. Nuclear
Regulatory Commission 625
Radiation Protection Standards of the U.S. Department of Energy 626
State Radiation Protection Standards 627
Current Recommendations of the ICRP, NCRP, and IAEA 627
Summary of Radiation Protection Standards for the Public 628
Standards for Specific Practices or Sources 629
Operations of Uranium Fuel-Cycle Facilities 630Radioactivity in Drinking Water 631
Radioactivity in Liquid Discharges 635
Uranium and Thorium Mill Tailings 636
Other Residual Radioactive Material 639
Radioactive Waste Management and Disposal 648
Airborne Emissions of Radionuclides 660
Indoor Radon 662
Risks Associated with Radiation Standards for the Public 664
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Contents xxiii
Consistency of Radiation Standards for the Public 666
Importance of ALARA Objective to Consistent Regulation 669
Exemption Levels for Radionuclides in the Environment 671
Concepts of Exemption 671De Minimis Level 671
Exempt or Below Regulatory Concern Level 671
Recommendations of Advisory Organizations 672
Recommendations of the NCRP 672
Recommendations of the IAEA 672
Exemptions Established by the U.S. Nuclear Regulatory Commission 673
Exemptions in U.S. Nuclear Regulatory Commission Regulations 673
U.S. Nuclear Regulatory Commission Guidance on Disposal ofThorium or Uranium 674
Protective Action Guides for Accidents 674
Purpose and Scope of Protective Action Guides 675
Time Phases for Defining Protective Actions 675
Protective Action Guides Established by Federal Agencies 676
Recommendations of the U.S. Environmental Protection Agency 676
Recommendations of the U.S. Food and Drug Administration 676
Proposed Recommendations of the U.S. Department of
Homeland Security 677 U.S. Nuclear Regulatory Commissions Reactor Siting Criteria 679
ICRP Recommendations on Responses to Accidents 680
IAEA Guidelines for Intervention Levels in Emergency Exposure
Situations 681
Conclusions 682
References 683
Index 689
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Contributors
Steven M. Bartell, PhD
Principal Scientist and Manager
E2 Consulting Engineers, Inc.
339 Whitecrest Drive
Maryville, Tennessee 37801
Roger H. Clarke
Emeritus Member, InternationalCommission on Radiological
Protection
Corner Cottage, Woolton Hill
Newbury, RG209XJ
United Kingdom
Richard B. Codell, PhD
Consultant to the U.S. NuclearRegulatory Commission
4 Quietwood Lane
Sandy, Utah 84092
Todd V. Crawford, PhDa
Consultant
Owen J. Devine, PhD
National Center on Birth Defects
and Developmental Disabilities
MS E-87
Centers for Disease Control and
Prevention
1600 Clifton Road
Atlanta, Georgia 30333
James O. Duguid, PhD
JK Research Associates
29 Touchstone Lane
Amissville, Virginia 20106
Ying Feng, PhD
7047 Dean Farm RoadNew Albany, Ohio 43504
John R. Ford, PhD
Department of Nuclear Engineering
Texas A&M University
3133 TAMU
a Deceased.
xxv
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xxvi Contributors
College Station,
Texas 77843-3133
Paul L. Garbe, DVMNational Center for Environmental
Health MS E-17
Centers for Disease
Control and Prevention
1600 Clifton Road
Atlanta, Georgia 30333
Helen A. Grogan, PhDCascade Scientific, Inc.
1678 NW Albany Avenue
Bend, Oregon 97701
Thomas B. Kirchner, PhD
Carlsbad Environmental Monitoring
and Research Center
New Mexico State University
1400 University Drive
Carlsbad, New Mexico 88220
David C. Kocher, PhD
SENES Oak Ridge, Inc.
102 Donner Drive
Oak Ridge, Tennessee 37830
Charles W. Miller, PhDChief, Radiation Studies Branch
Division of Environmental
Hazards and Health Effects
National Center for
Environmental Health
Centers for Disease Control
and Prevention
2400 Century ParkwayAtlanta, Georgia 30345
Yasuo Onishi, PhD
Yasuo Onishi Consulting, LLC
Adjunct Full Professor,
Washington State University
144 Spengler Street
Richland, Washington 99354
John W. Poston, Sr., PhD
Department of Nuclear Engineering
Texas A&M University
3133 TAMU
College Station, Texas 77843-3133
James R. Rocco
Sage Risk Solutions, LLC
360 Heritage Road
Aurora, Ohio 44202
Arthur S. Rood, MS
K-Spar, Inc.
4835 W. Foxtrail LaneIdaho Falls, Idaho 83402
Elisabeth A. Stetar, CHP
Performance Technology Group, Inc.
1210 Seventh Avenue North
Nashville, Tennessee 37208-2606
John E. Till, PhD
Risk Assessment Corporation
417 Till Road
Neeses, South Carolina 29107
Paul G. Voillequ, MS
MJP Risk Assessment, Inc.
P.O. Box 200937
Denver, Colorado 80220-0937
Allen H. Weber, PhD
Consultant
820 Jackson Avenue
North Augusta, Georgia 29841
F. Ward Whicker, PhD
Department of Radiological
Health SciencesColorado State University
Fort Collins, Colorado 80523
Lesley Hay Wilson, PhD
Sage Risk Solutions, LLC
3267 Bee Caves Road, Suite 107
PMB 96
Austin, Texas 78746
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1
The Radiological AssessmentProcess
John E. Till
R adiological assessment is defined as the process of estimating dose and risk
to humans from radioactive materials in the environment. These radioactive
materials are generally released from a source that may be either man-made or
natural. The materials may be transported through the environment and appear as
concentrations in environmental media. These concentrations can be converted to
dose and risk by making assumptions about exposure to people.
The chapters in this book explain the basic steps of radiological assessment thatare typically followed. There is some logic to the order of the chapters and to the
sequence of steps generally undertaken in radiological assessment. Some of this
logic comes from my own experience over the years, and some of it is defined by
the calculation process in radiological assessment because certain information must
be known before proceeding to the next step. This logic is explained in the sections
that follow.
Over the years, some scientists have suggested that the term radiological assess-
ment does not quantitatively express the intense level of computational science that
is necessary to estimate dose or risk. As a result, scientists have instead used the
terms environmental risk assessment or environmental risk analysis to describe
the process. Regardless of what it is called, radiological assessment has matured
significantly over the past three decades. It has become the foundation of many reg-
ulations and legal cases and has provided a means for decision makers to take action
on important issues such as cleanup of contaminated sites and control of emissions
to the environment from nuclear facilities. Additionally, radiological assessment has
increasingly become a fundamental element in communicating information to stake-
holders about exposure to radioactive materials in the environment. Stakeholders
1
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2 Radiological Risk Assessment and Environmental Analysis
are people who have an interest in the assessment process and the policies or recom-
mendations that result from it. This term is defined in greater detail later in this
chapter.
Radiological assessment requires the merging of a number of scientific disciplinesto provide quantitative estimates of risk to humans. The book focuses on humans
as an end point because decision makers typically use that end point to allocate
resources and resolve issues. More specifically, the targeted person is a member
of the public. The public is usually the objective in the assessment because when
radioactive materials are released to the environment, it is members of the public
who are or will be exposed. Although the primary target of exposure in the book is
a member of the public, many of the technical methods described here also address
occupational exposures.Although the book focuses on how we estimate dose and risk to a member of
the public, it is becoming more evident that impacts to the environment must also
be taken into account. Whicker et al. (2004) stress that care must be taken to avoid
destroying ecological systems in the interest of reducing inconsequential human
health risks. This is an essential point for everyone to understand. Although we do
not address ecological impacts in this book, many of the same principles described
could be used to consider these impacts. In the end, both impacts on humans and
impacts on the environment must be taken into account before good decisions can
be made.A number of examples are used in this chapter to illustrate key points being made
about specific areas of radiological assessment. These examples are taken from
work I and my research team, Risk Assessment Corporation, have performed over
the years.Although specific reports are cited, it must be emphasized that radiological
assessment can rarely be performed by a single individual. It generally requires the
skills of people across several scientific disciplines.
Radiological Assessment Process
Contemporary radiological assessment began with the testing of nuclear weapons as
scientists tried to predict the path of radioactive fallout and the dose to people who
lived downwind. Early research in this area, more than any other, laid the foundation
for the methods we still use today to estimate risk to people from radioactive mate-
rials in the environment. More recently, research to reconstruct historical releases
of radionuclides to the environment from atmospheric nuclear weapons testing and
from nuclear weapons facilities resulted in significant improvements in methods to
estimate risk (Till 1990; Till et al. 2000, 2002). This research included many new
areas of investigation, such as estimation of source terms, transport of radioactive
materials in the environment, uptake of radionuclides by humans and biota, and the
development of dose and risk coefficients.
Radiological assessment is not confined to a specific time frame; it can address the
past, present, or future. Dose and risk can be estimated for possible future releases
of materials (prospective), for present-day releases, or for releases that occurred in
the past (retrospective). Risk can be estimated for present-day or potential future
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The Radiological Assessment Process 3
releases of materials at existing or planned facilities. Such assessments are typically
designed to demonstrate compliance with standards. Risk can also be estimated for
releases that occurred in the past to help understand the impact of those releases.
The dose reconstruction studies conducted on the weapons complex facilities in theUnited States provide excellent examples of retrospective risk assessments, as do the
studies of populations exposed following the Chernobyl reactor accident. Although
the risk assessments may be undertaken somewhat differently in their methods, there
are many similarities in the techniques applied to each.
The components that comprise radiological assessment today evolved from indi-
vidual sciences that have been merged gradually (and lately, more frequently) to
form the computational methods we now use to estimate dose and risk to humans.
In explaining the process of radiological assessment to colleagues and to the public,I often use the following illustrative equation to express the interdisciplinary nature
of this research:
Risk= (S TE D R)uvcp (1.1)where
S= source termT= environmental transport
E= exposure factorsD= conversion to doseR = conversion of dose to risku= uncertainty analysisv= validationc= communication of results
p= participation of stakeholdersIn the sections that follow, each of these components of radiological assessment
is discussed, with emphasis on several key concepts that are important to keep inmind.
Source Term
The source term is the characterization and quantification of the material released
to the environment. It is the heart of a risk assessment. We frequently give too
little attention to the derivation of the source term, and yet this step is where the
greatest potential lies for losing scientific and stakeholder credibility. This is also
the component of radiological assessment that typically requires the most resources
relative to the other steps. Therefore, it is important that development of the source
term be given highest priority and that the source term be carefully estimated before
moving to the next step of radiological assessment.
Chapter 2 of this book, contributed by Paul Voillequ, addresses source terms.
The chapter covers an expansive scope of which nuclear materials are typically
released to the environment and how to quantify them. Issues such as chemical
form, particle size, temporal trends, and estimating releases when measurement
data are not available are discussed.
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4 Radiological Risk Assessment and Environmental Analysis
Two key points need to be mentioned about how source terms should be derived in
radiological assessment. The first point is that uncertainties should be included with
the estimates of releases if a realistic estimate of the source is to be used. Methods
for estimating uncertainties in the source term and other aspects of radiologicalassessment are discussed in chapter 11. This aspect has been overlooked in the past,
with release estimates being reported as point values when in reality we know there
is a range of possible values that exist even when good monitoring data are available
on which to base the source term.
An alternative approach to addressing uncertainties in the source term that may be
useful for screening or providing preliminary estimates to determine the significance
of a particular source is the use of an upper bound, deterministic value. The upper
bound approach is designed to provide doses and risks that are significantly greaterthan what is expected to occur. This approach may be useful for screening or making
preliminary comparisons of the impact of different sources.
The second key point is that the source term should be derived using as many dif-
ferent independent approaches as possible to increase the confidence that a credible
estimate has been made. This is especially important in historical dose reconstruc-
tion, where sources that may have occurred many years ago are being estimated.
This point is illustrated in the work of Meyer et al. (1996) that reconstructed source
terms for the Fernald Feed Materials Production Center (FMPC), near Cincinnati,
Ohio, which was formerly a part of the U.S. nuclear weapons complex that pro-cessed uranium ore. The facility has now been decommissioned and cleaned up.
This study estimated the release of uranium from the FMPC using two methods.
The first method, which could be called the inside-out approach, considered the
amounts of material being processed at the site and estimated the fractional release
of uranium to the atmosphere through various effluent treatment systems (primar-
ily scrubbers and dust collectors). Using this approach, it was determined that the
median quantity of uranium released to the atmosphere was 310,000 kg, with the
5th and 95th percentiles ranging between 270,000 kg and 360,000 kg, respectively.These results are shown in table 1.1.
An alternative calculation, called the outside-in approach, was performed as
a check to verify the calculation, looking at the amount of uranium deposited on
soil within 7.5 km of the site based on soil samples that had been collected over
time. Taking into account environmental removal of some of the uranium and the
amount of uranium that would have been deposited from the atmosphere, it was
estimated that the source term for uranium released from the site to the atmosphere
Table 1.1 Uranium and radon source terms for the Fernald FeedMaterials Production Center for 19511988a (Voillequ et al. 1995)
Source: uranium Median release 5th percentile 95th percentile
to atmosphere estimate
Primary estimate 310,000 270,000 360,000
Alternative calculation 212,000 78,000 390,000
aValues are in kilograms of uranium.
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The Radiological Assessment Process 5
would lie between 78,000 kg and 390,000 kg, with a median value of 212,000 kg.
This alternative calculation, although the uncertainties are large, provides additional
confidence that the source term estimate for uranium is reasonable.
Without a defensible estimate of the source term, it is not possible to provide adefensible estimate of dose or risk, and the credibility of the assessment is lost. There-
fore, it is critical to carefully and thoroughly address this first step in radiological
assessment.
Environmental Transport
Once the source term has been estimated, the next step is to determine where in the
environment the radioactive materials go and what are the resulting concentrationsin environmental media. This step is called environmental transport.
One of the first tasks in evaluating environmental transport is to identify the
relevant exposure pathways. Figure 1.1 illustrates possible pathways typically con-
sidered in radiological assessment. However, not all pathways shown in the diagram
typically apply to every site or radionuclide. Special pathways of concern may also
exist that are not shown here. Determining important pathways and eliminating
those that are not important is a critical step that can help focus resources. This pro-
cess can be accomplished using screening models or other techniques that are easy
Airborne Effluents
AirS
ubm
ersio
n
Inh
alationand
T
ranspiration
D
epositio
n
toG
round
Dep
ositio
n
toCrop
s
Irrigation Crop
Ingestion
Ingestion
Meat
MilkIngestion
WaterImmersion
andWaterSurface
ShorelineExposure
WaterIngestion
AquaticFoo
d
Ingestion
Uptake by
Aquatic Plants
Resuspensionof Deposited
Materials
Liquid Effluents
to Surface Water
and Groundwater
Figure 1.1 Diagram illustrating pathways typically considered in radiological assessment.
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6 Radiological Risk Assessment and Environmental Analysis
to use and help set priorities for the focus of the assessment (NCRP 1996; Mohler
et al. 2004).
Environmental transport necessarily involves individuals from a number of sci-
entific disciplines because different pathways of exposure have to be considered.Consequently, environmental transport of materials in the environment makes up a
large part of this book. Chapter 3, contributed by Todd Crawford, Charles Miller,
and Allen Weber, focuses on the transport of radioactive materials through the
atmosphere. Chapter 4, contributed by Yasuo Onishi, discusses the transport of
radioactive materials in surface water. Chapter 5, contributed by Richard Codell and
James Duguid, looks at transport in groundwater. Chapter 6, contributed by Ward
Whicker and Arthur Rood, provides methods for evaluating transport of radioac-
tive materials in terrestrial food chain pathways. Chapter 7, contributed by StevenBartell and Ying Feng, considers the transport of materials in aquatic food chain
pathways. These chapters present a comprehensive look at the state-of-the-art sci-
ence today in estimating environmental transport techniques used in radiological
assessment.
Transport of radioactive materials in the environment can be determined in sev-
eral ways. If there are measurement data in the environment that are sufficiently
thorough, these measurements may be used directly to determine concentrations
in media. The more data that are available to characterize the environment around
a site, the more defensible will be the estimates of dose and risk. In fact, mea-surements of environmental concentrations are always preferable to modeling. It
is rare, however, that measured data can be used in place of models. This is espe-
cially true when radiological assessment is being undertaken for a new facility
where releases of materials will occur at some point in the future. In most cases,
environmental transport is determined using a combination of both modeling and
measurement data.
To illustrate environmental transport, I use the work performed by our research
team during the historical dose reconstruction for the Rocky Flats EnvironmentalTechnology Site near Denver, Colorado. This site has been decommissioned and is
now a wildlife refuge. The goal of the project was to reconstruct risks to members
of the public from releases of plutonium and other materials at the site. Most of
the plutonium was released to the atmosphere, and the most significant release was
during a fire that occurred in September 1957. Understanding the risks associated
with this source of plutonium and where it went in the environment was crucial to
the success of the study (Rood et al. 2002; Till et al. 2002).
Environmental Transport of Plutonium in Air Duringthe 1957 Fire at Rocky Flats
In order to determine environmental transport of plutonium during and after the
1957 fire at Rocky Flats, several critical pieces of information had to be obtained.
First, a source term was needed that estimated the amount of plutonium released,
the distribution of the release over time, the heat generated by the fire (to account
for the rise of the plume), and the size of the particles released. This impor-
tant part of the puzzle controlled the concentrations of plutonium in the plume
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The Radiological Assessment Process 7
as it moved downwind from the site. Since inhalation was determined to be the
only major pathway of exposure, air concentration coupled with where people
were located during the fire and their breathing rate determined the resulting
risk. The source term was reconstructed (Voillequ 1999) by reviewing histori-cal records detailing the fire and interviewing fire experts. Quantities of plutonium
released to the atmosphere were estimated for 15-min intervals during the time
of the fire, along with the physical and chemical form of plutonium that was
dispersed.
The next critical piece of information we needed was data describing the meteoro-
logical conditions during the fire. Information such as wind direction, wind velocity,
and atmospheric conditions was required if the transport of the plutonium through air
was to be understood. Fortunately, these data were collected and could be found inhistorical records. This information was used as input to RATCHET, an atmospheric
dispersion model (Ramsdell 1994) that could take advantage of the resolution of
meteorological data and the temporal distribution of the source.
Once these steps were taken, time-integrated concentration valueswere combined
with scenario exposure information and risk coefficients to yield the incremental
lifetime cancer incidence risk to hypothetical individuals in the model domain. Pluto-
nium released during the 1957 fire was modeled as puffs that entered the atmosphere
every 15 min from 10:00 p.m. September 11 until 2:00 a.m. September 12, 1957
(Rood and Grogan 1999). The transport calculations were continued until 7:00 a.m.
September 12, 1957, to allow all the released plutonium to disperse throughout the
model domain. The computer code simulations performed using RATCHET cov-
ered a 9-h period. Because the effluent release temperature was estimated to be
near 400C, there was significant plume rise, and maximum plutonium concen-trations in ground-level air were estimated some distance southeast of the Rocky
Flats Plant, not adjacent to it. The concentration in air at ground level, typically at
a height of 1 m, represents the air concentration to which people would have been
exposed.At the time the fire started, the plume was transported in a westerly direction for
a few kilometers. Around 10:45 p.m., the wind direction at the Rocky Flats plant
shifted so that it blew out of the northwest and continued to blow from that direc-
tion until about 4:00 a.m., September 12. Those winds transported the bulk of the
airborne plutonium to the suburb of Arvada and toward the Denver metropolitan
area. Near southern Arvada, the air mass converged with air flowing from the south-
west in the Platte River Valley, which resulted in a northeasterly plume trajectory.
Figure 1.2 shows the median (50th percentile) estimated time-integrated plutonium
concentrations in air near ground level.
This example of environmental transport of plutonium during the 1957 fire at
Rocky Flats illustrates a very important point. First, without meteorological data
collected at the time of the event, it would have been difficult to understand where
the plume carried the plutonium and who may have been exposed. Obtaining these
data that characterized atmospheric conditions at the precise time and location of
the accident was essential to the assessment. In radiological assessment, a signifi-
cant amount of time will be spent obtaining site-specific data that characterize the
situation being investigated.
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8 Radiological Risk Assessment and Environmental Analysis
Figure 1.2 Estimated nine-hour average plutonium concentration in air one meter aboveground at the 50th percentile level during the 1957 fire (Till et al. 2002).
Exposure Factors
The dose or risk to a person depends upon a number of characteristics, called
exposure factors, such as time, location, transport of radionuclides through the
environment, and the traits of the individual. These traits include physiologi-
cal parameters (e.g., breathing rate), dietary information (e.g., consumption rate
of various foods), residence data (e.g., type of dwelling), use of local resources
(e.g., agricultural resources), recreational activities (e.g., swimming), and any other
individual-specific information that is necessary to estimate dose or risk. In radio-
logical assessment, a specific set of these characteristics is referred to as an exposure
scenario.
The target of radiological assessment may be real individuals or representative
individuals. Real individuals are those who are or were actually exposed. Their
characteristics should be defined as closely as possible to those that actually exist.
Representative, or hypothetical, individuals are not characterized by specific persons
but have characteristics similar to people in the area who are or were exposed in the
past or who may be exposed in the future.
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The Radiological Assessment Process 9
Exposure scenarios are described in a site conceptual exposure model (SCEM)
that contains information about exposure factors specific for a given source and
location. Chapter 8, contributed by James Rocco, Elisabeth Stetar, and Lesley Hay
Wilson, addresses exposure factors and the SCEM.There is no prescribed approach for defining and presenting scenarios of exposure
in radiological assessment. This decision must fit the particular assessment being
undertaken, the type of individual (real or representative) being evaluated, and the
goals of the assessment. Two examples follow that come from studies we performed
at Rocky Flats and at the Hanford Site, a nuclear weapons production facility in
Washington State.
Rocky Flats Representative Exposure ScenariosA key component of the Rocky Flats dose reconstruction work was estimating the
health impacts to representative individuals in the model domain. In this case, the
cancer risk to people depended upon a number of factors, such as where the person
lived and worked, when and how long that person lived near the site, the age and
gender of the person, and lifestyle. It was not possible to create an exposure sce-
nario that fit every person in the exposed population. To consider the many factors
that influence exposure, exposure scenarios were developed for residents for whom
representative risk estimates could be made, incorporating typical lifestyles, ages,genders, and times in the area. The scenarios provided a range of potential profiles
and included a laborer, an office worker, a homemaker, an infant-child, and a stu-
dent. The infant-child scenario represented a single individual who matured during
the exposure period. Table 1.2 lists key features of the exposure scenarios used in
the analysis.
The five exposure scenarios were organized according to occupational and
nonoccupational activities. Occupational activities included work, school, and
extracurricular activities away from the home. Nonoccupational activities included
time spent at home doing chores, sleeping, and leisure activities (e.g., watching
television). In these calculations, the receptor was assumed to perform occupational
and nonoccupational activities at the same location. The age of the individual during
which exposure occurred was also considered when calculating risk.
Risks were reported for these scenarios at various locations in the domain as
illustrated in figure 1.3, which shows risks estimated for the laborer scenario.
Table 1.2 Exposure scenario descriptions for Rocky Flats
Exposure scenario Gender Year of Year beginning Year ending Days per year
birth exposure exposure exposed
Laborer Male 1934 1953 1989 365
Homemaker Female 1934 1953 1989 350
Office worker Female 1940 1965 1989 350
Infant-child Female 1953 1953 1960 350
Student Male 1957 1964 1974 350
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10 Radiological Risk Assessment and Environmental Analysis
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
1953-59
1960-69
1970-79
1980-89
106
105
104
103
102
101
100
101
102
103
IncrementalLifetimeCancerIncidenceRisk1
06 L
eydon
RFPEastE
ntrance
IndianaStreet&64th
CoalCreek
I-70andSh
eridanBlvd
StandleyLa
keEast
Broomfield
Superior
Denver
Boulder
Decade of Exposure
Figure 1.3 Lifetime cancer incidence risk from plutonium inhalation for the laborer scenarioat selected locations in the model domain. Dots represent the 50th percentile value; horizontal
bars represent the 5th and 95th percentile range. Cancer risks have been sorted by decade of
exposure.
Hanford Site Scenarios for Native Americans
In almost every risk assessment, there are special population groups who do not
fit the usage factors for the general public. One example of this occurred in the
dose reconstruction project for the Hanford Site. The Hanford facility released large
amounts of radionuclides, 131I in particular, to the atmosphere. Significant quantities
of materials were also released directly into the Columbia River, which was used
for cooling the production reactors at the site (Farris et al. 1994a).
Pathways of exposure from the river were investigated thoroughly. Members of
the general public who lived near the river received relatively small doses (estimated
to be 15 mSv over about 40 years) from consumption of river water, consumption
of fish from the river (150 kg of fish per year), and activities in and around the river.
Special attention, however, had to be given to Native Americans who relied on the
river for fish, a major component of their food (Grogan et al. 2002).
There was concern by NativeAmericans that because their unique lifestyles relied
more heavily on natural sources of local foods and materials and because they hadunique pathways of exposure, their risk may have been significantly greater than that
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The Radiological Assessment Process 11
of non-Native American people. Working with nine tribes in the region, scientists
collected and summarized data, which allowed specific exposure factor information
on diet, lifestyle, and special cultural ceremonies to be included to assess risk.
There were several pathways for which few data were available to estimate expo-sure and for which Native Americans were concerned about risk. Examples of these
included exposure from shoreline sediment used for paints and medicinal purposes,
sweat lodges using Columbia River water, and inhalation of river water spray during
fishing. The pathway of most concern was that of fish consumption, not only because
of the large quantities of fish consumed but also because they consumed the whole
fish, which could significantly increase the dose and risk since some radionuclides
concentrate in the bones of fish. Table 1.3 shows fish consumption data gathered
with the involvement of Native Americans in the area and used in our risk estimates.The results of the study indicated that except for the consumption of fish from the
river, the risks from all other pathways would be small. In the case of consumption
of fish, risks to Native Americans could have been substantially greater than those of
non-Native American people. Since this study was a screening analysis, it is evident
that the only pathway that deserved more detailed analysis, if quantitative estimates
of risks were warranted, was consumption of fish from the Columbia River.
This discussion related to exposure factors illustrates several important points.
The individual who is the target of exposure must be clearly defined in the beginning
of the assessment. This step will help determine the scenarios of exposure and helpidentify specific exposure factors needed for the assessment. It must also be decided
how the scenarios of exposure will be presented in the end so that people can
understand what dose or risk they may have received.
The design of exposure scenarios and the data used to describe them are important
to the credibility of the study. In some cases, generic information will be sufficient to
characterize individuals for whom dose or risk is being calculated. It may be neces-
sary, however, to undertake surveys or other methods for collecting exposure factor
data when generic information is not available for specific groups of individualswith uncommon habits.
Table 1.3 Fish consumption of Native Americans for the Columbia River nearthe Hanford Site as reported by Walker and Pritchard (1999)
Fish categorya Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Total Holdupb
(days)
Omnivore 4 4 4 2 2 2 2 2 2 2 4 4 34 3
First-order 0 0
predator
Second-order 4 4 4 2 2 2 2 2 2 2 4 4 34 3
predator
Salmon 3 3 3 22 22 22 22 22 22 22 3 3 169 14
a Omnivorous fish include bullhead, catfish, suckers, whitefish, chiselmouth, chub, sturgeon, minnows, and shiners.
First-order predators include perch, crappie, punkinseed, and bluegill. Second-order predators include bass, trout,
and squawfish.b The time between obtaining fish from the river and consuming it.
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12 Radiological Risk Assessment and Environmental Analysis
Conversion to Dose
The conversion of radioactive materials taken into the body or the conversion of
external radiation to dose has become a routine process because of the large effortput into deriving and publishing dose coefficients over the past several decades.
Two chapters in the book address conversion to dose. Chapter 9, contributed by
John Poston and John Ford, describes concepts of internal dosimetry. Chapter 10,
contributed by David Kocher, focuses on external dosimetry.
There are two brief issues about conversion to dose I wish to make in this introduc-
tory chapter. The first is the importance of uncertainties related to dose coefficients
and when, or when not, to take this uncertainty into account. The second issue is
relatively new (ICRP 2007) and is related to the appropriate use of dose coefficientsfor compliance as a function of age.
Uncertainty in Dose Coefficients
Until recently, little was understood about uncertainties associated with dose coef-
ficients, and these values were typically used as single point values even when the
radiological assessment was performed probabilistically. The use of single values
probably came about because dose coefficients were first introduced as a means fordetermining compliance with a regulatory standard rather than for determining dose
to individuals in a population. However, as more emphasis was placed on studies
of populations where dose to specific individuals for use in epidemiology was the
objective, it became clear that more information was needed on the uncertainty of
these coefficients to properly address uncertainties in the calculation. As a result,
considerable attention has been given to this important area of dosimetry over the
past 10 years.
In the Hanford Environmental Dose Reconstruction Project (Farris et al. 1994b),it was determined that one of the two most important contributors to overall uncer-
tainty was the dose coefficient for 131I; the other key component of uncertainty
was the feed-to-milk transfer coefficient. In this analysis, it was pointed out that
the uncertainty in the iodine dose coefficient was due primarily to variability in the
mass of the thyroid, uptake of iodine in the gastrointestinal tract, transfer of iodine
to the thyroid, and the biological half-time of iodine.
It is generally assumed that uncertainties associated with external dose coeffi-
cients are much less than those for internal dose coefficients and that there is little
variability in dose per unit of exposure with age (Golikov et al. 1999, 2000). One
reason for this low variability is because external radiation fields can be measured,
and if measurements are carried out properly, the variability is small for a given
location. Determining uncertainty of internal dose coefficients is a much more com-
plicated process because radionuclides disperse after being taken into the body, and
it is not possible to quantify precisely where they go and to measure the resulting
dose. Therefore, it becomes an intensive computational process involving many
assumptions. Nevertheless, much progress is being made in this area of dosimetry,
and it will continue to be a viable area for research in the future.
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The Radiological Assessment Process 13
Appropriate Use of Dose Coefficients as a Function of Age
The International Commission on Radiological Protection (ICRP) has issued age-
specific dose coefficients (dose per unit intake, Sv Bq1) for members of the publicin six age ranges covering the time period from the newborn infant to 70 years of
age (ICRP 1995, 1996a, 1996b). Additional refinements of these coefficients are
also available for the embryo/fetus (ICRP 2001, 2005). The ICRP (2007) points out
that application of dose coefficients for the six age groups should be weighed in
relation to the ability to predict concentrations in the environment from a source and
the ability to account for uncertainties in habit data for individuals exposed. This
is an important statement to consider in radiological assessment, especially when
the assessment is being made for prospective calculations. It implies that a carefulbalance is needed between the resolution of dose coefficients being applied and the
overall uncertainty in assessment.
Most likely, scientists will continue to refine dose coefficients into more discrete
categories of age; however, this increased resolution will not likely give a better
estimate of dose. As a result, ICRP (2007) recommends that some consolidation of
dose coefficients is justified when the coefficients are being used for the purpose
of determining compliance. There are a number of reasons the ICRP changed its
policy, including the idea that compliance is generally determined by a dose standard
that is typically set at a level to protect individuals from exposure to a continuing
source over the lifetime of an individual. Table 1.4 lists the three age groups now
recommended by the ICRP for compliance calculations.
The ICRP does recommend the use of specific age-group categories for ret-
rospective calculations of dose and in addressing accidents. The reason for this
recommendation is that specific information about age, diet, lifestyle, and other
habit data is generally known.
Internal dosimetry and external dosimetry continue to be important areas of
research. Too frequently, we assume that work in this area of radiological assessmentis essentially complete; this assumption is not correct.
Conversion of Dose to Risk
If the objective of radiological assessment is to estimate risk, then converting dose to
risk is the next step. This step is generally accomplished by applying risk coefficients
to doses that have been calculated for individuals. Increasingly, the intermediate
Table 1.4 Dose coefficients recommended by ICRP (2007)for compliance calculations
Age category (years) Name of age category Dose coefficient and
habit data to be used
05 Infant 1-year-old
615 Child 10-year-old
1670 Adult Adult
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14 Radiological Risk Assessment and Environmental Analysis
step of calculating dose is subsumed into the calculation that converts exposure to
risk. For example, Federal Guidance Report 13 (Eckerman et al. 1999) presents
risk coefficients in terms of risk per unit intake via inhalation or ingestion. Risk
coefficients and their foundation are covered in chapters 12 and 13. Chapter 12,contributed by Roger Clarke, addresses exposure standards, risk coefficients, and
how these coefficients were developed over the years. Chapter 13, contributed by
Owen Devine and Paul Garbe, explains how epidemiological studies to investigate
the effects of exposure on populations can be designed to help determine if there
are effects in populations following radiation exposure and if those effects can be
attributed to the exposure.
Until the past decade, the end point of radiological assessment was typically dose,
and conversion to risk was not routinely undertaken. Converting dose to risk, how-ever, is becoming more important and useful for several reasons that are discussed
below.
Why Risk?
In the context of this chapter and this book, risk refers to risk of adverse health
effects, primarily cancer, to humans from exposure to radioactive materials in the
environment. Unfortunately, in radiological assessment, people are exposed not only
to radioactive materials but also to chemicals. By using risk as an end point for thecalculation in radiological assessment, one can compare the effects of radioactive
materials with chemicals that may be present. Risk is the most fundamental common
denominator in an assessment that can be estimated to help people understand cur-
rent and prospective effects on humans and the environment from both radioactive
materials and chemicals. If people have a better understanding of the risk imposed
from exposure to these materials, it gives them a starting point for making decisions
about potential cleanup or remediation.
There are other reasons to estimate risk in radiological assessment. The termrisk is becoming more common in our language today. Medications are often
described as having a risk of side effects. We discuss the risk posed by potential bad
weather. Farmers refer to the risk of investing in an expensive crop. Of course, the
type of risk referred to in this book could be described as a chance of harm from
being exposed to radioactive materials in the environment. More specifically, risk
is quantified in radiological assessment as a risk of the incidence of, or dying from,
cancer following exposure.
Risk Coefficients
As with conversion of intake or external exposure to dose, conversion of dose to risk
is a straightforward process involving risk factors published by a number of different
groups (UNSCEAR 2000). The current risk estimates of cancer following exposure
to ionizing radiation are based primarily upon analyses of Japanese survivors of
the atomic bombings at Hiroshima and Nagasaki. These risk estimates essentially
relate to uniform whole-body exposures to predominantly low linear energy transfer
radiation doses ranging from 0.01 Gy to 4 Gy delivered at high dose rate.
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The Radiological Assessment Process 15
Risk coefficients and dose coefficients are similar with regard to our lack of
understanding about the uncertainties associated with them. Typically, risk coeffi-
cients are applied with a single, deterministic value. We know that such a value is
not valid and that the range of uncertainty associated with risk coefficients oftenmay be quite large. Little work has been done to try to quantify this uncertainty,
although scientists are working to quantify uncertainties in risk coefficients and to
apply these uncertainties in their results.
Uncertainty in the risk factors for radiation was described by Sinclair (1993)
and investigated more thoroughly by Grogan et al. (2001) as having five primary
components:
Epidemiological uncertainties Dosimetric uncertainties
Projection to lifetime
Transfer between populations
Extrapolation to low dose and dose rate
Epidemiological uncertainties include statistical uncertainties associated with quan-
tifying the relatively small number of excess cancers attributable to ionizing
radiation from the background cancers resulting from all causes. Also included
in epidemiological uncertainties are uncertainties from underreporting of cancers
per unit population and nonrepresentativeness of populations used to determine
risk. Dosimetric uncertainties include those from random errors in individual dose
estimates arising from errors in the input parameters used to compute doses, and
systematic errors due to the presence of more thermal neutrons at Hiroshima than
originally estimated. Risk projection includes uncertainties associated with extrap-
olating beyond the time period covered by the observed population. Transfer of
estimates of risk from one population (Japanese) to another introduces an additional
source of uncertainty that must be considered. Finally, since the exposures for theA-bomb population were at relatively high dose rate, uncertainty is introduced when
we extrapolate estimates of risk to low-dose, low-dose-rate situations common in
most risk assessments. This area of risk assessment research is very important for
the future, and the ideas introduced by Sinclair (1993) must be pursued. Indeed, we
may find that the risk factors themselves introduce more uncertainty into the overall
estimate of risk than does any other single component.
Uncertainty Analysis
Uncertainty has been mentioned frequently up to this point, but it has not been
explained or tied to the other components of radiological assessment. Uncertainty is
covered in chapter 11, contributed by Thomas Kirchner. Uncertainty analysis is an
essential element of risk assessment. Of all the steps in radiological assessment, this
is the area where the greatest progress has been made over the past three decades.
This success has been partly due to advances in techniques that are used to propagate
uncertainties in calculations, but it is mainly due to the rapid improvements in
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16 Radiological Risk Assessment and Environmental Analysis
computer technology. Today, uncertainties can be readily estimated with off-the-
shelf software and laptop computers. This success was not imaginable even a decade
ago.
Methods for quantification of uncertainty have been well established. Today, itis expected that when one carries out a risk assessment, the best estimate of risk is
reported along with associated uncertainties.
The most common method for uncertainty analysis uses Monte Carlo statistical
techniques incorporating a random sampling of distributions of the various models
and parameters involved (see figure 1.4). In this simplified illustration, Ais an input
Parametic Uncertainty Analysisof Mathematical Models
Deterministic Application
A(Parameter)
Y(Result)
Stochastic (Monte Carlo) Application
Distribution of A Distribution of Y
Sample randomly from A...
Apply themodel to
eachrandomvalue...
A1
Y1
Y2
Y3
Y4
YN
A2
A3
A4
AN
Assemblethe results...
Model
ConstructY
Model
Model
Model
Model
Model
Figure 1.4 Schematic presentation of Monte Carlo methods for propagating a parametric
uncertainty distribution through a model to its results.
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The Radiological Assessment Process 17
parameter to the model, andYis the result, or output, corresponding toA. For each
specific value ofA, the model produces a unique outputY. Such an application of
the model is deterministic because A determinesY. ButA may not be known with
certainty. If uncertainty aboutA is represented by a distribution, such as the triangularone in the figure, repeatedly sampling the distribution at random and applying the
model to each of the sample input valuesA1,A2 . . . gives a set of outputsY1,Y2, . . .,
which can be arranged into a distribution for Y. The distribution ofY is then the
estimate of the uncertainty inYthat is attributable to the uncertainty inA. This is a
stochastic, or probabilistic, application of the model.
Proposed distributions may be based on measurements or on scientific judgment
when data are not available. Site-specific data are used when such measurements
exist for relevant times, locations, and processes, but often surrogate data basedon other times or locations must be used. The most difficult aspect of uncertainty
analysis is the selection of parameters and distributions to be used in the analysis.
Use of Uncertainty for Determining Compliance with Standards
Little attention has been given to how uncertainties might be considered when radi-
ological assessment was being used to determine compliance with environmental
regulations. Until recently, deterministic calculations were used as the comparison
value without regard to the uncertainties associated with them. ICRP (2007) clarifies
this matter for exposures to the public in prospective situations.
The difficulty in applying uncertainties in determining compliance with a stan-
dard arises from the fact that, in almost all cases, some members of the population
exposed will exceed the dose benchmark (e.g., 50th percentile, 95th percentile) that
is used as the basis for comparison. The number of people who exceed the criterion
for comparison and the level of dose they may receive are important to consider. As
a result, ICRP (2007) recommends the following:
In a prospective probabilistic assessment of dose to individuals, whether from a planned
facility or an existing situation, the ICRP recommendsthat the representative individual
be defined such that the probability is less than about 5% that a person drawn at random
from the population will receive a greater dose. In a large population, many individuals
will have doses greater than that of the representative individual, because of the nature
of distributions in probabilistic assessments. This need not be an issue if the doses are
less than the relevant dose constraint. However, if such an assessment indicates that
a few tens of persons or more could receive doses above the relevant constraint, then
the characteristics of these people need to be explored. If, following further analysis,it is shown that doses to a few tens of persons are indeed likely to exceed the relevant
dose constraint, actions to modify the exposure should be considered.
This recommendation by the ICRP illustrates some of the problems that will be
encountered as uncertainties are accounted for in future radiological assessments.
Other difficult issues will be encountered, as well. These include the acceptance and
understanding of uncertainty by the public and the misuse of uncertainty to argue
the presence of an upper bound (e.g., 99th percentile) dose or risk to an individual as
being the basis for a legal decision. Regardless of the difficulties introduced when
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18 Radiological Risk Assessment and Environmental Analysis
uncertainties are accounted for in radiological assessment, the benefits far outweigh
the problems, and the result is a more realistic understanding of dose and risk.
Validation
The term validation is used here to mean efforts taken to verify the estimates
made in radiological assessment. Since direct measurements of dose to people
exposed cannot be readily taken, validation typically involves comparing pre-
dicted concentrations in the environment with measurement data. Validation in
radiological assessment is discussed in chapter 14, contributed by my co-editor,
Helen Gro