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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.

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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

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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

vii


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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

ix


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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 69 437 ICRP Publication 71 437



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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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

radiological risk assessment

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