NCRP report no 152 performance assessment of near surface facilities for disposal of low level radioactive waste

Performance Assessment of Near-Surface Facilities for Disposal of Low-Level Radioactive Waste Recommendations of the NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS December

National Council on Radiation Protection and Measurements

N C R P

Performance Assessment

of Near-Surface Facilities for Disposal of Low-Level Radioactive Waste

Recommendations of the

NATIONAL COUNCIL ON RADIATION

PROTECTION AND MEASUREMENTS

December 31, 2005

National Council on Radiation Protection and Measurements

7910 Woodmont Avenue, Suite 400 / Bethesda, MD 20814-3095

Measurements (NCRP) The Council strives to provide accurate, complete and ful information in its documents However, neither NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this Report, nor any person acting on the behalf of any of these parties: (a) makes any warranty or rep- resentation, express or implied, with respect to the accuracy, completeness or use- fulness of the information contained in this Report, or that the use of any information, method or process disclosed in this Report may not infringe on pri- vately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or process disclosed in

use-this Report, under the Civil Rights Act of 1964, Section 701 et seq as amended 42 U.S.C Section 2000e et seq (Title VII) or any other statutory or common law theory governing liability.

Disclaimer

Any mention of commercial products within NCRP publications is for tion only; it does not imply recommendation or endorsement by NCRP.

informa-Library of Congress Cataloging-in-Publication Data

National Council on Radiation Protection and Measurements.

Performance assessment of near-surface facilities for disposal of low-level radioactive waste.

p cm — (NCRP report ; no 152)

Includes bibliographical references and index.

ISBN-13: 978-0-929600-89-5

ISBN-10: 0-929600-89-4

1 Low level radioactive waste disposal facilities—United States—Evaluation

2 Radioactive waste disposal in the ground—United States—Evaluation I National Council on Radiation Protection and Measurements.

[For detailed information on the availability of NCRP publications see page 448.]

The search for solutions to the challenges posed by the need forlong-term disposal and isolation of low-level radioactive waste hasbeen long and complex The Low-Level Radioactive Waste PolicyAct, passed in 1980 and amended in 1985, specified that the dis-posal of most low-level waste not generated at U.S Department ofEnergy sites is the responsibility of states or State Compacts Acritical factor in the process of determining acceptable disposalpractices for low-level waste at any site is a demonstration of com-pliance with regulatory performance objectives NCRP was asked

to evaluate current approaches to performance assessment fornear-surface disposal facilities for low-level radioactive waste, andScientific Committee 87-3 was established to prepare a report onthis subject

This Report provides a review of concepts underlying mance assessments of near-surface disposal facilities for low-levelradioactive waste and approaches to conducting such assessments.This review includes discussions on the nature and scope of perfor-mance assessment, accepted approaches to conducting all aspects

perfor-of a performance assessment, and unresolved issues in conductingperformance assessments and applying the results The Reportalso discusses a number of policy issues that affect conduct of per-formance assessment Examples of these issues include the timeperiod for complying with performance objectives, application ofdrinking water standards, and interpretation of performance objec-tives for compliance purposes It is not the objective of this Report

to present recommendations for resolution of policy issues,although the importance of such issues and other social, politicaland economic factors is recognized Serving on the Committeewere:

Chairmen

David C Kocher (1999–2006)

SENES Oak Ridge, Inc

Oak Ridge, Tennessee

Matthew W Kozak (1992–1999)

Monitor Scientific LLCRichland, Washington

NCRP Secretariat

E Ivan White, Staff Consultant Cindy L O’Brien, Managing Editor David A Schauer, Executive Director

The Council is grateful for the financial support provided bythe U.S Department of Energy and the U.S Nuclear RegulatoryCommission at various times during the preparation of this Report.The Council also wishes to express its appreciation to the Com-mittee members for the time and effort devoted to the preparation

Preface iii

Executive Summary 1

1 Introduction 11

1.1 Purpose of Report 11

1.2 Scope of Report 12

1.3 Related NCRP Recommendations 14

2 Definition and Principles of Performance Assessment 16

2.1 Nature of Performance Assessment 16

2.2 Definition of Performance Assessment 18

2.3 General Principles of Performance Assessment 20

2.3.1 Performance Assessment as an Iterative Process 21

2.3.2 Performance Assessment as a Decision Tool 22 2.3.3 Uncertainty in Results of Performance Assessment 23

2.3.4 Integration and Interpretation of Results 23

2.3.5 Summary 24

2.4 Balance Between Conservatism and Realism in Performance Assessment 25

3 Context for Performance Assessment 28

3.1 Definition of Low-Level Radioactive Waste 28

3.1.1 Earliest Descriptions of Low-Level Waste 28

3.1.2 Current Definition of Low-Level Waste 29

3.2 Sources and Properties of Low-Level Waste 34

3.3 ICRP Recommendations on Disposal of Radioactive Waste 35

3.3.1 General Recommendations on Radiation Protection 35

3.3.2 General Policy on Application of Protection Principles to Radioactive Waste Disposal 36

3.3.3 Application of Protection Principles to Disposal of Solid Radioactive Wastes 37

3.3.4 Discussion of ICRP Recommendations 45

3.4 Requirements for Near-Surface Disposal of Low-Level

Waste 46

3.4.1 Authorized Disposal Systems 47

3.4.1.1 Legal and Regulatory Specifications 47 3.4.1.2 Historical Development of Disposal Technologies 48

3.4.2 Requirements for Protection of the Public 49

3.4.2.1 Licensing Criteria Established by NRC 49

3.4.2.2 Requirements Established by DOE 52

3.4.2.3 Implementation of the ALARA Requirement 56

3.4.2.4 EPA Views on Requirements for Disposal of Low-Level Waste 57

3.4.2.5 Implications of Performance Objectives 59

3.4.2.6 Requirements of States and State Compacts 61

3.4.3 Unresolved Issues in Performance Objectives for Low-Level Waste Disposal 61

3.4.3.1 Time Period for Compliance 62

3.4.3.2 Inclusion of Doses Due to Radon 63

3.4.3.3 Performance Objective for Protection of Groundwater 64

3.4.3.4 Interpretation of Performance Objectives for Compliance Purposes 66 3.4.4 Other Approaches to Regulating Waste Disposal 67

3.4.4.1 Approaches to Regulating Radioactive Waste Disposal 67

3.4.4.2 Approach to Regulating Disposal of Hazardous Chemical Waste 69

3.4.5 Requirements for Protection of the Environment 73

3.5 Other Concepts in Performance Assessment 74

3.5.1 Institutional Controls 74

3.5.1.1 Active Institutional Controls 74

3.5.1.2 Passive Institutional Controls 75

3.5.2 Model Validation and Confidence in Model Outcomes 76

3.5.2.1 Quality Assurance 78

3.5.2.2 Model Calibration 79

3.5.2.3 Evaluation of Conservative Bias 80

3.5.3 Concept of Reasonable Assurance 81

3.5.3.1 Description and Interpretation of

4.1.2 Development of Conceptual Models 91

4.1.3 Selection and Implementation of

4.2.2.2 Description of Disposal System 103

4.2.2.3 Development and Justification of

5.2 Cover Performance and Infiltration 117

5.2.1 Introduction 117

5.2.2 Types of Covers 121

5.2.3 Degradation of Covers 123

5.2.4 Approaches to Estimating Infiltration 125

5.2.5 Summary and Conclusions 129

5.3 Performance of Concrete Barriers 130

5.3.1 General Approach to Modeling of Concrete Barriers 132

5.3.2 Water Flow Through Concrete 132

5.3.3 Degradation of Concrete 134

5.3.3.1 Sulfate Attack 135

5.3.3.2 Freeze/Thaw Cycling 136

5.3.3.3 Calcium Leaching 136

5.3.3.4 Alkali-Aggregate Reaction 137

5.3.3.5 Corrosion of Reinforcing Steel 137

5.3.3.6 Combination of Reactions 139

5.3.4 Application of Models 140

5.3.5 Example Analyses of Long-Term Performance of Concrete Barriers 140

5.3.6 Summary 143

5.4 Source Term 143

5.4.1 Inventories of Radionuclides 144

5.4.2 Radionuclide Release Rates (Source Term) 147

5.4.3 Disposal Facility Concepts 153

5.4.4 Waste Containers 155

5.4.5 Waste Forms 158

5.4.5.1 Waste-Form Performance: Aqueous Phase 160

5.4.5.1.1 Surface Rinse with Partitioning 160

5.4.5.1.2 Diffusion-Controlled Release 162

5.4.5.1.3 Dissolution (Constant) Release 165

5.4.5.1.4 Solubility-Limited Release 166

5.4.5.2 Waste-Form Performance: Gas Phase 167

5.4.5.3 Ingrowth of Radionuclides 167

5.4.6 Transport in Disposal Facility 168

5.4.6.1 Aqueous-Phase Transport 168

5.4.6.2 Gas-Phase Transport 170

5.4.7 Interfaces with Other Performance Assessment Models 171

5.4.8 Source-Term Issues 171

5.4.8.1 Radionuclide Inventory Issues 172

5.4.8.1.1 Unit Source Term 172

5.4.8.1.2 Inaccurate Estimation of Inventories 173

5.4.8.2 Waste-Container Issues 174

5.4.8.2.1 Insufficient Characterization of Containers 175

5.4.8.2.2 Distributed Failure of Containers 175

5.4.8.3 Waste-Form Issues 175

5.4.8.3.1 Changes in Waste Types and Characteristics 176

5.4.8.3.2 Insufficient Waste-Form Characterization 176

5.4.8.3.3 Insufficient Data on Release Rates 177

5.4.8.3.4 Homogeneity of Wastes 177 5.4.8.3.5 Issues of Geochemistry and Solubility 178

5.4.8.4 Issues of Radionuclide Transport 179

5.4.8.4.1 Steady-State Flow 179

5.4.8.4.2 Uniform Flow Fields 179

5.4.8.4.3 Role of Geochemistry in Transport 180

5.4.8.4.4 Role of Microbial Processes 181

5.4.8.4.5 Role of Colloids 181

5.4.9 Summary 182

5.5 Unsaturated Zone Flow and Transport 183

5.5.1 Introduction 183

5.5.2 Interfaces with Other Performance Assessment Models 185

5.5.3 General Discussion of Unsaturated Zone Flow

and Transport 186

5.5.4 Data Requirements 191

5.5.5 Modeling of Unsaturated Flow 194

5.5.6 Summary 196

5.6 Aquifer Flow 197

5.6.1 Modeling of Aquifer Flow 197

5.6.2 Issues in Solving Flow Equation 200

5.6.2.1 Development of Steady-State Conditions 200

5.6.2.2 Scale and Heterogeneity 200

5.6.2.3 Boundary Conditions 202

5.6.2.4 Flow in Fractured Media 203

5.6.3 Summary 204

5.7 Radionuclide Transport in Groundwater and Surface Water 205

5.7.1 Phenomena That Influence Transport in Groundwater 206

5.7.1.1 Sorption 206

5.7.1.2 Advection 208

5.7.1.3 Diffusion 209

5.7.1.4 Dispersion 210

5.7.2 Combination of Phenomena That Influence Transport in Groundwater 213

5.7.3 Methods of Solution of Groundwater Transport Equation 215

5.7.3.1 Analytical Solutions 216

5.7.3.2 Green's Function (Semi-Analytical) Solutions 216

5.7.3.3 Finite-Element and Finite-Difference Solutions 217

5.7.3.4 Stream-Tube Solutions 218

5.7.4 Boundary Conditions 219

5.7.5 Modeling of Transport in Surface Water 221

5.7.5.1 Modeling of Discharges to Surface Water 222

5.7.5.2 Modeling of Transport in Rivers and Streams 223

5.7.5.3 Modeling of Transport in Lakes 224

5.7.5.4 Modeling of Transport in Sediment 225

5.7.6 Summary 225

5.8 Atmospheric Transport Analysis 226

5.8.1 Models for Estimating Suspension of Particulates 227

5.8.1.1 Modified Mass Loading Model 227

5.8.1.2 Resuspension Factor Model 229

5.8.1.3 Resuspension Rate Model 233

5.8.2 Release of Gases by Diffusion 235

5.8.3 Advective Transport 236

5.8.4 Atmospheric Transport Models 237

5.8.5 Summary 240

5.9 Biotic Transport 240

5.9.1 Background 241

5.9.2 Biotic Transport Processes 242

5.9.2.1 Transport Enhancement 243

5.9.2.2 Intrusion and Active Transport 243

5.9.2.3 Secondary Transport 244

5.9.3 Pathways of Human Exposure 244

5.9.4 Summary 245

5.10 Exposure Pathways and Radiological Impacts 245

5.10.1 Introduction 245

5.10.2 General Recommendations 249

5.10.3 Exposure Scenarios for Off-Site Members of the Public 250

5.10.3.1 Definition of Environmental Conditions and Living Habits 250

5.10.3.2 Exposure Scenarios for Different Release and Transport Pathways 251

5.10.4 Exposure Pathway Models 252

5.10.4.1 General Considerations 252

5.10.4.1.1 Components of Exposure Pathway Models 253

5.10.4.1.2 Multiplicative-Chain Models 255

5.10.4.1.3 Specific-Activity Models 256 5.10.4.2 Models of Foodchain Pathways 258

5.10.4.2.1 Terrestrial Foodchain Pathways 258

5.10.4.2.2 Aquatic Foodchain Pathways 258

5.10.5 Selection of Model Parameter Values 259

5.10.6 Sources of Generic Data on Model Parameter Values 260

5.10.6.1 Dose Coefficients 260

5.10.6.1.1 Internal Exposure 261

5.10.6.1.2 External Exposure 268

5.10.6.1.3 Summary of Dose Coefficients 272

5.10.6.2 Usage Factors 272

5.10.6.3 Transfer Factors for Foodchain Pathways 274

5.10.6.3.1 Terrestrial Foodchain Pathways 274

5.10.6.3.2 Aquatic Foodchain Pathways 278

5.10.6.3.3 Summary of Transfer Factors 279

5.10.7 Uncertainties in Dose Assessment Models 279

5.10.7.1 Uncertainties in Transfer and Usage Factors 279

5.10.7.2 Significance of Uncertainties in Transfer and Usage Factors 280

5.10.7.3 Sources of Uncertainty in Dose Coefficients 282

5.10.7.4 Significance of Uncertainties in Dose Coefficients 282

5.10.8 Approaches to Estimating Risk 283

5.10.9 Summary 285

6 Inadvertent Human Intrusion 288

6.1 Introduction 288

6.2 Role of Inadvertent Human Intrusion in Radioactive Waste Disposal 291

6.2.1 Historical Perspective 291

6.2.2 Regulatory Requirements for Near-Surface Disposal 292

6.2.2.1 NRC Requirements 292

6.2.2.2 DOE Requirements 294

6.3 Widely Used Scenarios for Inadvertent Intrusion 295

6.3.1 Scenarios for Acute Exposure 295

6.3.1.1 Construction Scenario 295

6.3.1.2 Discovery Scenario 296

6.3.1.3 Drilling Scenario 297

6.3.2 Scenarios for Chronic Exposure 297

6.3.2.1 Agriculture Scenario 297

6.3.2.2 Resident, Nonagriculture Scenario 299 6.3.2.3 Postdrilling Scenario 299

6.3.2.4 Groundwater Pathway for Chronic Intrusion Scenarios 300

6.3.3 Comparison of Standard Scenarios for Inadvertent Intrusion 301

6.3.4 Other Scenarios for Inadvertent Intrusion 302

6.4 Selection of Site-Specific Scenarios for Inadvertent Intrusion 304

6.4.1 Application of Widely Used Scenarios to Site-Specific Assessments 304

6.4.2 Judgmental Factors in Selecting Exposure Scenarios 305

6.4.3 Summary of Principles of Scenario Selection 308 6.5 Inputs to Dose Analyses for Inadvertent Intruders 308 6.5.1 Time of Occurrence of Intrusion 309

6.5.2 Radioactive Decay 309

6.5.3 Waste Dilution Following Disposal 310

6.5.4 Consideration of Radionuclide Transport 312

6.6 Outputs of Dose Analyses for Inadvertent Intruders 313 6.6.1 Scenario Dose Conversion Factors 313

6.6.2 Waste Acceptance Criteria Based on Intruder Dose Assessment 314

6.7 Effects of Inadvertent Intrusion on Off-Site Releases of Radionuclides 315

6.8 Summary 317

7 Uncertainty, Sensitivity and Importance Analysis 320

7.1 Introduction 320

7.2 Description of Importance Analysis 321

7.3 Purpose of Importance Analysis 322

7.4 Nature of Uncertainties in Performance Assessment 323 7.4.1 Characteristics of Uncertainties 323

7.4.1.1 Type-A and Type-B Uncertainties 324 7.4.1.2 Classification of Model Uncertainties 325

7.4.2 Uncertainty in Models 326

7.4.3 Uncertainty in Future Site Conditions 327

7.4.4 Uncertainty in Model Parameters 328

7.4.4.1 Measurement Errors 328

7.4.4.2 Insufficient Data 329

7.4.4.3 Dependence of Measurements on Scale 330

7.5 Mathematical Methods of Treating Uncertainty 330

7.5.1 Introduction to Mathematical Methods 331

7.5.2 Propagation of Model Uncertainty 332

7.5.3 Propagation of Future Uncertainty 334

7.5.4 Propagation of Parameter Uncertainty 338

7.5.4.1 Deterministic Methods 338

7.5.4.2 Probabilistic Methods 340

7.5.4.2.1 Monte-Carlo Analysis 340

7.5.4.2.2 Perturbation Analysis 343

7.5.4.2.3 Possibilistic Analysis 344

7.6 Role of Sensitivity Analysis in Importance Analysis 344 7.6.1 Need for Sensitivity Analysis 344

7.6.2 Methods of Parameter Sensitivity Analysis 345 7.7 Application of Uncertainty Analysis to Importance Analysis 346

7.7.1 General Structure of Uncertainty Analysis 346

7.7.2 Evaluation of Different Methods of Uncertainty Analysis 347

7.7.3 Evaluation of Approaches to Importance Analysis 350

7.8 Summary 351

8 Summary 353

8.1 Purpose and Scope of Performance Assessment 353

8.2 Basic Elements of Performance Assessment 354

8.2.1 Development of Conceptual Models 355

8.2.2 Development and Application of Mathematical and Physical Models 356

8.2.3 Integration and Interpretation of Results 357

8.3 Components of Performance Assessment Modeling 358

8.3.1 Cover Performance and Infiltration 358

8.3.2 Performance of Concrete Barriers 359

8.3.3 Source Term 360

8.3.4 Unsaturated Zone Flow and Transport 361

8.3.5 Aquifer Flow 363

8.3.6 Radionuclide Transport in Groundwater and

Surface Water 364

8.3.7 Atmospheric Transport 366

8.3.8 Biotic Transport 366

8.3.9 Exposure Pathways and Radiological Impacts 367

8.3.10 Overview of Components of Performance Assessment 368

8.4 Inadvertent Human Intrusion 369

8.5 Uncertainty, Sensitivity and Importance Analysis 370

8.6 Iterative Nature of Performance Assessment 371

8.7 Reasonable Assurance of Compliance with Performance Objectives 372

Glossary 373

References 396

The NCRP 439

NCRP Publications 448

Index 458

In the United States, low-level radioactive waste is defined asany radioactive waste arising from operations of the nuclear fuelcycle that is not classified as high-level waste (including spentfuel when it is declared to be waste), transuranic waste, or uranium

or thorium mill tailings Low-level waste is generated in many mercial, defense-related, medical, and research activities Owing

com-to its definition only by exclusion and its many sources, low-levelwaste occurs in a wide variety of physical and chemical forms,and it contains a wide range of concentrations of many differentradionuclides

Most low-level waste, except relatively small volumes that tain high concentrations of radionuclides with half-lives on theorder of 30 y or longer, is intended for disposal in facilities located

con-on or near the ground surface Decisicon-ons about acceptable surface disposals of low-level waste are based in large part on theneed to comply with regulatory requirements that have been estab-lished by the U.S Nuclear Regulatory Commission (NRC) and theU.S Department of Energy (DOE) These regulatory requirementsinclude performance objectives that define allowable radiationexposures of the public at future times due to releases of radionu-clides to the environment

near-In order to determine whether a particular facility complieswith regulatory performance objectives for long-term protection of

the public, a performance assessment of the facility must be

con-ducted As used in this Report:

Performance assessment is an iterative process involvingsite-specific, prospective modeling evaluations of the post-closure time phase of near-surface disposal systems forlow-level waste with two primary objectives:

• to determine whether reasonable assurance of ance with quantitative performance objectives can bedemonstrated; and

compli-• to identify critical data, facility design, and model opment needs for defensible and cost-effective licensingdecisions and to develop and maintain operating limits

devel-(i.e., waste acceptance criteria).

This definition emphasizes that performance assessment focusesprimarily on a decision about compliance with performance objec-tives, rather than the much more difficult problem of predictingactual radiological impacts on the public at far future times.The purpose of this Report is provide a review of concepts under-lying performance assessments of near-surface disposal facilitiesfor low-level waste and approaches to conducting such assess-ments This review includes discussions on the nature and scope ofperformance assessment, accepted approaches to conducting allaspects of a performance assessment, and unresolved issues in con-ducting performance assessments and applying the results Chal-lenges in conducting and defending performance assessments atspecific sites also are emphasized.

An understanding of general principles of performance ment is important, because such understanding can lead to an effi-cient process and defensible product and can reduce the potentialfor misinterpretation of results General principles of performanceassessment are summarized as follows:

assess-• Performance assessment should be an iterative, flexibleprocess of integrating modeling, data collection, and designactivities in a manner that identifies those aspects of engi-neered and natural barriers in a disposal system of impor-tance to a decision about compliance with regulatoryperformance objectives The performance assessment pro-cess is important during all time phases of a facility fromsite selection and facility design through operations andpostclosure monitoring and surveillance

• Performance assessment is a process that is intended to

pro-vide reasonable assurance of compliance with performance

objectives; absolute assurance of compliance generally is notattainable by any means unless disposal of only very smallquantities of radionuclides is allowed

• Since there is substantial uncertainty in models and tant parameters used in performance assessment and somephysical, chemical, and biological processes that affectthe long-term performance of a disposal system may not bewell understood, use of subjective scientific judgment is anessential aspect of performance assessment Therefore, avariety of results that investigate the consequences of differ-ent plausible assumptions should be presented, rather than

impor-a single projected outcome

• An integration and interpretation of assumptions and results,

in which conceptual models of the performance of a disposal

system and their bases and the results of calculations are sented in a manner that reflects the many judgments involvedand the importance of different aspects of an assessment to ademonstration of compliance with regulatory performanceobjectives, is a critical aspect of performance assessment.Quantitative performance objectives for near-surface disposal oflow-level waste in NRC and DOE regulations are expressed interms of limits on annual dose to members of the public, eitherequivalent dose to specific organs or tissues or effective dose equiv-alent Therefore, projections of maximum concentrations of radio-nuclides in the environment at assumed locations of exposure arerequired Although regulations are well established, a number ofissues regarding performance objectives and their implementationare not yet fully resolved or are controversial These include (1) thetime period for compliance and the weight, if any, that should begiven to projections of performance beyond the compliance period

pre-in determpre-inpre-ing acceptable disposals, (2) whether projected dosesdue to radon are included in performance objectives, which ispotentially important in determining acceptable disposals of wastethat contains radium, thorium, or uranium, (3) whether perfor-mance objectives should include a separate requirement for protec-tion of groundwater resources in accordance with drinking waterstandards, which is important when drinking water standardswould be more restrictive in determining allowable releases ofmany radionuclides than the existing performance objectives thatapply to all exposure pathways combined, and (4) interpretation of

performance objectives for compliance purposes—i.e., how highly

uncertain results of performance assessments should be comparedwith fixed performance objectives in judging compliance

At most sites, movement of water is considered to be the mostimportant means by which radionuclides may be released from adisposal facility and transported to locations where exposures ofthe public could occur Even for the simplest types of near-surface

facilities (e.g., an unlined trench with backfill and cap consisting of

earthen materials), performance assessment requires an tion of results of a number of different types of models to provide anoverall description of the performance of a disposal system Theusual approach to performance assessment is to model variouscomponents of the system separately and then link the components

integra-in a sequential fashion, with appropriate boundary and contintegra-inuityconditions, to describe overall system performance The differentcomponents that generally must be considered in a performanceassessment are the following:

• an analysis of cover performance and infiltration, the

primary purpose of which is to estimate the flux of water

(i.e., incident precipitation) that infiltrates through a

natu-ral or engineered cover system to locations of disposed waste

or an engineered barrier (e.g., a concrete structure) above

the waste; the performance of a cover system in inhibitingatmospheric releases of radionuclides in gaseous form alsocan be important at some sites and for some wastes;

• an analysis of the performance of concrete barriers (e.g.,

vaults, modular canisters, or bunkers) that are used inmany disposal facilities to enhance containment of low-levelwaste by (1) providing structural support for an earthen orengineered cover system, (2) delaying and inhibiting inflow

of water to locations of disposed waste, (3) supplying tional adsorbing materials to retard movement of radionu-clides into the surrounding environment, and (4) delayingand inhibiting release of radionuclides in leachate from afacility;

addi-• an analysis of the source term, the purpose of which is to

estimate the rate of release of radionuclides from a disposalfacility into the surrounding environment, either the vadosezone when releases are in liquid form or the atmospherewhen releases are in gaseous form, by considering rates ofrelease from waste forms and waste containers, transportwithin a disposal facility, and transport through any engi-neered barriers used in constructing the facility;

• an analysis of flow and transport in the unsaturated (vadose) zone, which uses results of source-term modeling

of liquid releases as input and provides estimates of releasesinto groundwater (zone of saturation);

• an analysis of flow and transport in groundwater (zone

of saturation), which uses results of modeling of flow and

transport in the vadose zone, as well as any additionaldirect recharge to an aquifer that may occur due to runofffrom a cover, as input and provides estimates of concentra-tions of radionuclides in groundwater at assumed locations

of exposure; an analysis of flow and transport in surface water also may be required when groundwater into which

releases occur discharges to the surface at locations close to

a facility;

• an analysis of atmospheric transport, which uses results

of source-term modeling of gaseous releases or other means

of transport of buried waste to the ground surface andrelease to the atmosphere as input and provides estimates

of airborne concentrations of radionuclides at assumed tions of exposure;

loca-• an analysis of biotic transport, which considers actions of

plants and animals that could affect transport of buriedwaste to assumed locations of exposure; biotic transport isnot often considered explicitly in performance assessment,but it can serve to enhance release and transport of radionu-clides and can be important at some sites; and

• an analysis of exposure pathways and radiological impacts, which uses results of environmental transport

models (i.e., groundwater or surface water flow and

trans-port, atmospheric transtrans-port, or biotic transport models) thatestimate concentrations of radionuclides in environmentalmedia at assumed locations of exposure as input and pro-vides estimates of transport through various exposure path-ways to human receptors and the resulting radiation doses.Atmospheric and biotic transport often are considered to be unim-portant compared with transport in water However, these pro-cesses can be important in some environments and for some facilitydesigns, and their importance generally should be evaluated insite-specific analyses

NRC and DOE regulations also require that near-surface posal facilities provide protection of hypothetical inadvertentintruders who are assumed to come onto a disposal site after loss ofinstitutional control and access disposed waste by such means asexcavating to construct a foundation for a home or drilling Protec-tion of inadvertent intruders at sites licensed under NRC regula-tions is provided by the NRC’s waste classification system, whichspecifies limits on concentrations of radionuclides in Class-A, -B,and -C waste and technical requirements on disposal of waste ineach class At DOE sites, a site-specific assessment of potentialimpacts on inadvertent intruders is required for the purpose ofestablishing limits on concentrations of radionuclides; these limitscan vary greatly depending on site conditions and the design of afacility Under either regulations, compliance with a requirement

dis-to protect inadvertent intruders is based on analyses of potentialradiological impacts in assumed intrusion scenarios Standard sce-narios that are often used are reviewed in this Report Given thatthere are separate requirements to protect members of the publicand inadvertent intruders, determinations of acceptable near-sur-face disposals of low-level waste essentially involve achieving a bal-ance between acceptable releases of radionuclides beyond the siteboundary and acceptable residual concentrations in a disposal

facility after loss of institutional control If excavation into wasteand residence on a disposal site in a homesteader scenario are con-sidered to be credible occurrences, criteria that define adequateprotection of inadvertent intruders usually are more restrictive indetermining acceptable disposals than performance objectives forprotection of the public or the environment Therefore, selection ofcredible intrusion scenarios can be very important in determiningacceptable disposals in near-surface facilities.

Performance assessments generally must consider uncertainties

in models and parameter values, which result in uncertainty in

results of modeling (e.g., projected doses to the public), and the

sen-sitivity of model outputs to changes in assumptions and variations

in parameters Since the primary purpose of performance ment is to provide a demonstration of compliance with regulatoryperformance objectives, a particular kind of uncertainty and sensi-

assess-tivity analysis, which is termed importance analysis, is

empha-sized in this Report Importance analysis is an integration andinterpretation of results obtained from the performance assessmentprocess for the purpose of identifying assumptions and parameterswhich, when changed within credible bounds, can affect a decisionabout regulatory compliance This type of analysis, which focuses

on uncertainties and sensitivities that are important to a decisionabout compliance with performance objectives, is different frommore traditional uncertainty and sensitivity analyses, which areconcerned with representing uncertainty in the actual behavior of adisposal system and outcomes of waste disposal An understanding

of the distinction between importance analysis and traditional sitivity and uncertainty analysis is important in conducting perfor-mance assessments efficiently and defending the results

sen-Models of varying degrees of sophistication and complexity havebeen developed for all components of a performance assessment.However, detailed modeling of all aspects of the performance of adisposal system is beyond current capabilities and, indeed, is notrequired to achieve defensible results and robust decisions aboutthe acceptability of waste disposals Since there are a large number

of radionuclides in low-level waste and a large number of potentialpathways for transport and exposure, simple screening analyses toselect for further analysis only those radionuclides and pathwaysthat contribute significantly to projected doses to the public are animportant initial step in making a performance assessment tracta-ble Once radionuclides and pathways are selected for further anal-ysis, many stylized and simplifying assumptions normally are used

in performance assessment in the interest of expediency Examples

of such assumptions include the following:

• The potential importance of future climate change on tration and release and transport in water either is not con-sidered or is modeled by assuming an abrupt change to anexpected climate at some future time.

infil-• Infiltration through natural or engineered cover systemsusually is modeled by assuming steady-state conditions, andtransients that might occur as a result of episodic precipita-tion events are ignored Failure of a cover system usually ismodeled by assuming an instantaneous change or a series ofinstantaneous changes to natural conditions at some futuretime

• Degradation or failure of engineered barriers, either cal or chemical, usually is modeled by assuming an instan-taneous change from an initial condition to a failed state atsome future time or a constant rate of failure

physi-• When there are highly heterogeneous distributions of nuclides and a multiplicity of waste forms in a disposalfacility, source terms usually are modeled by averaging radio-nuclide distributions over individual disposal units andassuming no more than a few idealized representations ofwaste forms

radio-• The physical structure of unsaturated and saturated logic media and their geochemical properties usually areassumed to be homogeneous and isotropic

geo-• A graded approach to modeling flow and transport in theunsaturated (vadose) zone may be taken in which all or

parts of the unsaturated zone are ignored (e.g., releases

from a disposal facility are assumed to directly enter anunderlying aquifer) or, less conservatively, a unit-gradientmodel that assumes steady-state flow and a flow rate equal

to the infiltration rate is used

• The interface between models of flow in the vadose and urated zones often is represented by simple boundary condi-

sat-tions (e.g., zero pressure head).

• A linear sorption isotherm, described by the equilibrium

solid/solution distribution coefficient (Kd), usually is assumed

to represent all geochemical effects on transport in the posal facility following release from a waste form and wastepackage and transport in the vadose and saturated zones

dis-• All physical and chemical processes that affect release andtransport of radionuclides often are assumed to be indepen-dent of radionuclide concentrations in waste, and the perfor-

mance of the disposal system (e.g., projected dose to the

public) is assumed to depend linearly on those concentrations

Challenges in conducting performance assessments and ing the results in a regulatory setting, which is tantamount todefending important assumptions, increase as the quantities ofradionuclides that are intended for disposal in a facility increaseand compliance with performance objectives cannot be demon-strated by using clearly conservative (pessimistic) models forhighly uncertain components of performance The lack of relevantsite-specific data or data over time and spatial scales of importance

defend-is a frequent concern Some of the challenges in modeling the formance of a disposal system when water is the medium in whichreleases are assumed to occur and a high level of performance ofnatural and engineered barriers is required to demonstrate compli-ance with performance objectives are summarized as follows:

per-• Cover performance and infiltration: It can be difficult to

jus-tify that an engineered cover will perform as designed andconstructed to control infiltration over time periods muchbeyond the period of institutional control

• Concrete barriers: There is little relevant data to predict the

structural integrity and load-bearing capabilities of concreteover long time periods, so it is difficult to justify assump-tions about structural integrity at times beyond severalhundred years Assumptions about infiltration throughdegraded concrete structures, including the relative impor-tance of flow in fractures and pores, can be an importantissue

• Source term: Inventories of radionuclides that often are

expected to be the most important in releases to

groundwa-ter (e.g., 14C, 99Tc, and 129I) can be difficult to estimate eling of releases can be challenging when radionuclidedistributions are heterogenous and several waste forms areused Although grout provides a homogeneous waste formfor liquid wastes, modeling of long-term changes in hydro-logic and geochemical properties of grout waste forms can bedifficult when there are little relevant data and judgmentmust be relied on

Mod-• Unsaturated (vadose) zone flow and transport: Modeling at a

detailed level is data-intensive and difficult to defend at cific sites owing to the complex, nonlinear relationshipsbetween moisture content, pressure (suction) head, andhydraulic conductivity and their dependence on soil type Adefensible conceptual model of flow and transport in unsat-urated fractured rock is not yet available

spe-• Saturated zone flow and transport: A groundwater velocity

field is not directly measurable but must be generated using

a model that is based on data on hydraulic head from toring wells and data on pump or core hydraulic conductiv-ity tests A velocity field so generated is non-unique, andmultiple interpretations of data, with different effects onperformance, may be reasonable at any site Other issues ofpotential concern in modeling saturated zone flow includeassumptions about boundary conditions, use of transientdata to model steady-state conditions, the spatial scale andheterogeneity of a velocity field, modeling of flow in frac-tured rock, and the applicability (scaling) of laboratory data

moni-to field conditions Issues in modeling transport include the

simplistic nature of the Kd concept, justification of assumed

Kd values at specific sites, treatments of diffusion and persion, and a lack of site-specific data on dispersivities

dis-In contrast, modeling of atmospheric transport and exposure ways and radiological impacts rarely is difficult or controversial, inpart because the relevant processes are well understood and thereare extensive studies to validate models normally used in perfor-mance assessments

path-A central issue that must be confronted in all performanceassessments is whether simple and clearly conservative modelsshould be used in demonstrating compliance with performanceobjectives or whether more complex and rigorous modeling should

be undertaken in an effort to provide more realistic projections ofoutcomes at times far into the future Either approach may bedesirable for many reasons, and both have their difficulties Thepoint of view taken in this Report is that an appropriate balancebetween conservatism and more realistic approaches to perfor-mance assessment is largely a matter of judgment that should beapplied on a site-specific basis All performance assessmentsshould attempt to incorporate some degree of realism to demon-strate an appropriate level of understanding of the long-term per-formance of disposal systems The goal should be to provide acost-effective and defensible assessment that is commensuratewith the hazards posed by wastes that are intended for disposal at

a specific site At some sites with highly desirable characteristics,use of simple and conservative models for some aspects of systemperformance may not affect a decision about regulatory compli-ance At other sites, however, efforts at more realistic modelingmay be required At any site, it is important to recognize thatperformance assessment is conducted to inform decisions about the

acceptability of waste disposals, and that it is not necessary toobtain realistic projections of outcomes to render such decisions in

a defensible manner Although performance assessment involves asignificant amount of subjective scientific judgment and there areimportant limitations in regard to predicting actual outcomes,these factors do not compromise the essential role of performanceassessment in regulatory decision making

Low-level radioactive wastes are generated in a variety of mercial, defense-related, medical, and research activities Mostlow-level waste1 generated in the United States, except relativelysmall volumes that contain the highest concentrations of radionu-clides, is intended for disposal in facilities located on or near theground surface This intention is based largely on generic analyses,such as those performed by the U.S Nuclear Regulatory Commis-sion (NRC, 1981a; 1982a), which indicated that near-surface facili-ties should be capable of providing adequate protection of publichealth and the environment at times far into the future

com-Decisions about acceptable near-surface disposals of low-levelwaste at specific sites are made on the basis of applicable laws andregulations Such decisions take into account many scientific, tech-nical, economic and social factors Regulatory requirements thatapply to disposal of low-level waste in near-surface facilitiesinclude performance objectives that define allowable radiationexposures of the public at future times In order to determinewhether a particular disposal facility complies with performanceobjectives, a performance assessment of the facility must be con-ducted A performance assessment essentially is a prospectiveevaluation of potential radiation exposures of the public at timesafter a disposal facility is closed

1.1 Purpose of Report

The purpose of this Report is to provide a review of conceptsunderlying performance assessments of near-surface disposal facil-ities for low-level waste and approaches to conducting such assess-ments This review includes discussions on the nature and scope ofperformance assessment, accepted approaches to conducting allaspects of a performance assessment, and unresolved issues in con-ducting performance assessments and applying the results

1In this Report, the term “low-level waste” is used to refer to low-levelradioactive waste, and similarly with “high-level waste.” Since the terms

“low-level waste” and “high-level waste” are not used to describe wastesthat contain hazardous chemicals, their use to describe different radioac-tive wastes should not cause confusion

Many discussions in this Report also apply to deep geologicrepositories, which are intended for disposal of high-level and tran-suranic radioactive wastes In particular, the conceptual founda-tions and general principles of performance assessment described

in this Report are applicable to geologic repositories, even thoughmodels used in performance assessments for geologic repositoriesand applicable regulatory criteria may differ from those for near-surface facilities Discussions in this Report generally do not apply

to disposal of large volumes of uranium mill tailings, since thisactivity is conducted in accordance with a different set of legal andregulatory requirements that do not call for prospective evalua-tions of long-term radiological impacts on the public in determiningacceptable disposal practices at specific sites.2

This Report focuses on performance assessments of proposed orcurrently operating facilities for near-surface disposal of low-levelwaste Assessments to evaluate potential radiological impacts ofpast disposal practices at inactive or abandoned sites for the pur-pose of determining whether there is a need for site remediation tomitigate those impacts are outside the scope of this Report.Although technical aspects of such assessments may have much incommon with performance assessments discussed in this Report,remediation of past disposal practices is addressed under a differ-ent legal and regulatory framework, and the need to assess poten-tial impacts of past disposal practices at times far into the future isnot yet established

1.2 Scope of Report

This Report is intended to provide a general overview of mance assessment, and to identify sources of current informationabout performance assessment and the different scientific disci-plines involved in conducting performance assessments An impor-tant emphasis is the level of detail that has been shown by pastexperience to be required in performance assessments of specificdisposal facilities It is not the intent of this Report to provide acomprehensive review of all aspects of performance assessmentthat may be important at a particular facility

perfor-2Performance assessments to evaluate potential radiation exposures

of the public at mill-tailings disposal sites were conducted by the U.S.Environmental Protection Agency in developing regulatory requirements

in the form of design objectives that apply at all sites (EPA, 1982; 1983).The acceptability of mill-tailings disposal at any site then is demonstrated

by meeting the design objectives

Discussions in this Report also consider a number of policyissues that affect conduct of performance assessment Examplesinclude the time period for complying with performance objectives,application of drinking water standards to protection of ground-water resources, and whether impacts of inadvertent human intru-sion on the normal performance of a near-surface disposal facilityare evaluated in demonstrating compliance with performanceobjectives It is not the purpose of this Report, however, to presentrecommendations for resolution of any such issues Similarly,although the importance of social, political and economic factors isrecognized and discussed to a limited extent, this Report is not con-cerned with addressing those factors and integrating them into theprocess of developing and licensing new facilities.

This Report is organized as follows Sections 2 and 3 providebackground information of importance to understanding discus-sions on technical approaches to performance assessment, includ-ing a definition of performance assessment and a discussion ofgoals and principles of performance assessment (Section 2) anddiscussion of the broader context for performance assessment (Sec-tion 3) Sections 4 and 5 present current views on suitable technicalapproaches to conducting performance assessments Section 4 pre-sents a conceptual framework for conducting performance assess-ments, including general discussions of a recommended approach

to performance assessment, principles for treatment of uncertainty,and development of confidence in results as part of the performanceassessment process These discussions emphasize advantages of aniterative approach to performance assessment involving interac-tions among site characterization and data collection, facilitydesign, and modeling activities Section 5 discusses models anddatabases that are considered suitable for use in performanceassessments of near-surface disposal facilities for low-level waste,

as well as important sources of uncertainty in those models Theapproach taken in this Report is to divide performance assessmentinto several modules, each of which represents a particular aspect

of a disposal system (e.g., longevity of engineered barriers, release

of radionuclides from a disposal facility, transport of radionuclides

in the environment, pathways of human exposure) Specific puter codes that may be used in performance assessment are rarelydiscussed in this Report A code is no more than a particularnumerical representation of models embodied in it, and it is farmore important to focus on selection of credible models than on par-ticular implementations of those models

com-Sections 6 and 7 present discussions on topics that are tant to performance assessment but do not fit in discussions in

impor-Section 5 on modeling of particular aspects of a disposal system.These topics include assessments of inadvertent human intrusion(Section 6) and treatment of uncertainty and sensitivity by means

of importance analysis (Section 7) Finally, Section 8 providessummary comments on performance assessment, with particularemphasis on technical challenges in conducting performanceassessments

1.3 Related NCRP Recommendations

The National Council on Radiation Protection and ments (NCRP) previously issued three reports that are relevant toperformance assessments of near-surface disposal facilities forlow-level waste NCRP Report No 76 (NCRP, 1984a) presents rec-ommendations on models and databases for use in assessing radi-ation doses to the public following release of radionuclides to theatmosphere, surface water, or groundwater Models of transport inthe environment and exposure pathways discussed in Report

Measure-No 76 are relevant to performance assessment However, someapproaches to modeling environmental transport and exposurepathways discussed in that report may be more detailed than nec-essary or useful in performance assessment For example, model-ing of episodic releases and seasonal dependencies of exposures,which can be important in assessing doses resulting from actual orexpected releases from operating facilities and in reconstructingdoses from past releases, are not needed in prospective assess-ments of the performance of disposal facilities at far future times.NCRP Report No 123 (NCRP, 1996a; 1996b) presents recom-mendations on screening models to assess impacts of releases

of radionuclides to the environment, including screening models ofatmospheric transport, transport in surface water, disposal of radi-onuclides in the ground, and transport in terrestrial and aquaticfood chains Those models are generic and are intended to beapplied at any site Screening involves use of simple models thatemploy clearly conservative (pessimistic) assumptions Such mod-els can be used to demonstrate that operating facilities comply withregulatory requirements or to eliminate unimportant radionu-clides and exposure pathways from further consideration in a doseassessment Screening models described in Report No 123 may, inmany cases, provide a suitable first iteration for related aspects ofperformance assessments of low-level waste disposal facilities, to

be followed by use of more site-specific models of increasing tication However, when generic screening models are used in per-formance assessment, including models recommended by NCRP,

sophis-proper justification for their use at a disposal site of concern must

be provided Screening models for disposal of radionuclides in theground, which include models for releases to groundwater andexposures to buried waste by several pathways assuming loss ofinstitutional control at 10 y after facility closure, and transport

of radionuclides in terrestrial and aquatic food chains could be ticularly useful in performance assessment Models of transport insurface water, which are discussed in this Report, also could be use-ful at some disposal sites

par-NCRP Report No 129 (par-NCRP, 1999) presents recommendations

on screening levels (concentrations) of radionuclides in surface soil.For each of several assumptions about future uses of contaminatedland and assumptions about exposure pathways and pathwaymodels in each land-use scenario, radionuclide-specific screeningfactors expressed as annual effective doses per unit activity concen-tration in surface soil [Sv (Bq kg)–1] are derived The review andevaluation of exposure pathway models used to derive screeninglevels in surface soil essentially updates recommendations inNCRP Report No 76 (NCRP, 1984a) In contrast to the approach toscreening in NCRP Report No 123 (NCRP, 1996a; 1996b), whichuses point values of all parameters, the screening analysis inReport No 129 incorporates assumptions about uncertainties in allparameters used in estimating annual effective doses Median val-ues and 95th percentiles of uncertain screening factors are derived,and the 95th percentiles are used to obtain screening levels in sur-face soil that correspond to an annual effective dose of 0.25 mSv.Thus, the intent is that at any site at which a particular land-usescenario is considered appropriate, application of the derivedscreening levels should ensure that, with a high level of confidence,annual doses to members of the public who might occupy contami-nated land would be less than the assumed dose criterion Screen-ing levels developed in Report No 129 could be particularly useful

in performance assessments in regard to assessing doses following

dispersal of radionuclides over the land surface (e.g., by irrigation

with contaminated groundwater) and in assessing doses from vertent intrusion into a facility

2.1 Nature of Performance Assessment

Performance assessment is concerned with prospective tions of waste disposal systems A disposal system is comprised ofmultiple engineered and natural barriers (ICRP, 1998), which areintended to inhibit movement of radionuclides from locations ofwaste emplacement into the general environment beyond theboundary of a disposal site and to inhibit intrusion into waste bywater, plants, burrowing animals, and humans Examples of engi-neered barriers include concrete disposal vaults, impermeable capsabove buried waste that are constructed with man-made or naturalmaterials, and grouting of waste to control ingress and egress ofwater and release of radionuclides An important natural barrier atany near-surface disposal site is the ability of native soils to sorbradionuclides and, thus, retard their migration

evalua-Performance assessment is an essential activity in developingdisposal facilities and gaining approval by regulatory authorities,because it provides the only available link between measurableproperties of a disposal facility or waste and potential long-termradiological impacts of waste disposal on the public This link iscrucial because, unlike many more familiar engineered systems,there usually are not intuitively evident relationships betweenmeasurable properties of a disposal system and consequences ofwaste disposal For instance, one cannot draw general conclusionsthat a geologic stratum with high permeability is either favorable

or unfavorable as a disposal site Performance assessment isused to integrate available information about the long-term behav-ior of a disposal facility for the purpose of obtaining a defensible

conclusion regarding the ability of the facility to protect the public

in accordance with applicable regulatory criteria Given the plexities associated with assessments of natural and engineereddisposal systems, a critical part of this integration process involvesiterative feedback to identify data, modeling and design needs forthe process of gaining regulatory approval of a facility

com-Performance assessment also can be important after regulatoryauthorities have approved a disposal facility Additional data may

be gathered during disposal operations or after facility closure, and

a performance assessment may need to be maintained and updateduntil institutional control is relinquished Thus, performanceassessment should be viewed as a tool for risk management, notjust as a means of gaining approval of a facility by regulatoryauthorities

Unlike safety analyses of nuclear reactors, airplanes andbridges, for example, that address relatively short-term behavior

of engineered systems on the basis of large amounts of empiricaldata, performance assessment addresses the long-term behavior

of complex systems for which relatively little empirical data areavailable.3 It is, therefore, important to distinguish performanceassessment from other, more standard engineering problems Theprimary distinction is that performance assessment relies moreheavily on judgments, which are necessitated by a lack of observa-tions of the long-term performance of disposal facilities Indeed,judgments are essential and form the basis of calculations oflong-term performance There needs to be a frank acknowledgmentthat required judgments do not always have a firm basis in rele-vant measurements on real systems Some judgments will be based

on established principles of science and engineering, but others will

be little more than informed opinion

Performance assessments commonly involve a combination ofsimple screening models and more complex analyses Screeningmodels may be used, for example, to eliminate unimportant radio-nuclides and transport or exposure pathways from further consid-eration in an assessment or to address in a clearly conservative

3Experience with low-level waste disposal facilities that have operated

in the past can provide useful information for performance assessments ofcurrently operating or planned facilities For example, observations at his-torical facilities that did not perform adequately can provide information

on designs and environmental conditions that are not likely to prove isfactory However, the number of such facilities is limited, and most ofthem did not include engineered barriers and waste forms of the type fre-quently used in currently operating or planned facilities

sat-manner aspects of the performance of a disposal system for whichlittle information is available and modeling is difficult, such astransport of radionuclides in unsaturated soil In conducting aperformance assessment, considerable effort often is spent indeveloping an appropriate balance between use of realistic andconservative models; this important issue is discussed further inSection 2.4 The key to conducting performance assessments is

to structure an analysis so that it is defensible on the basis ofavailable information on the long-term performance of a disposalfacility

2.2 Definition of Performance Assessment

The definition of performance assessment in this Report sizes several concepts that are important to the conduct, interpre-tation and use of performance assessments, and that distinguishperformance assessment from typical engineering analyses Asused in this Report:

empha-Performance assessment is an iterative process involvingsite-specific, prospective modeling evaluations of the postclo-sure time phase of near-surface disposal systems for low-levelwaste with two primary objectives:

• to determine whether reasonable assurance of compliancewith quantitative performance objectives can be demon-strated; and

• to identify critical data, facility design, and model ment needs for defensible and cost-effective licensing deci-

develop-sions and to develop and maintain operating limits (i.e.,

waste acceptance criteria)

This definition emphasizes that, for purposes of this Report, mance assessment is a process that focuses primarily on a decisionabout compliance with regulatory requirements, rather than themuch more difficult problem of predicting actual outcomes of waste

perfor-disposal (i.e., actual radiological impacts on the public and the

envi-ronment) The following paragraphs summarize key aspects of thisdefinition Further discussions are contained in the followingsection

The term “iterative process” refers to an expectation that mance assessment probably will require two or more sequentialsets of calculations as additional data are collected during sitecharacterization activities and, perhaps, during facility operationsand postclosure monitoring of a facility In general, an iterative

perfor-approach helps to minimize a tendency for performance ment to become focused on details of models without consideringthe relative importance of specific aspects of an assessment to theprimary concern of demonstrating compliance with regulatoryrequirements; it also emphasizes that performance assessment ismore than a modeling exercise Modeling of disposal systems must

assess-be conducted in coordination with data collection, facility designactivities, and long-term monitoring When applied efficiently, theperformance assessment process provides input to managementdecisions regarding needs for additional data collection, designactivities, and monitoring In general, iteration of performanceassessments is necessary when new information becomes availablethat could affect a licensing decision, but care should be taken

to avoid unnecessary iterations that would impede the licensingprocess.4

The term “site specific” refers to the need to base performanceassessments on relevant data for a facility being considered.Generic performance assessments have been useful in developingregulations, investigating the cost-effectiveness of various designs,and building confidence that near-surface disposal facilities atwell-chosen sites should be protective of public health and the envi-ronment, but models used in generic assessments should not beapplied to specific sites and facility designs without proper justifi-cation The optimum approach to collecting site-specific datadepends primarily on conditions at a site, the design of a facility,and the need for additional data in demonstrating compliance withperformance objectives

The term “postclosure” emphasizes that performance ment is concerned only with the consequences of waste disposalfollowing emplacement of all waste and closure of a facility.5 Rou-tine and accidental releases of radionuclides prior to closure arebetter addressed using standard safety assessments of site opera-tions and monitoring However, performance assessment can behelpful in guiding monitoring activities during the preclosure timephase, and such monitoring can provide useful information on post-closure performance

assess-4In this Report, the term “licensing” refers generally to a process ofobtaining approval of a disposal facility by any regulatory authorities; itdoes not necessarily refer only to approval by NRC or an AgreementState

5This may not be the case at a geologic repository for disposal of spentnuclear fuel and high-level wastes, because such a facility may remainopen for decades after waste emplacement is complete

The term “prospective modeling evaluations” is used ratherthan “modeling predictions” to emphasize that performance assess-ment is directed primarily at building sufficient understanding ofthe long-term behavior of a disposal system to make a defensibledecision about compliance with performance objectives Such anunderstanding is achieved by identifying those assumptions aboutthe performance of engineered and natural barriers in a disposalsystem that are most important to obtaining the projected outcomeand then showing that plausible changes in those assumptionswould not affect a decision about compliance.

The term “reasonable assurance” emphasizes the inexactnature of performance assessment and the crucial role of judgment

in conducting performance assessments and in evaluating results.Further discussion of the important concept of reasonable assur-ance is provided in Section 3.5.3

The term “operating limits” is included in the definition

to emphasize that results of performance assessment are used toidentify acceptable operating conditions at a disposal facility, espe-cially waste acceptance criteria in the form of limits on concentra-tions or inventories of radionuclides and requirements on physicaland chemical properties of waste forms Operating limits maychange during the period of waste disposal in response to improvedunderstanding of system behavior or changes in facility design,properties of waste, waste containers, waste emplacement, or theclosure concept Any changes in operating limits must be supported

by revisions of a performance assessment to incorporate such newinformation

2.3 General Principles of Performance Assessment

This Section presents a discussion of general principles of formance assessment, based on the definition given in the previoussection It is important to recognize that performance assessment

per-is essential to management of a dper-isposal site and waste dper-isposaloperations An understanding of principles of performance assess-ment can reduce the potential for misinterpretation of results, andcan lead to an efficient process and defensible product Generalprinciples of performance assessment discussed in this Section areidentified as follows:

• Performance assessment should be an iterative, flexibleprocess of integrating modeling, data collection, and designactivities in a manner that identifies those aspects of

engineered and natural barriers in a disposal system ofimportance to a decision about compliance with perfor-mance objectives.

• Performance assessment is a process intended to providereasonable assurance of compliance with performanceobjectives

• Since models and parameters used in performance ment are uncertain and some processes that affect systemperformance may not be well understood, a variety ofresults should be presented rather than a single projectedoutcome

assess-• An integration and interpretation of assumptions andresults, in which bases for conceptual models and results ofcalculations are presented in a manner that reflects thejudgments involved and the importance of different aspects

of an assessment to a licensing decision, is a critical aspect

of performance assessment

In general, the performance assessment process provides a means

of building confidence in judgments and models used to determinewhether reasonable assurance of compliance with performanceobjectives can be obtained

2.3.1 Performance Assessment as an Iterative Process

Characterization of performance assessment as a process,rather than a set of calculations, is important An early view wasthat site characterization should be completed and a conceptualfacility design developed, after which a performance assessment

would be conducted to support a license application (Starmer et al.,

1988)

However, experience has shown that efficient application of theperformance assessment process involves substantial interactionbetween modeling and data collection and facility design activities.These interactions occur during the period prior to licensing andduring facility operations and are dynamically linked That is, sitedata and information on facility design are needed to make model-ing assumptions and to assign parameter values, evaluation ofmodeling results can lead to an identification of additional dataneeds or design changes, and these in turn can lead to use of alteredassumptions and parameter values in modeling Introduction ofunanticipated wastes can lead to changes in modeling assumptionsand facility design In general, feedback between different activi-ties identifies aspects of the site, design or models for which altered

assumptions are needed to improve projected performance in adefensible manner Thus, performance assessment is a manage-ment tool to be used throughout the preclosure phase of facilityoperations Performance assessment also is a useful tool for riskmanagement during the postclosure time phase It can be used todetermine needs for institutional control at a site, and it can beused to guide monitoring activities that normally are undertakenafter facility closure Such monitoring activities can provide addi-tional information on the validity of a performance assessment insupporting a licensing decision.

Important aspects of data collection, facility design, and ing are best identified using a flexible, iterative process that allowsfeedback from each activity to be incorporated in subsequent itera-tions of other activities (Case and Otis, 1988; DOE, 1985; IAEA,

model-1997a; 1999; Kozak, 1994a; Kozak et al., 1993; NRC, 2000; Seitz

et al., 1992a) Efficiency is improved by starting with simple

anal-yses and using findings of those analanal-yses to identify areas thatrequire more detailed consideration in subsequent analyses Sensi-tivity analysis is an important contributor to this feedback.Sensitivity analysis provides an increased understanding of partic-ular aspects of a disposal system that influence overall perfor-mance, and it helps to provide justification for those aspectsfor which additional effort would provide the most benefit to aperformance assessment and an appropriate basis for a licensingdecision

2.3.2 Performance Assessment as a Decision Tool

Performance assessment is an essential tool for regulatory sion making Performance assessment is used to identify conditions

deci-at a disposal facility thdeci-at support a finding of reasonable assurance

of compliance with performance objectives An understanding ofhow this use of performance assessment differs from its application

to the more difficult problem of predicting the actual long-term formance of disposal systems is critical to the conduct of perfor-mance assessment and associated data-collection activities and toproper interpretation of results

per-In the context of performance assessment, it is desirable todevelop conceptual models of the long-term performance of disposalsystems that bound the range of reasonably foreseeable conditions.Thus, performance assessment can be used, in an inverse sense,

to identify conditions that may cause performance objectives to beexceeded This places the emphasis on defending why such condi-tions are not likely to occur or on needed changes in facility design

to minimize the potential for such conditions to occur Because

of inherent uncertainties in any performance assessment, ment will be necessary in assessing the defensibility of conceptualmodels

judg-2.3.3 Uncertainty in Results of Performance Assessment

Models and parameters used in performance assessment arecharacterized by varying degrees of uncertainty Thus, perfor-mance assessment generally should provide more than a singleresult in demonstrating compliance with performance objectives,with different results developed on the basis of a variety of plausi-ble assumptions For example, although a single set of results mayprovide the primary basis for a comparison with performance objec-tives, additional results should be provided in the context of asensitivity and uncertainty analysis to lend credence to the demon-stration of compliance

The type of uncertainty of primary concern to this Report is

uncertainty of importance to a licensing decision, rather

than uncertainty in the actual outcome (projected dose) To size this distinction, this Report uses the term “importanceanalysis” (Section 7) Importance analysis is used to identifyassumptions and parameter values in a performance assessmentthat have an impact on a decision regarding compliance with per-formance objectives This information can then be used to identifyareas where further data, design enhancements, or modeling areneeded to reduce uncertainty (increase confidence) in a decision.Since results of performance assessment depend on assumptions,data and design of a facility, changes in any of these can result inchanges in conclusions resulting from an analysis

empha-2.3.4 Integration and Interpretation of Results

Given the inexact nature of performance assessment and theneed to consider a variety of conditions in an assessment and, thus,

to produce a range of projected outcomes, it is logical to ask howthose outcomes can be interpreted for comparison with fixed perfor-mance objectives An essential but highly challenging aspect

of performance assessment is an integration and presentation ofresults in a manner that facilitates this interpretation

The term “integration” refers to a need to summarize a variety

of results and to identify assumptions and data that are important

to a licensing decision, as determined from the analyses performed.The term “interpretation” refers to a need to present resultsand critical assumptions in a manner that provides the basis for