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NuREG/cR-5550 LA-UR-90-732

Passive Nondestructive Assay of Nuclear Materials

Date Published: March 1991

Edited bv: Doug Reilly, Norbert Ensslh and Hastings smith) Jr. Technical Edit&: Sarah Kreiner

Contractor: Los Alamos National Laboratory Post Office Box 1663, MS E540 Los Alamos, NM 87545

Prepared for: Office of Nuclear Regulatory Research U.S. Nuclear Regula~ory Commission e --Washington, DC 20555

NRC FIN A724

.K

NOTfCE ThisbookwaspreparedasanaccountofworksponsoredbyanagencyoftheUnitedStatesGovernment.Neitherthe UnitedStatesGovernmentnorany agencythereof,or anyof theiremployees,makesanywarranty,expressedor implied,orassumesanylegalIiabilityorresponsibilityforanythirdparty’suae,or the resultsof suchuse,of anyinformation,apparatusproductor processdisclosedinthisbook,or representsthat itsusebysuchthirdpartywouldnotinfringeprivatelyownedrights.

Availableas NUREG/CR-555t)from, The Superintendentof Documents U.S.GovernmentPrintingOffice

P.O.~X 37082 Washington,DC20013-7082

NationalTechnicalInformationService 5285Port RoyalRoadSpringfield,VA 22161

ISBN 0-16-032724-5

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PREFACE

This book is a general reference on the theory and application of passive non-destructive assay (NDA) techniques, or PANDA. It is part of a four-volume set on nuclear material measurement and accountability sponsored by the US Nuclear ,Reg-ulatory Commission (NRC). Although we discuss a few active NDA techniques, they have been treated in detail in another book in the NRC series authored by T. Gozani.

The book’s intended audience ranges from NDA neophytes to experienced practi-tioners. While the major motivation to write this book was, provided by the NRC, there has long been a desire at Los Alrunos to prepare a text of this kind. Many of the techniques and instruments described herein were developed at I+ Alamos, and we welcome the opportunity to describe the techniques ,more completely than is possible in reports or papers.

We hope that you will find this text a useful and lasting reference to the interesting subject of passive NDA.

Doug Reilly, Norbert Ensslin, and Hastings Smith, Jr.

... 111

ACKNOWLEDGMENTS

The writing and preparation of this book was funded by the Office of Nuclear Regulatory Research, U. S. Nuclear Regulatory Commission, and we would like to gratefully acknowledge their support. We would especially like to thank Dr. Sandra Frattali of the Division of Safeguards for her patience and help in guiding and nurturing this project over many years. We would also like to acknowledge the ongoing support of the U.S. Department of Energy, Office of Safeguards and Security, under which the techniques and instrumentation described in this book were developed.

Many ~eople have been involved in the production of this book. It was truly a team effort in which every participant played a vital role. The majority of the participants belong to the Safeguards Assay Group at Los Alamos National Laboratory; however, significant contributions were also made by members of the Advanced Nuclear Tech-nology Group of the Laboratory, the University of Arizona, and Fort Lewis College. The contributing authdrs are listed at the beginning of the book and on the individual chapters: We would like to acknowledge the help of William B. Wilson (Applied Nuclear Science Group) on Chapter 11 and Stanley Simmonds on Chapter 14.’ Walt Strohm of Mound Laboratory and Ron Perry of Argonne National Laboratory provided essential information to Chapters 21 and 22 on calorimetry.

This book has benefited greatly from the peer reviewers who provided the con-tributing authors with corrections and suggestions for improvement. We would like to thank the following reviewers: James Cadieux (Westinghouse Savannah River Corp.), Paul Cloessner (WSRC), Raymond Dewbemy (WSRC), John Fleissner (Rocky Flats Plant), Wllly Higinbotham (Brookhaven National Laboratory), WMiam Laing (Oak Ridge National Laboratory), and Samuel Untermeyer (Private Consultant).

Douglas Reilly, Norbert Ensslin, and Hastings Smith served as project leaders and principal technical reviewers as well as writers of several chapters. Sarah Kreiner was the technical editor throughout the long course of producing the volume. Much of the credit for the readabilityy of the text goes to Sarah’s careful and meticulous editing. Any volume of this size demands a great deal of work from word processors. We were ably served by Sophia Howard and Celina Ortiz who graciously put up with the many changes to the text. The photocomposition and final editing were done by Joyce A. Martinez of the Systems Technology Support Group and Martha Lee DeLanoy of the Writing and Editing Group at Los Alamos.

To all of these people we wish to express our heartfelt thanks.

iv

CONTRIBUTING AUTHORS

Unless otherwise noted, the authors are affiliated with the Los Alamos National Laboratory.

Michael P. Baker (Chapter 13)

Thomas W. Crane (Chapter 13)

Norbert Ensslin (Chapters 11, 16, 20, 23)

Paul E. Fehlau (Chapter 19)

Robert N. Likes, Fort Lewis College, Durango, Colorado (Chapters 21, 22)

Marcia C. Lucas (Chapter 3)

Howard O. Menlove (Chapter 17)

Michael C. Miller (Chapter 10)

George W. Nelson, University of Arizona, Tucson, Arizona (Chapter 2)

Jack L. Parker (Chapters 4, 5, 6)

John R. Phillips (Chapter 18)

T. Douglas Reilly (Chapters 1, 2, 23)

Phil M. Rinard (Chapter 12)

Phyllis A. Russo (Chapter 9)

Thomas E. Sampson (Chapter 8)

Hastings A. Smith, Jr. (Chapters 3,7,9,20, 23)

James K. Sprinkle (Chapter 15)

James E. Stewart (Chapter 14)

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Introduction

The term nondestructive assay (NDA) is applied to a series of measurement tech-niques for nuclear fuel materials. The techniques measure radiation induced or emitted spontaneously from the nuclear material; the me~urements are nondestructive in that they do not alter the physical or chemical state of the nuclear material. In some cases, the emitted radiation is unique to the isotope(~) of interest and the radiation intensity can often be related to the mass of the isotopes. Gther techniques to mea-sure nuclear material involve sampling the material and analyzing the sample with destructive chemical procedures. NDA obviates the need for sampling, reduces oper-ator exposure, and is much faster than chemical assay unfortunately NDA is usually less accurate than chemical assay. The development of NDA reflects a trend toward automation and workforce reduction that is occurring throughout our society. NDA measurements are applied in all fuel-cycle facilities for material accounting, process control, criticality control, and perimeter monitoring.

The original impetus for NDA development was the need for increased nuclear material safeguards. As safeguards agencies throughout the world needed more nu-clear material measurements,’ it became clear that rapid measurement methods were required that would not alter the state of nuclear materird items. Development efforts to address these needs were supported by the US Nuclear Regulatory Commission, the Depamnent of Energy, and the International Atomic Energy Agency. Rapid nonde-structive measurement techniques are required by the safeguards inspectors who must veriij’ the inventories of nuclear material held throughout the world.

NDA techniques are characterized as passive or active depending on whether they measure radiation from the spontaneous decay of the nuclear material or radiation induced by an external source. This book emphasizes passive NDA techniques, al-though certain active techniques like gamma-ray absorption densitometry and x-ray fluorescence are discussed here because of their intimate relation to passive assay techniques.

The principal NDA techniques are classified as gamma-ray assay, neutron assay, and calorimetry. Gamma-ray assay techniques are treated in Chapters 1-10. Chapters 1-6 deal with basic subjects including the origin of gamma rays, gamma-ray interac-tions, detectors, instrumentation, and general measurement principles. Chapters 7-10 cover applications to uranium enrichment, plutonium isotopic composition, absorption densitometry, and x-ray fluorescence.

vii

Neutron assay techniques are the subject of Chapters 11-17. Chapters 11–13 cover the ongin of neutrons, neutron interactions, arid neutron detectors. Chapters 14-17 cover the theory and applications of total and coincidence neutron counting.

Chapter 18 deals with the assay of irradated nuclear fuel, which uses both gamma-ray and neutron assay techniques. Chapter 19 covers perimeter monitoring, which uses gamma-ray and neutron detectors of high sensitivity to check that no unauthorized nuclear material crosses a facility boundary. The subject of Chapter 20 is attribute and semiquantitative measurements. The goal of these measurements is a rapid verification of the contents of nuclear material containers to assist physic+ inventory verifications. Waste and holdup measurements are also treated in this chapter. Chapters 21 and 22 cover calorimetry theory and application, and Chapter 23 is a brief application guide to illustrate which tecliniques can be used to solve certain measurement problems. Appendices A-C contain information on statistical treatment of assay data, radiation safety, and on criticality safety.’

...VIII

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Contents

TABLE OF CONTENTS

Chapter 1: The Origin of Gamma Rays

1.1 Gamma Rays and the Electromagnetic Spectrum 1 1.2 Characteristics of Nuclear Decay 3

1.2.1 Nuclear Decay Processes Geneml 3 1.2.2 Alpha Decay 4 1.2.3 Beta My 5

1.3 X-Ray Production 8 1.3.1 The Bohr Model of the Atom 8 1.3.2 X-Ray Production Mechanisms 9 1.3.3 Characteristic X-Ray Spectra 10

1.4 Major Gamma Rays from Nuclear Material 10 1.4.1 Typical Spectra 10 1.4.2 Major Gamma-Ray Signatures for Nuclear Material Assay 11 1.4.3 Fission-Product Gamma Rays 18 1.4.4 Background Radiation 19

1.5 Additional Gamma-Ray Production Reactions 22 1.5.1 Bremsstrahlung (Braking Radiation) 22 1.5.2 Particle Reactions 24

Chapter 2: Gamma-Ray Interactions with Matter

2.1 Introduction 27 2.2 Exponential Attenuation 27

2.2.1 The Fun&mental Law of Gamma-Ray Attenuation 27 2.2.2 Mass Attenuation Coefficient 30

2.3 Interaction Processes 31 2.3.1 Photoelectric Absorption 31 2.3.2 Compton Scattering 33 2.3.3 Pair Production 36 2.3.4 Total Mass Attenuation Coefficient 38

2.4 Filters 40 2.5 Shielding 41

Chapter 3: Gamma-Ray Detectors

3.1 Introduction 43 3.2 Types of Detectors 43

3.2.1 Gas-Filled Detectors 43

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Contents

3.2.2 Scintillation Detectors 45

3.2.3 Solid-State Detectors 46

3.3 Characteristics of Detected Spectra 51 3.3.1 Generic Detector Response 51 3.3.2 Spectml Features 53 3.3.3 Det@or Resolution 55 3.3.4 Detector Efficiency 58

3.4 Detector selection 62

Chapter 4: Instrumentation for Gamma-Ray Spectroscopy

4.1 Introduction 65 4.2 Selection of Detector 66 4.3 High-Voltage Bias Supply 68 4.4 Warnplifier 69 4.5 Amplifier 73

4.5.1 Pole-Zero Compensation Circuit 76 4.5.2 Baseline Restoration Circuit 76 4.5.3 Pileup Rejection Ckuit 78 4,5.4 Advanced Concepts in Amplitler Design 80

4.6 Single-Channel Analyzer 80 4.7 Counters, Scalers, Timers, and Ratemeters 82 4.8 Multichannel Analyzer 84

4.8.1 Analog-to-Digital Converter 85 4.8.2 Spectrum Stabilizers 88 4.8.3 Multichannel Analyzer Memory, Display, and Data Analysis 89

4.9 Auxiliary Electronic Equipment 91 4.10 Concluding Remarks 92

Chapter 5: General Topics in Passive Gamma-Ray Assay

5.1 Energy Calibration and Determination of Peak Position 95

5.1.1 Introduction 95

5.1.2 Linear Energy Calibration 100

5.1.3 Determination of Peak Position (Centroid) 101

5.1.4 Visual Determination of Peak Position 104

5.1.5 Graphical Determination of Peak Position 104

5.1.6 Determination of Peak Position by the First Moment Method 105

5.1.7 Determination of Peak Position by the Five-Channel Method 105

5.1.8 Determination of Peak Position by a Linearized Gaussian Fit 106

5.1.9 Determination of Peak Position Using a Parabolized Gaussian Fit 109

5.1.10 Determination of Peak Position Using Complex Spectral Fitting Codes 113

Conlenls

5.2 Detector Resolution Measurements 113 5.2.1 Intrwiuction 113 5.2.2 Determination of Peak Width by Visual Estimation from

MCA Display 115 5.2.3 Gmphical Determination of Peak Width 116 5.2.4 Determination of Peak Width Using Analytical Interpolation 117 5.2.5 Determination of Peak Width Using the Second-Moment

Method 119 5.2.6 Determination of Peak Width Using a Linearized Gaussian Fit 119 5.2.7 Determination ofPeak Width Using a Parabolized Gaussian Fit 120

5.3 Determination of Full-Energy-Peak Area 120 5.3,1 Introduction 120 5.3.2 Selection of Regions of Interest (ROI) 120 5.3.3 Subtraction of Straight-Line Compton Continuum 121 5.3.4 Subtraction of Smoothed-step Compton Continuum 124 5.3.5 Subtraction of Compton Continuum Using a Single

Region of Interest 124 5.3.6 Subtraction of Compton Continuum Using Two-Standard

Procedures 126 5.3.7 Using Region-of-Interest Sums to Measure Peak Areas 127 5.3.8 Using Simple Gaussian Fits to Measure Peak Areas 130 5.3.9 Using Known Shape Parameters to Measure Peak Areas

in Multiples 131 5.3.10 Using Complex Fitting Codes to Measure Peak Area 133

5.4 Rate-Related Losses and Corrections 134 5.4.1 Introduction 134 5.4.2 Counting Loss as a Function of Input Rate 135 5.4.3 Actual Data Throughput 138 5.4,4 Correction Methods General 142 5.4.5 Pileup Correction Methods: Electronic 142 5.4.6 Pulser-Bhsed Corrections for Deadtime and Pileup 143 5.4.7 Referende-Source Method for Deadtime-Pileup Confections 146

5.5 Effects of the Inverse-Square Law 150 5.6 Detector Efficiency Measurements 153

5.6.1 Absolute Full-Energy-Peak Eftlciency 153 5.6.2 Intrinsic Full-Energy Efficiency 153 5.6.3 Relative Efficiency 155 5.6.4 Efficiency Relative to a 7.65-cm by 7.65-cm NaI(Tl) Detector 156 5.6.5 Efficiency as Function of Energy and Position 156

Chapter 6: Attenuation Correction Procedures

6.1 Introduction 159 6.2 Pmcedum 160

6.2.1 Preliminary Remarks 160

xi

Contents

6.2.2 General Description of Assay Procedure 160 6.2.3 Necessary Assumptions for Determining the Self-Attenuation

Correction Factor 161 6.2.4 Methods for Determining the Sample Lhwar Attenuation

Coefficient 164 6.3 Formal Definition of Conection for Self-Attenuation 165

6.3.1 The General Definition 165 6.3.2 Useful Speeified Shapes 166

,6.4 Important ~ameters of the Self-Attenuation Correction Factor 167 6.5 Analytic Far-Field Forms for the Self-Attenuation Correction Factor 168

6.5.1 Box-Shaped Samples 169 6.5.2 Cylindrical Samples 170 6.5.3 Spherical Samples 171

6.6 Numeric Computation in the Near Field 171 6.6.1 General Discussion 171 6.6.2 A Useful One-Dimensional Model 172 6.6.3 A Useful Two-Dimensional Model 174 6.6.4 A ThreUlmensional Model 177 6.6.5 Approximate Forms and Interpolation 178 6.6.6 The Effects of Absolute and Relative Error in the

Self-Attenuation Correction Factor 179 6.6.7 Precision of Self-Attenuation Correction Factor

and Total Correeted Rate 181 6.7 Factors Governing the Required Number of Standards 182 6.8 Gamma-Ray Ratio Methods 184 6.9 Assay Exanqies 185

6.9.1 Useful Gamma-ray Combinations for Assay, Transmission, and Refemnee Peaks 185

6.9.2 Interpolation and Extrapolation of Transmission 185 6.9.3 Uranium-235 Assay in Far-Field Geometry 187 6.9.4 Plutonium-239 Solution Assay in Near-Field Geometry 189 6.9.5 Transmission-Corrected Segmented Scanning 190

Chapter 7: The Measurement of Uranium Enrichment

7,1 Introduction 195 7.2 Radiations fkom Uranium Samples 196 7.3 Infhite-Sample Gamma Measurement Technique 196

7.3.1 One-Component Example (Uranium Metal) 200 7.3.2 Two-Component Example (Uranium and Matrix Material) 200 7.3.3 Instrumentation and Infinite-Sample Technique Applications 202

7.4 Peak-Ratio Techniques 205 7.4.1 ‘fheory 205 7.4.2 Applications 206

xii

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Contents

7.4.3 Summary of Peak-Ratio Techniques 207 7.5 Gas-Phase Uranium Enrichment Measurement Techniques 207 7.6 Neutron-Based Enrichment Measurements 210 7.7 Container Wall Attenuation Corrections 211

7.7.1 Direct Measurementof Wall Thickness 211 7.7.2 Intemal-Lme Ratio Technique 212 7.7.3 Measurement of U’F6Cylinders 214

7.8 Extension of the Enrichment Meter Principle to other Applications 215 7.8.1 Concentration Meter 215 7.8.2 Mixing (Blend) Ratio in Mixed-Oxide Fuel 216

Chapter 8: Plutonium Isotopic Composition by Gamma-Ray Spectroscopy

8.1 Introduction 221 8.2 Background 221

8.2.1 Decay Characteristics of Plutonium Isotopes 221

8.2.2 Decay Characteristics of 241Pu 221

8.2.3 Determination of 242Pu Concentration 223 8.2.4 Spectral Interferences 223 8.2.5 Applications of Plutonium Isotopic Measurements 223

8.3 Spectral Regions Useful for Isotopic Measurements 225 8.3.1 The 40-keV Region 225 8.3.2 The 100-keV Region 230 8.3.3 The 125-keV Region 233 8.3.4 The 148-keV Region 234 8.3.5 The 160-keV Region 235 8.3.6 The 208-keV Region 238 8.3.7 The 332-keV Region 238 8.3.8 The 375-keV Region 240 8.3.9 The 640-keV Region 242

8A Measurement Principles 243 8.4.1 Measurement of Isotopic Ratios 243 8.4.2 Measurement of Absolute Isotopic Mass 248

8.4.3 M2Pu Isotopic Correlation 248 8.5 Dara Acquisition 249

8.5.1 Electronics 249 8.5.2 Deteetors 250 8.5.3 Filters 250 8.5.4 Count Rate and Samph#Detector Geometry 251 8.5.5 Count Time 251

8.6 Speetral Analysis 251 8.6.1 Region-of-Interest Summation “ 252

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Contents

8.6.2 Peak Fitting 252 8.6.3 Response-Function Analysis 254

8.7 Implemented Systems , 254 8.7.1 Rockwell-Hanford Company 255 8,7.2 Los Akunos National Laboratory 256 8.7.3 Mound Facility 261 8.7.4 Lawrence Livennore National Laboratory 263 8.7.5 Summary of Measunment Precision 267

Chapter 9: Densitometry

9.1 Introduction 273 9.2 Single-Line Densitometry 274

9.2.1 Concentration and Thickness Gauges 274 9.2.2 Measurement Precision 275

9.3 Multiple-Energy Densitometry 277 9.3.1 Analysis of Two-Energy Case 277 9.3.2 Measurement Precision 278 9.3.3 Extension to More Energies 278

9.4 Absorption-Edge Densitometry 278 9.4.1 Description of Measurement Technique 280 9.4.2 Measurement Precision 282 9.4.3 Measurement Sensitivity 285 9.4.4 hhliX Effects 286 9.4.5 Choice of Measurement Technique 288 9.4.6 Transmission Sources 288

9.5 Single-Line Densitometers 289 9.6 Dual-Line Densitometers 290 9.7 Absorption-Edge Densitometers 291

9.7.1 K-Absoqnion-Edge Densitometers 291 1. Los Alamos National Laboratory 294 2. Oak Ridge Y-12 Plant 294 3. Allied Genwal Nuclear Services (AGNS) 295 4. Power Reactor and Nuclear Fuel Development

Corporation (PNC) 295 5. Savannah River Plant (SRP) 297 6. IAEA Safeguards Analytical Laboratory 299 7, Kernforschungszentrum Karlsruhe (KfK) 301

9.7.2 L~-Absorption-Edge Densitometers 303

1. Savamjah River Laboratory (SRL) 303 2. New Brunswick Laboratory (NBL) 304 3. Allied General Nuclear Services (AGNS) 304 4. Los Alamos Nationat Laboratory 306

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313

Contents

Chapter

10.1 10.2

10.3 10.4

10.5

Chapter

11.1 11.2 11.3 11.4 11.5 11.6 11,7

Chapter

12.1 12.2

12.3

12.4 12.5 12.6 12.7

10: X-Ray Fluorescence

Introduction Theory 314 10.2.1 X-Ray Production 10.2.2 Fluomcence Yield 10.2.3 Photon Transmission 10.2.4 Measurement Geometry Types of Sources Correction for Sample Attenuation 10.4.1 Effects of Sample Attenuation 10.4.2 Geneml Assay Equation 10.4.3 Attenuation Correction Methods Applications and Instrumentation

11: Origin of Neutron Radiation

Introduction Spontaneous and Induced Nuclear Fkion Neutrons and Gamma Rays from Fkssion Neutrons from (a, n) Reactions Neutrons from Other Nuclear Reactions Isotopic Neutron Sources Conclusions

12: Neutron Interactions with Matter

Introduction Microscopic Interactions 12.2.1 The Cross-Section Concept 12.2.2 The Energy-Velocity Relationship for Neutrons 12.2.3 Types of Interactions 12.2.4 Energy Dependence of Cross Sections Macroscopic Interactions 12.3.1 Macroscopic Cross Sections 12.3.2 Mean Free Path and Reaction Rate Effects of Moderation in Bulk Matter Effects of Multiplication in Bulk Matter Neutron Shielding Transport Calculational Techniques 12.7.1 Monte Carlo Techniques 12.7.2 Discrete Ordinates Techniques

xv

314 315 315 318 320 324 324 324 327 329

337 337 340 344 349 351 353

357 357 357 358 359 361 363 363 366 367 372 374 375 375 377

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Chapter 13: Neutron Detectors

13. I M@hanisms for Neutron Detection ,13.2 General Properties of Gas-FWxl Deteetors

13.3 Gamma-Ray Sensitivity of Neutron Detectors 13.4 Gas-Filled Detectors

13.4.1 3Hc and BF3 Thermal-Neutron Detectors

13.4.2 4He and CI-LIFast-Neutron Detectors

13.4.3 Fission Chambers

13.4.4 10B-Lined Detectors 13.5 Plastic and Liquid Scintillators

13,5.1 Background 13.5.2 Neutron and Gamma-Ray Interaction Mechanisms 13.5.3 Pulse-Shape Discrimination

13.6 Other Types of Neutron Detectors 13.7 Measurement of Neutron Energy Spectra

13.7.1 Background )3.7.2 Techniques

Chapter 14: Principles of Total Neutron Counting

14.1 Introduction 14.1.1 Theory of Total Neutron Counting 14.1.2 Comparison of Total and Coincidence Counting

14.2 Primary Neutron Production Sources 14.2.1 Plutonium Compounds 14.2.2 Uranium Compounds 14.2.3 Impurities 14.2.4 Neutron Energy Spectrum Effects 14.2.5 Th-Target Effects

14.3 Neutron Transport in the Sample 14.3.1 Leakage Multiplication 14.3.2 Leakage Spectra

14.4 Neutron Detection Efficiency

14.4.1 3He Counter Arrangement 14.4.2 Moderator Design 14.4.3 Effect of Neutron Energy Spectrum

Chapter 15: Total Neutron Counting Instruments and

15.1 Introduction 15.2 The Shielded Neutron Assay Probe (SNAP)

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Contenls

379 380 384 386

386

391

393

395 396 396 396 398 401 404 404 404

407 407 408 409 409 412

415 418 419 420 422 425 427

427 428 429

Applications

435 435

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Contents

15.2.1 Verification of ,Plutonium Metal 437 15.2.2 Verification of ~ti Cylinders 438

15.2.3 Holdup Measurements 439 15.2.4 Other Applications 440

15.3 Slab Detectors 440 15.3.1 Monitoring of ~6 Enrichment 440

15.3.2 Holdup Measurements 442

15.4 The 47cCounter 443

15.4.1 Box Counter 443

15.4.2 Measurement of 238Pu Heat Sources 444 15.5 Measurement of Low-Level Waste 445

15.5.1 Detection Sensitivity 446 15.5.2 Assay of 55-Gallon Drums 447 15.5.3 Assay of Large Crates 447

15.6 Special Applications 448

15.6.1 The 252Cf Hydrogen Analyzer 449 15.6.2 Moisture Determination by Detector Ring Ratio 450 15.6.3 Energy-Independent Long Counter 451

Chapter 16: Principles of Neutron Coincidence Counting

16.1 Introduction 457 16.2 Characteristics of Neut.mn Pulse Trains 458

16.2.1 Ideal and Actual Pulse Trains 458 16.2.2 The Interval Distribution 460 16.2.3 The Rossi-Alpha Distribution 461

16.3 Basic Featurq of Coincidence Circuits 461 16.3.1 Electronic Gates 461 16.3.2 Updating and Nonupdating Deadtimes 462 16.3.3 Cross-Correlation and AutoCorrelation Circuits 463

16.4 Three Common Coincidence Circuits 464 16.4.1 Variable Deadtime Circuit 464 16.4.2 Updating One-Shot Circuit 464 16.4.3 Reduced Variance Logic 465

16.5 The Shift Register Coincidence Circuit 466 16.5.1 Prhciples of Shift Register Operation 466 16.5.2 The R+A Gate 467 16.5.3 The A Gate 468 16.5.4 Net Coincidence Response R 469

16.6 Deadtime Corrections for the Shift Register 471 16.6.1 Detector and Amplifier Deadtimes 471 16.6.2 Bias Resulting from Pulse Pileup 471

xvii

Contents

16.6.3 Digital Deadtimes 472 16.6.4 Empirical Deadtime Correction Formulas 474 16.6.5 AMPTEK Electronics and Derandomizing Buffer 475

16.7 Uncertainties Resulting from Counting Statistics 476 16.7.1 Simple Emor Equation for the Shift Register 477 16.7.2 Uncertainties for Passive and Active Counting 477

16.8 Effects of Sample Self-Multiplication 478 16.8.1 Origin of Self-Multiplication Effects 479 16.8.2 Calculational Results 479 16.8.3 Effects on Shift Register Response 483 16.8.4 A Simple Correction Factor for Self-Multiplication 484 16.8.5 Applications and Limitations of the Simple Correction 485

16.9 Other Matrix Effects 486

Chapter 17: Neutron Coincidence Instruments and Applications

17.1 Neutron Coincidence System Design Principles 493 17.2 Passive Neutron Coincidence Systems 494

17.2.1 The 55-Oallon Barrel Counter 495 17.2.2 The High-Level Neutron Coincidence Counter (HLNCC) 497 17.2.3 The Upgraded High-Level Neutron Coincidence

Counter (HLNCC-11) 499 17.2.4 Special Detector Heads for FCA Coupons 502 17.2.5 Special Detector Heads for FBR Fuel 504 17.2.6 Inventory Sample Coincidence Counter (ISCC) 506 17.2.7 Counters for Bulk Plutonium Nitrate Solutions 510 17,2.8 The Dual-Range Coincidence Counter (DRCC) 512

17.3 Active Neutron Coincidence Systems 514 17.3.1 The Active Well Coincidence Counter (AWCC) 515 17.3.2 The Uranium Neutron Coincidence Collar (UNCC) 520 17.3.3 The Passive Neutron Coincidence Collar (PNCC) 521 17.3.4 Receipts Assay Monitor (RAM) for UF6 Cylinders 523

Chapter 18: Irradiated Fuel Measurements

18.1 Introduction 529 18.2 Characteristics of Reactor Fuel 530

18.2.1 Physical Description 530 18.2.2 Definition of Burnup and Exposure 531 18.2.3 Fission Product Yields 532

18.3 Indirect Signatures for Fuel Burnup 537 18.3.1 Physical Attributes 537 18.3.2 Cerenkov Radiation 537

. . .XVIII

Contents

18.3.3 Single Fission Product Gamma-Ray Activity 538 18.3.4 Total Gamma-Ray Activity 540 18.3.5 Fission Product Activity Ratios 541 18.3.6 Total Neutron Output 543

18.4 Gamma-Ray Measurement of Irradiated Fuel 546 18.4.1 Total Gamma-Ray-Activity Measurements 546 18.4.2 High-Energy Gamma-Ray Activity Measurements 547 18.4.3 High-Resolution Gamma-Ray Spectxuscopy 547 18.4.4 Cerenkov Radiation Measurements 549

18.5 Neutron Measurement of Irradiated Fuel 550 18.5.1 The Fork Detector and ION-1 Electronics Package 551 18.5.2 Neutron Meamuement of Burnup 552

18.6 Determination of the Fissile Content of Irradmted Fuel 554 18.6.1 Indhect Determination from Passive Burnup Measurements 555 18.6.2 Determination by Active Neutron interrogation 556

18.7 Summary of Nondestructive Techniques fbr Verification of hadia~ Fuel 557

Chapter 19: Perimeter Radiation Monitors

19.1 Introduction 563 19.2 Background Radiation Effects 564 19.3 Characteristics of Diversion and Contamination Signals 565

19.3.1 Radiation Sources 565 19.3.2 Time Varying Signals 569

19.4 Signal Analysis 569 19.4.1 Detecting Radiation Signals 569 19.4.2 Anrdog Detection Methods 570 19.4.3 Digital Detection Methods 571 19.4.4 Long-Term Monitoring 573

19.5 Radiation Detectom 573 19.5.1 Plastic Scintillators 573 19.5.2 (k+now Detectors 575

19.6 Perimeter Monitor Components 575 19.6.1 Components and Their Functions 575 19.6.2 Signal Electronics 576 19.6.3 Power Supplies 578 19.6.4 Diagnostic Tests 578

19.7 Monitor Calibration 579 19.7.1 Scintillation Detector Calibration 579 19.7.2 Single-Channel AnaIyzer Calibration 579 19.7.3 Periodc Calibration Checks 580

19.8 Monitor Evaluation Methods 580 19.9 Examples of Perimeter Monitors 581

xix

. .

Chapter

20.1 20.2 20.3

20.4

20.5

20.6

Chapter

21.1 21.2

21.3

21.4

21.5

19.9.1 Hand-Held Perimeter Monitors 19.9.2 Automatic Pedestian Monitors 19.9.3 Automatic Vehicle Monitors 19.9.4 Monitor Performance Summary

20: Attribute and Semiquantitative Measurements

Introduction Measurement of Nuclear Material Attributes Quantitative Screening of Waste 20.3.1 Purpose 20.3.2 Gamma-Ray and Neutron Sensitivities Confmatory Measurements 20.4.1 Purpose 20.4.2 Nondestructive Assay Options 20.4.3 Recent Expience Nuclear Material Holdup 20.5.1 Causes and Mechanisms 20.5.2 Magnitude of Holdup 20,5.3 Statistical Modeling The Art and Science of Holdup Measurements 20.6.1 Useful Radiation Signatures 20.6.2 Detectors and ReadoutInstrumentation 20.6.3 Holdup Measurement procedures 20.6.4 Point, Line, and Area Calibrations 20.6.5 Calibration Standards and Check Sources 20.6.6 Self-Absorption and Attenuation Corrections 20.6.7 Error Estimation

21: Principles of’ Calorimetric Assay

Introduction Heat Production in Radionuclides 21.2.1 Alpha Particles 21.2.2 Beta Particles 21.2.3 Gamma Rays 21.2.4 Other Emissions Specific Power Determination 21.3.1 Definition of Specific Power 21.3.2 Effective Specific Power Heat Measurement by Calorimetry 21.4.1 Adiabatic Calorimeters 21.4.2 Isothermal Calorimeters Types of Heat Flow Calorimeter

xx

Contents

581 582 583 584

589 590 591 591 592 593 593 593 595 596 596 598 599 601 602 604 606 607 609 610 611

617 618 618 619 619 620 620 620 621 623 624 624 625

Contenls

21.5.1 Common Electrical Features 625 21.5.2 Full Twin-Bridge Calorimeters 626 21.5.3 Over/Under Bridge Calorimeter 627 21.5.4 Gmdient Bridge Calorimeter 628

21.6 Methods of Operation for Heat Flow Calorimeter 631 21.6.1 Replacement Method 631 21.6.2 Differential Method 632 21.6.3 Constant Temperature Servo-Control Method 633

21.7 Assay Time Considerations 633 21.7.1 Calorimeter Design and Operating Method 634 21.7.2 Sample Preconditioning 635 21.7.3 End-Point Prediction 635

21.8 Calorimeter Calibration 636 21.8.1 Electrical Calibration 636 21.8.2 Calibration with Radbactive Heat Sources 637

21.9 Sources of Error 638

Chapter 22: Calorimetric Assay Instruments 22.1 Introduction 641

22.1.1 Calorimetric Assay System Components 641 22.1.2 Calorimeter Design Considerations 642

22.2 Small Calorimeters for Laboratory Samples 642 22.2.1 Mound Analytical Calorimeter 642 22.2.2 Argonne Small-Sample Calorimeter 644

22.3 Transportable Calorimeters 645 22.3.1 .Mound Transportable Calorimeter 645 22.3.2 Argonne Bulk Assay Calorimeter 647

22.4 In-Plant Calorimeters 649 22.4.1 Mound Twin-Bridge Production Calorimeter 649 22.4.2 Mound Calorimeter for Simultaneous Isotopics Measurements 649 22.4.3 Rocky Flats Production Calorimeters 652 22.4.4 Mound Gradient Bridge Calorimeters 653

22.5 Fuel Rod Calorimeters 655 22.5.1 Argonne One-Meter Fuel Rod Calorimeter 655 22.5.2 General Electric ti~atcd Fuel Assembly Calorimeter 656

Chapter 23: Application Guide 23.1 Introduction 661 23.2 Capabilities of Selected Passive and Active NDA Techniques 661 23.3 Nuclear Material Types and Applicable NDA Techniques 668

Appendix A: Statistical Treatment of Assay Data 679 Appendix B: Radiation Safety 685 Appendix C: Criticality Safety 689

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