Delayed Gamma-Ray Assay for Nuclear Safeguards By Vladimir Mozin A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering – Nuclear Engineering in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Jasmina Vujic, Chair Dr. Stephen J. Tobin Professor Kai Vetter Professor James Siegrist Fall 2011 Delayed Gamma-Ray Assay for Nuclear Safeguards Copyright 2011 by Vladimir Mozin 1 Abstract Delayed Gamma-Ray Assay for Nuclear Safeguards by Vladimir Mozin Doctor of Philosophy in Engineering – Nuclear Engineering University of California, Berkeley Professor Jasmina Vujic, Chair This dissertation addresses the need for new non-destructive assay instruments capable of quantifying the fissile isotopic composition of spent nuclear fuel and of independently verifying the declared amounts of special nuclear materials at various stages of the nuclear fuel cycle. High-energy delayed gamma-ray spectroscopy can provide the ability to directly assay fissile and fertile isotopes in the highly radioactive environment of the spent fuel assemblies and to achieve the safeguards goal of measuring nuclear material inventories for spent fuel handling, interim storage, reprocessing facilities, and final disposal and repository sites. The delayed gamma-ray assay concept is investigated within this context with the objective of assessing whether the delayed gamma-ray assay instrument can provide sufficient sensitivity, isotope specificity and accuracy as required in nuclear material safeguards applications. Preliminary system design analysis indicates that the delayed gamma-ray response is affected by multiple parameters: type and intensity of the interrogating source, the configuration of the interrogation setup, the time pattern of the interrogation, and the resolution and count rate limit of the gamma-ray detection system. 2 In order to handle the variety of factors associated with the delayed gamma-ray assay of spent nuclear fuel, a high-fidelity response modeling technique is introduced. The new algorithm seamlessly combines transport calculations with analytical decay/depletion, and discrete gamma-ray source reconstruction codes. Its performance was benchmarked in the dedicated experimental campaign involving accelerator-driven photo-neutron sources and samples containing fissile and fertile isotopes. Analytical estimations of the intensity of the delayed gamma-ray response and the passive background rate are utilized to develop a concept of the non-destructive instrument for the assay of spent nuclear fuel. The modeling technique is then applied to more detailed parametric study. These simulations included extensive spent fuel inventories, and accounted for realistic assay configurations and instrumentation. The results of this preliminary analysis indicate that the delayed gamma-ray assay of spent nuclear fuel assemblies can be performed with available neutron generator and detection technology. The sensitivity of the delayed gamma-ray spectra to the actinide content of the spent nuclear fuel is investigated. The simplest analysis of the delayed gamma-ray response is based on the analysis of integrated count rates and peak ratios. More powerful analytical and numerical methods are likely needed for determining the relative concentrations of fissile and fertile isotopes in samples with complex compositions. i Contents List of Figures ........................................................................................................ iv List of Tables ....................................................................................................... viii 1 Introduction ......................................................................................................... 1 1.1 Nuclear Nonproliferation and Safeguards ...................................................... 1 1.2 Spent Nuclear Fuel Safeguards ....................................................................... 3 1.3 Importance of Non-destructive Assay ............................................................ 7 1.4 Delayed Gamma-Ray Assay Potential............................................................ 9 1.5 Dissertation Outline ........................................................................................ 9 2 Theoretical Considerations .............................................................................. 13 2.1 Fission Process .............................................................................................. 13 2.2 Fission Yields ............................................................................................... 17 2.3 Delayed Gamma-Ray Time Dependence ..................................................... 21 2.4 Delayed Gamma-Ray Assay Concept .......................................................... 24 2.4.1 Interrogating Source ............................................................................... 28 2.4.2 Detector System ..................................................................................... 29 2.4.3 Interrogation Time Regime .................................................................... 31 3 Modeling approach ........................................................................................... 33 3.1 Algorithm description ................................................................................... 33 Contents ii 3.2 CINDER method .......................................................................................... 36 3.3 DGSDEF method .......................................................................................... 41 4 Experimental Benchmarking ........................................................................... 43 4.1 Motivation ..................................................................................................... 43 4.2 Experimental Arrangement ........................................................................... 46 4.3 Depleted Uranium Plate Experiment ............................................................ 51 4.4 Three Pu Coupons Experiment ..................................................................... 55 4.5 DU + 3 Pu Coupons Experiment .................................................................. 59 4.6 Passive Source Experiments ......................................................................... 64 5 Spent Fuel Assay Instrument Design Considerations .................................... 69 5.1 Design Methods and Limitations .................................................................. 69 5.2 Detector System Limitations ........................................................................ 75 5.3 Assay Instrument Parameters Evaluation ..................................................... 76 5.3.1 Delayed Gamma-Ray Response Rate Estimation .................................. 78 5.3.2 Passive Background Rate Estimation ..................................................... 84 5.4 Modeling Results .......................................................................................... 88 6 Delayed Gamma-Ray Response Analysis ...................................................... 104 6.1 Preliminary Considerations ........................................................................ 104 6.2 Total Fission Rate Methods ........................................................................ 105 6.3 Individual Isotopic Content Method ........................................................... 109 6.3.1 Derivation of the Exact Delayed Gamma-Ray Peak Ratio Expression112 6.3.2 Analysis of Delayed Gamma-Ray Peak Ratios .................................... 116 6.3.3 Numerical Approach to the Response Analysis ................................... 121 Contents iii Conclusions and Outlook ................................................................................... 127 Bibliography ....................................................................................................... 131 iv List of Figures 2.1 Thermal neutron fission cross-section for primary actinides ........................... 14 2.2 Photo-fission cross-sections for primary actinides .......................................... 15 2.3 Schematic of the fission process. ..................................................................... 16 2.4 Thermal chain fission yields for primary actinides .......................................... 19 2.5 Chain fission yields of primary actinides for fast and 14 MeV neutrons ......... 20 2.6 Fission products mass distributions for Th-232 photofissions at various incident photon energies......................................................................................... 20 2.7 Fission product half-lives temporal distribution. ............................................. 23 2.8 Cumulative thermal neutron fission yields for U-235 and Pu-239 .................. 25 2.9 ENDF/B-VII U-235 and Pu-239 thermal neutron induced fission product yields. Each point corresponds to an absolute yield of an isotope with unique A and Z ....................................................................................................................... 26 2.10 Passive gamma-ray spectrum of a PWR assembly with 32 GWd/t burnup and 9 months cooling time ............................................................................................ 30 3.1 Four-step delayed gamma-ray response modeling algorithm .......................... 34 4.1 Literature data benchmarking of the HEU delayed gamma-ray response: