Monte Carlo Simulation Research on the Spontaneous Fission Yield of 240Pu A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Nuclear & Radiological Engineering of the College of Engineering and Applied Sciences by Tianyou Xie University of Cincinnati Nov 2015 Committee Chair: Henry B. Spitz, Ph.D. 1 Abstract This research describes results of mathematical simulations to predict the spontaneous fission yield of 240Pu thereby guiding selection of a suitable fission product for use as a radiochronometer for weapons grade plutonium. The PUREX process is effective in separating plutonium and uranium in irradiated nuclear fuel from fission products, which may make it possible to use one or more of the spontaneous fission products from 240Pu as a radiochronometer. However, the fission product inventory from the spontaneous fission of 240Pu reported in the literature is quite variable. The non-uniform induced fission probability at multiple neutron energies makes the induced fission yield too complex to be listed as a single value. The neutron flux distribution from a source containing 240Pu is a combination of both spontaneous and induced fissions. Fission products generated in this source are due to a combination of spontaneous and induced fission processes, each with a unique neutron energy distribution and fission product yield. The physical conditions of the source (i.e., size, shape, etc.) will have a significant influence on the neutron flux distribution 2 and the induced fission probability associated with 240Pu. Thus, it is likely that there is a relationship between the combined (spontaneous and induced) fission yield and the physical conditions of the source. Exploring this relationship, a table of values will be generated to predict the combined fission product yield for any source geometry containing 240Pu. Minimizing variations in the source geometry should stabilize the combined fission product yield and provide a means to determine the spontaneous fission probability for 240Pu. This research has identified that the combined fission product yields from 97Zr and 138Xe exhibit less sensitivity to the physical source parameters than other fission products making them good candidates as radiochronometers for age dating weapons-grade plutonium. 3 4 Contents Abstract ............................................................................................................................... 2 Chapter 1 Introduction ........................................................................................................ 7 1.1 Introduction ............................................................................................................... 7 1.1.1 Weapons-grade Plutonium ................................................................................. 8 1.1.2 Spontaneous Fission........................................................................................... 9 1.1.3 Chronometer .................................................................................................... 11 1.1.4 Monte Carlo Method & MCNPX .................................................................... 13 1.1.5 Current Study ................................................................................................... 15 1.1.6 Challenge of Chronometer Study..................................................................... 20 1.2 Objectives and Specific Aims ................................................................................. 22 Chapter 2 Neutron Energy Effect on Fission .................................................................... 24 2.1 Background ............................................................................................................. 24 2.1.1 Fission Neutron Energy ................................................................................... 30 2.1.2 Simulations Using Fission Product Photopeaks .............................................. 31 2.1.3 Independent Fission Probability Study ............................................................ 32 2.2 Method .................................................................................................................... 34 2.2.1 Single Neutron Energy Model ......................................................................... 34 2.2.2 Varied Energy Selection .................................................................................. 35 2.3 Result & Discussion ................................................................................................ 36 2.4 Conclusions ............................................................................................................. 39 Chapter 3 Micro Position Model....................................................................................... 41 3.1 Background ............................................................................................................. 41 3.2 Method .................................................................................................................... 44 3.2.1 Uniform Thickness Model ............................................................................... 47 3.2.2 Uniform Volume Model .................................................................................. 51 3.2.3 Neutron Energy Distribution............................................................................ 52 3.3 Result & Discussion ................................................................................................ 53 3.3.1 Neutron Energy Distribution............................................................................ 53 3.3.2 Equal Thickness Model.................................................................................... 55 3.3.3 Equal Volume Model ....................................................................................... 58 3.3.4 Fission Yield Combination .............................................................................. 61 5 3.4 Conclusion .............................................................................................................. 62 Chapter 4 Macroscopic Parameter Model......................................................................... 64 4.1 Background ............................................................................................................. 64 4.1.1 Plutonium Fission Yield Curve ........................................................................ 64 4.2 Method .................................................................................................................... 65 4.2.1 Simulation Parameters Selection ..................................................................... 68 4.2.2 Macro Parameter Model .................................................................................. 69 4.3 Result & Discussion ................................................................................................ 73 4.4 Conclusion .............................................................................................................. 83 Chapter 5 Conclusion ........................................................................................................ 85 5.1 Research Accomplishments .................................................................................... 85 5.2 Future Work ............................................................................................................ 86 References ......................................................................................................................... 88 Appendix A ....................................................................................................................... 90 Appendix B ....................................................................................................................... 98 Appendix C ..................................................................................................................... 102 6 Chapter 1 Introduction 1.1 Introduction Plutonium is a radioactive actinide metal whose isotope, plutonium-239, is one of the three primary fissile isotopes1 (uranium-233 and uranium-235 are the other two), 2 Plutonium-241 is also highly fissile. A fissile isotope has a nucleus that may fission when struck by a thermal neutron. Twenty radioactive isotopes of plutonium have been characterized. The longest-lived are plutonium-244, with a half-life of 80.8 million years, plutonium-242, with a half-life of 373,300 years, and plutonium-239, with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight metastable states, all with have half-lives less than one second.3 7 1.1.1 Weapons-grade Plutonium The primary component of nuclear weapons is uranium or plutonium metal, enriched in a fissile isotope (233U, 235U or 239Pu). These isotopes are defined as Special Nuclear Material (SNM).4 The development of nuclear weapons has relied predominantly on the production of high-enriched uranium (HEU) or weapons-grade plutonium for fission primaries. However, spontaneous fission neutrons from 240Pu may cause a premature detonation of the device unless the weapon design accommodates this neutron source.4 The commercial nuclear power industry is a significant contemporary source of SNM. Plutonium is created by irradiation of 238U in a nuclear reactor, under tailored conditions