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Monte Carlo Simulation Research on the Spontaneous Fission Yield of 240Pu PDF

104 Pages·2015·2.16 MB·English
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Monte Carlo Simulation Research on the 240 Spontaneous Fission Yield of Pu 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 240 spontaneous fission yield of Pu 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 240 use one or more of the spontaneous fission products from Pu as a radiochronometer. However, the fission product inventory from the 240 spontaneous fission of Pu 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. 240 The neutron flux distribution from a source containing Pu 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 2 240 and the induced fission probability associated with Pu. 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 240 fission product yield for any source geometry containing Pu. Minimizing variations in the source geometry should stabilize the combined fission product yield and provide a means to determine the spontaneous fission 240 probability for Pu. This research has identified that the combined fission 97 138 product yields from Zr and Xe 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 1 one of the three primary fissile isotopes (uranium-233 and uranium-235 are 2 the other two), 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 3 metastable states, all with have half-lives less than one second. 7 1.1.1 Weapons-grade Plutonium The primary component of nuclear weapons is uranium or plutonium metal, 233 235 239 enriched in a fissile isotope ( U, U or Pu). These isotopes are defined 4 as Special Nuclear Material (SNM). 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 240 fission neutrons from Pu may cause a premature detonation of the device 4 unless the weapon design accommodates this neutron source. The commercial nuclear power industry is a significant contemporary source 238 of SNM. Plutonium is created by irradiation of U in a nuclear reactor, under tailored conditions of neutron bombardment and post irradiation decay, followed by radiochemical reprocessing. A major difference in used commercial nuclear fuel and weapons-grade plutonium is the concentration 240 of Pu. Plutonium is considered as “weapons grade” if it contains at least 239 240 5 93% Pu and <7% Pu. Reactor grade plutonium contains more than 8% 240 Pu. Plutonium is also used for civilian nuclear power production in 239 mixed-oxide (MOX) fuel, which nominally contains 60% Pu and 25% 240 6 Pu. 8 Weapons grade plutonium is generated in nuclear fuel that has been irradiated for no more than 7 months. The economics of commercial nuclear power plants dictates that fuel be retained in a reactor core for about 4 years to achieve optimum performance and cost effectiveness. The typical 1-GW e nuclear reactor uses about 1 ton of fissile material per year which generates about 200 kg of plutonium. The annual global production of plutonium in 7 commercial nuclear power reactors is about 70 t. 1.1.2 Spontaneous Fission In nuclear physics and chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei). The fission process also produces fast neutrons and photons (as gamma rays) with the release of a large amount of energy. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass 9

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