Mass and Shielding Optimization Studies for a Low Enrichment Uranium Fueled Kilopower Space Nuclear Reactor

Mass and Shielding Optimization Studies for a Low Enrichment Uranium Fueled Kilopower Space Nuclear Reactor

MASS AND SHIELDING OPTIMIZATION STUDIES FOR A LOW ENRICHMENT URANIUM FUELED KILOPOWER SPACE NUCLEAR REACTOR by Leonardo de Holanda Mencarini A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Nuclear Engineering). Golden, Colorado Date: Signed: Leonardo de Holanda Mencarini Signed: Dr. Jeffrey C. King Thesis Advisor Golden, Colorado Date: Signed: Dr. Mark Jensen Professor and Director Nuclear Science and Engineering Program ii ABSTRACT A Low-Enriched Uranium (LEU) fueled space reactor would avoid the security concerns inherent with Highly Enriched Uranium (HEU) fuel and could be attractive to signatory countries of the Non-Proliferation Treaty (NPT) or commercial interests. This thesis considers the feasibility of an LEU-fueled kilopower-class space reactor based on mass-optimization and shielding considerations. The HEU-fueled Kilowatt Reactor Using Stirling TechnologY (KRUSTY) serves as a basis for a similar reactor fueled with LEU fuel. Zirconium hydride moderator is added to the core in four different configurations (a homogeneous fuel/moderator mixture and spherical, disc, and helical fuel geometries) to reduce the mass of uranium required to produce the same excess reactivity, decreasing the size of the reactor. All three heterogeneous geometries yield a minimum mass reactor using a moderator/fuel ratio of 80 wt%. The lifetime is directly proportional to the initial amount of fissile material in the core in all the cases. Based on the small differences in estimated masses, but large difference in estimated lifetimes, between the 60 wt% and 80 wt% moderated reactors, the 60 wt% moderated systems with disc or helical fuel geometries represent the best balance between total mass and operating lifetime. Based on the results of the mass-optimization study, the thesis considers shadow shield options for an unmoderated HEU-fueled space reactor and a moderated LEU-fueled space reactor. Both reactors are kilowatt-class reactors, producing 15 kWth of thermal power over a 5- year operational lifetime. Based on the shielding required to meet established dose limits (a neutron fluence of less than 1014 n/cm2 (>1 MeV equivalent in silicon) and a gamma ray dose of iii less the 1 Mrad in silicon), the moderated LEU-fueled space reactor will require a thicker shadow shield than the unmoderated HEU-fueled space reactor. The thinner reflector of the moderated LEU-fueled reactor results in more neutrons reaching the shadow shield at higher energies compared to the unmoderated HEU-fueled reactor. The presence of a significant reflector in most space reactor designs means that the core spectrum is relatively unimportant in terms of shadow shield design, as the reflector thickness has a much stronger impact on the neutrons and gamma rays reaching the shadow shield. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ..................................................................................................................... viii LIST OF TABLES ...........................................................................................................................x NOMENCLATURE ..................................................................................................................... xii ABBREVIATIONS ..................................................................................................................... xiii ACKNOWLEDGEMENTS...........................................................................................................xv CHAPTER 1 INTRODUCTION .....................................................................................................1 1.1 References ......................................................................................................................4 CHAPTER 2 BACKGROUND .......................................................................................................6 2.1 Fundamentals of Space Nuclear Fission Systems ..........................................................9 2.2 History of Space Nuclear Reactors Power Systems ....................................................14 2.2.1 The United States of America Space Nuclear Power Programs ...................15 2.2.2 Former Soviet Union Space Nuclear Power Programs .................................23 2.3 Space Nuclear Reactor Systems Policy Issues.............................................................26 2.4 Space Nuclear Fission Systems Programs Around the World .....................................30 2.4.1 Current The United States of America Programs .........................................32 2.4.2 Current South Korean Programs ..................................................................35 2.4.3 Current Brazilian Program ...........................................................................36 2.4.4 Current European Program ..........................................................................36 2.4.5 Current Russian Program .............................................................................36 2.5 References ....................................................................................................................37 v CHAPTER 3 FUEL GEOMETRY OPTIONS FOR A MODERATED LOW-ENRICHED URANIUM KILOWATT-CLASS SPACE NUCLEAR REACTOR…………....42 3.1 Introduction ..................................................................................................................43 3.2.Background ..................................................................................................................44 3.3 Model Description .......................................................................................................48 3.4 Results ..........................................................................................................................52 3.4.1 Unmoderated Reactor ...................................................................................52 3.4.2 Moderated Reactor ........................................................................................53 3.4.2.1 Homogeneously moderated core....................................................55 3.4.2.2 Heterogeneously moderated core ...................................................55 3.4.3 Reactor Core and Reflector Size Optimization .............................................57 3.4.3.1 Mass Optimization of the Unmoderated Reactor ..........................58 3.4.3.2 Mass Optimization of the Homogeneously Moderated Reactor ....58 3.4.3.3 Mass Optimization of the Heterogeneously Moderated Reactor with Spherical Geometry ..................................................61 3.4.3.4 Mass Optimization of the Heterogeneously Moderated Reactor with Disc Geometry .........................................................62 3.4.3.5 Mass Optimization of the Heterogeneously Moderated Reactor with Helical Geometry ....................................................64 3.4.3.6 Mass Optimization Results for the Heterogeneously Moderated Reactors ......................................................................66 3.4.4 Lifetime Estimates .......................................................................................67 3.5 Mass comparison.........................................................................................................69 3.6 Summary and conclusions...........................................................................................71 3.7 References...................................................................................................................72 vi CHAPTER 4 SHIELDING ANALYSIS FOR A MODERATED LOW-ENRICHED URANIUM FUELED KILOWATT-CLASS SPACE NUCLEAR REACTOR….74 4.1 Introduction ..................................................................................................................75 4.2 Background ..................................................................................................................76 4.2.1 Neutron Attenuation and Shielding ..............................................................79 4.2.2 Gamma Ray Attenuation and Shielding .......................................................81 4.3 Model Descriptions ......................................................................................................82 4.4 Results ..........................................................................................................................87 4.4.1 Unmoderated HEU-fueled reactor results .....................................................87 4.4.2 Moderated LEU-fueled reactor results ..........................................................90 4.4.3 Gamma Ray Shielding Comparison..............................................................92 4.5 Summary and Conclusions...........................................................................................98 4.6 References....................................................................................................................99 CHAPTER 5 SUMMARY AND CONCLUSIONS ....................................................................101 CHAPTER 6 RECOMMENDATIONS FOR FUTURE WORK ................................................105 APPENDIX A MCNP6 UNMODERATED HEU-FUELED REACTOR EXAMPLE INPUT FILE........................................................................................................107

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