Mass and shielding optimization studies for a low enrichment uranium fueled kilopower space nuclear reactor

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 10^14 n/cm2 (>1 MeV equivalent in silicon) and a gamma ray dose of 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.