Shielding Analysis for a Moderated Low-Enriched-Uranium–Fueled Kilopower Reactor

A low-enriched-uranium (LEU)–fueled space reactor could avoid the security and proliferation concerns inherent with highly enriched uranium (HEU)–fueled space nuclear reactors. Recent LEU-fueled space reactor designs include a moderator to reduce the size and mass of the reactor core. This paper 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 kW(thermal) 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 less then 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. Based on the results presented in this paper, the mass optimization of moderated LEU-fueled space nuclear reactors should always consider the coupled effects of the core, the reflector, and the shielding.