A neutronic feasibility study on a small LEU fueled reactor for space applications

Highlights: • The minimum critical reactor mass with a simple homogeneous core was investigated. • The minimum mass of 133.8 kg was achieved with LEU-ZrH_1.5 core and Be reflector. • The reactor mass can be reduced by introducing heterogeneous core configuration. • Three space reactor designs with a control rod system were presented. • The neutronic performance and the safety behavior of the reactors were investigated. - Abstract: In this paper, a neutronic feasibility study on a small space reactor with LEU fuel is presented. The minimum critical reactor mass of a simple homogeneous core model was investigated for the variety of fuel types, moderator materials, and reflector materials. Lithium hydride (LiH) with 99.99% enriched ⁷Li showed a very good performance in terms of reactor mass. However, further study should be conducted to resolve the problems that could rise from the poor thermal properties of LiH such as a low melting point, low thermal conductivity, and high thermal expansion coefficient. The combination of uranium metal fuel and zirconium hydride (ZrH_1.5) moderator with a Be reflector showed a good performance as well. Uranium hydride (UH₃) is a very attractive fuel from a neutronic point of view. However, it requires hydrogen pressurization for high temperature operation due to its low hydrogen decomposition temperature. It was also shown that the core reactivity can be maximized or the critical reactor mass can be further reduced by adopting a heterogeneous core configuration through stacking fuel and moderator plates. Three small space reactors with a control rod system and NaK coolant pipes have been designed and the neutronic performance of these reactors during their life time as well as their safety behavior during various accident scenarios have also been investigated. All the three reactors showed similar neutronic performance during their life time, nevertheless, the smallest reactor mass was achieved in case of a reactor with heterogeneous core configuration. It was also found that a thick reflector is required to keep the reactor subcritical during launch accident scenarios involving a loss of the reflector and the control rods. In "the worst-case accident scenarios" in which the control rods are missing without any damage in the reflector, the reactors became supercritical even in cases when thick reflectors are used. Such situation is inevitable for any space reactor as long as a conventional control rod system or control drum system is adopted as the reactivity control system of the reactor. Efforts should be made to overcome this drawback of the conventional reactivity control systems