Fuel geometry options for a moderated low-enriched uranium kilowatt-class space nuclear reactor

•Options for small, LEU-fueled, space nuclear reactors are considered.•At low powers, moderated LEU reactors are less massive than non-moderated LEU reactors.•An 80 wt% moderator/fuel ratio results in the smallest reactor.•Disk or helical fuel geometries are preferred over spherical fuel geometries. A 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. The HEU-fueled Kilowatt Reactor Using Stirling Technology (KRUSTY) serves as a basis for a similar reactor fueled with LEU fuel. Based on MCNP6™ neutronics performance estimates, the size of a 5 kWe reactor fueled with 19.75 wt% enriched uranium-10 wt% molybdenum alloy fuel is adjusted to match the excess reactivity of KRUSTY. Then, 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. The lowest mass reactor with a given moderator represents a balance between the reflector thickness and core diameter needed to maintain the multiplication factor equal to 1.035, with a H/D ratio of 1.81. 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.