Effects of decreasing fuel enrichment on the design of the Pellet Bed Reactor (PeBR) for lunar outposts

Space nuclear reactors usually use high enrichment (>93%) fuel in order to reduce the launch mass and cost. In addition to nuclear safeguard and non-proliferation concerns, high enrichment limits the fuel loading to ensure sufficient subcriticality, in case of a launch abort accident resulting in the bare reactor being flooded with seawater and submerged in wet sand. The core of the fast neutron spectrum PeBR is divided into three equal sectors that are neutronically and thermally coupled during reactor operation, but hydraulically decoupled. The PeBR design for lunar outposts, avoids launch criticality concern by being launched unfueled and loaded, after emplaced below grade on the Moon, with 1.0 cm diameter fuel pellets of ZrC-coated UC fuel particles dispersed in a ZrC matrix. The pellets are launched separately in specially designed canisters that satisfy launch sub-criticality requirements. This paper investigates the effects of decreasing the fuel enrichment on the size, mass, and the full power operation life of the PeBR and on the design, dimensions, and mass of the fuel pellets launch canisters. Decreasing the fuel enrichment from 93.96% in the base design, to 70%, 50%, 40%, 30% and 25% increases the total mass of the reactor and the fuel pellets by 48%, 133%, 229%, 443%, and 697%, respectively, and decreases the hot-clean excess reactivity from $6.34 to $5.30, $3.99, $3.02, $1.67 and $0.81, respectively. Despite the decrease in reactivity with decreased fuel enrichment, the operation life increases due to the increase in the total fissile inventory in the PeBR core. At its nominal power of 471 kWth, and inlet and exit temperatures of 758 K and 910 K, the estimated life of the PeBR with 50%, 40%, and 30% fuel enrichment is ∼84, 77, and 54 FPY, compared to ∼66 years for the base design. Furthermore, the volume and mass of the launch canisters for low enrichment fuel pellets are much larger than those for the base PeBR design.