Burnup Analysis of a NuScale-Like Small Modular Reactor Core with a Modified Beryllium Oxide Reflector

The reference NuScale-like small modular reactor (SMR) uses a stainless-steel heavy reflector, whose comparatively high neutron-absorption cross section can reduce neutron economy. This study evaluates a modified reflector that places beryllium oxide (BeO) in the high-leakage corner regions and uses a thinner BeO layer along the flat regions, while replacing the stainless-steel baffle with zircaloy. MCNP6.2 calculations with ENDF/B-VII.0 data were performed for the first and equilibrium fuel cycles. Relative to the original reflector, the modified design increased the calculated critical average burnup from 13.77 to 15.64 GWd/tHM (13.6%) in the first cycle and from 14.30 to 15.50 GWd/tHM (8.4%) in the equilibrium cycle. Its albedo was approximately 0.82, about 10% higher than that of the original reflector. In the first cycle, the assembly power peaking factor decreased from 1.537 to 1.409 at beginning of cycle and was also lower at middle and end of cycle. In the equilibrium cycle, the modified design reduced the peaking factor at beginning and middle of cycle, whereas the end-of-cycle value increased from 1.124 to 1.211 because power shifted toward peripheral fresh-fuel assemblies as burnable poison was depleted. The modified reflector also softened the peripheral neutron spectrum. Calculated moderator temperature coefficients remained negative over 520–570 K, ranging from −22 to −42 pcm/K at beginning of cycle and from −25 to −56 pcm/K at end of cycle. The results indicate that the modified reflector can improve neutron economy and first-cycle power flattening while maintaining the self-regulating behavior on criticality.