Coupled Neutronic and Thermal-Hydraulic Analysis of Inner Moderator Radius Effects in Dual-Cooled Annular UO2 and Thorium-Based Fuels for a LW-SMR

This study evaluated the effect of the inner moderator radius on the neutronic and thermal-hydraulic performance of dual-cooled annular UO2 and (Th-233U-235U)O2 fuels for a light water (LW) small modular reactor (SMR) derived from the AP300™ concept. The analysis combined lattice neutronic calculations and fuel channel thermal-hydraulic modeling to assess reactivity behavior, discharge burnup, cycle length, isotopic evolution, power peaking factor, pressure drop, coolant temperature rise, and departure from nucleate boiling ratio (DNBR). The results showed that the thorium-based fuel provided better fuel utilization than UO2 over the entire investigated range of inner moderator radii. Under single-batch operation, the discharge burnup of the thorium-based fuel reached 36.96 to 38.57 gigawatt-days per metric tonne (GWd/t), whereas the UO2 cases achieved 27.14 to 35.41 GWd/t. For a three-batch strategy, the thorium-based option attained 166.3 to 170.8 GWd/t and yielded longer operating cycles. It also significantly suppressed plutonium production and higher actinide accumulations, while reducing 135Xe and 149Sm poisoning. Increasing the inner moderator radius improved fertile-to-fissile conversion in both fuel systems; however, the thorium-based fuel exhibited much smaller performance degradation across the examined range. From the thermal-hydraulic standpoint, all the cases exhibited very similar behavior, with low power peaking factors, DNBRs near 2.3, and nearly unchanged pressure drop trends. Overall, the results demonstrated that thorium-based dual-cooled annular fuel is a promising option for improving fuel cycle performance while maintaining favorable thermal-hydraulic safety margins in LW SMRs. © 2026 American Nuclear Society.