article · Nuclear Science and Engineering
This research evaluates how varying the inner moderator radius affects the neutronic and thermal-hydraulic performance of dual-cooled annular fuel in a light water small modular reactor based on the AP300 concept. It compares standard uranium dioxide fuel against a thorium-based fuel containing thorium, uranium-233, and uranium-235. The findings show that thorium-based fuel delivers superior fuel utilisation across all tested inner moderator radii. In single-batch operations, discharge burnup reached up to 38.57 gigawatt-days per metric tonne for thorium, outperforming uranium dioxide. Under a three-batch strategy, the thorium fuel achieved up to 170.8 gigawatt-days per metric tonne alongside longer operating cycles. Furthermore, the thorium cycle suppressed plutonium and higher actinide build-up, and decreased neutron poisoning. Both fuel systems maintained similar, favourable thermal-hydraulic safety margins, including low power peaking factors and robust departure from nucleate boiling ratios.
Small modular reactors offer flexible, low-carbon nuclear energy, but improving fuel efficiency and managing waste remain critical challenges. Using thorium in an advanced dual-cooled annular geometry enables reactors to extract more energy over longer cycles while substantially suppressing long-lived plutonium and transuranic waste. Importantly, these gains are achieved while maintaining necessary core cooling and thermal-hydraulic safety margins.
This work is relevant to nuclear fuel developers and small modular reactor design organisations seeking longer operating cycles and reduced actinide waste profiles. Because the findings derive from coupled lattice neutronic calculations and thermal-hydraulic channel modelling, the technology is at an early computational research stage. Real-world adoption would require extensive physical testing, including fuel fabrication development, irradiation testing, and regulatory safety licensing before deployment in commercial reactors.
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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.
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DOI: 10.1080/00295639.2026.2715897
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