Comparative neutronic assessment of U3Si2-based accident-tolerant fuels in a two-batch APR-1400

Uranium silicide (U 3 Si 2 ) is a leading candidate for accident-tolerant fuel in light-water reactors due to its higher uranium density and superior thermal conductivity compared with conventional UO 2 . However, its limited oxidation resistance and phase stability under reactor-relevant conditions have motivated the development of alternative designs, including composite and doped fuel architectures. This study presents a comparative neutronic evaluation of several U 3 Si 2 -based fuel concepts for a 24-month APR-1400 equilibrium core using the CASMO4E/SIMULATE3 code system. The CASMO4E/SIMULATE3 calculation route for accident-tolerant fuel analysis is verified against the VERA ATF benchmark, proposed by ORNL. The investigated options include monolithic U 3 Si 2 , homogeneous U 3 Si 2 –UB 2 composite fuel, Cr-doped U 3 Si 2 , UO 2 –U 3 Si 2 composite fuel, and newly proposed multilayer U 3 Si 2 /UB 2 configurations using natural boron. These multilayer concepts are introduced to suppress excess reactivity, reduce dependence on costly B-11 enrichment, and, in the outer-layer configuration, limit direct exposure of U 3 Si 2 to water or steam. Relative to the reference UO 2 core, monolithic U 3 Si 2 achieved the target cycle length with approximately 17–19% lower U-235 enrichment, although introducing substantial beginning-of-cycle excess reactivity. A homogeneous 90/10 wt% U 3 Si 2 –UB 2 composite with 99% enriched B-11 increased the cycle length by approximately 0.7% while improving excess reactivity suppression. The multilayer U 3 Si 2 –UB 2 concepts using natural boron effectively suppressed excess reactivity without requiring boron isotopic enrichment, with cycle-length changes of approximately −0.4% and −2.8% relative to the reference, while still satisfying the 2-year cycle-length criterion. In contrast, 3 wt% Cr-doped U 3 Si 2 and 50/50 wt% UO 2 –U 3 Si 2 composite fuels showed cycle-length reductions of approximately 9.0% and 7.8%, respectively, reflecting the reduced heavy-metal loading relative to monolithic U 3 Si 2 . Across all investigated concepts, maximum power peaking factors remained within acceptable design limits. The boron and Doppler coefficients remained negative, and the moderator, isothermal, and power coefficients showed acceptable overall trends, although beginning-of-cycle MTC behavior requires attention for high-excess-reactivity cases. The layered U 3 Si 2 –UB 2 configurations also increased the control rod worth by approximately 12–14% relative to the UO 2 reference, indicating improved shutdown capability. Overall, the results demonstrate that natural-boron multilayer U 3 Si 2 /UB 2 architectures offer a promising pathway for integrating U 3 Si 2 -based accident-tolerant fuel into APR-1400 cores while preserving cycle length, reactivity control, and safety-relevant neutronic characteristics.