Neutronic Assessment of Advanced Fuels and Moderators for Enhanced Light Water Reactor (LWR) Safety and Performance

The development of Accident Tolerant Fuels (ATF) is critical for enhancing the safety and economic performance of next-generation nuclear reactors. This study presents a comprehensive, high-fidelity neutronic assessment of five leading fuel candidates: Uranium Dioxide (UO2), Uranium Carbide (UC), Uranium Nitride (UN), Mixed Oxide (MOX), and Thorium Dioxide (ThO2) to resolve fundamental trade-offs between intrinsic safety and neutronic efficiency. Utilizing the OpenMC Monte Carlo code, a systematic parametric analysis was performed to quantify key safety and performance parameters, including the effective multiplication factor (Kₑff), Fuel Temperature Coefficient (FTC), Moderator Temperature Coefficient (MTC), and Void Reactivity Coefficient (VRC). Each fuel was modeled in both light (H2O) and heavy water (D2O) environments, with sensitivity studies conducted over a wide range of operational conditions, including temperatures (300K–1200K), soluble boron concentrations (0–1600 ppm), and fuel densities ranging from 0. 90 to 0. 97 of theoretical density. Automation of the simulation loops was implemented to enable a robust and high-throughput analysis across this multi-dimensional parameter space. The results revealed that non-oxide fuels, UN and UC, exhibited the highest initial reactivity and the most strongly negative FTCs, demonstrating superior transient performance due to enhanced Doppler feedback. Conversely, the transition to D2O moderation, while improving neutron economy, significantly compromised passive safety margins. This was most evident for MOX and ThO2, which displayed a near-zero MTC and a potentially positive VRC, presenting a critical safety challenge. Ultimately, Uranium Nitride (UN) was identified as the most balanced candidate, maintaining robustly negative safety coefficients even in the challenging D2O spectrum. This work provides foundational benchmarks for ATF development and concludes that the synergy between fuel properties and moderator choice must be holistically optimized to ensure both safety and performance in advanced reactor designs. These findings provide a validated neutronic benchmark for guiding ATF selection in next-generation LWR designs.