Abstract This study evaluates the performance of four nuclear fuel types – UO 2 (Model 1), U-Th MOX (Model 2), UN (Model 3), and UC (Model 4) in a KLT-40S assembly model using OpenMC. The analysis identifies uranium nitride (UN) and uranium carbide (UC) as superior alternatives to conventional oxide fuel due to their enhanced neutron economy. These fuels exhibited significantly higher fissile inventory, leading to a 3.54–4.17 % increase in the effective multiplication factor, with UC achieving the highest initial k inf (∼1.21). To manage this high excess reactivity, both fuels were optimized using 18 large burnable poison rods, which also facilitated their exceptionally long operational cycles of 837 and 769 days, respectively. UC demonstrated the highest beta effective, indicating superior inherent stability from a greater delayed neutron fraction. Furthermore, both carbide and nitride fuels promoted a more even power distribution, evidenced by lower radial power peaking factors and fission reaction rates in central fuel rods compared to oxide models. Radiotoxicity analysis showed that all fuel types achieve substantial reduction (>95 %) in total radiotoxicity within 10 years of cooling, with short and medium-lived isotopes decaying rapidly and long-lived actinides dominating only over extended periods, providing guidance for safe interim storage and waste management. Based on their superior neutronics, extended lifespan, inherent stability, even power generation, and favorable radiotoxicity profile, UN and UC fuels are recommended as promising replacements for UO 2 in KLT-40S assemblies.
Hasan et al. (2026) studied this question.