Impact of uranium oxide (UO₂) fuel with molybdenum (Mo) inserts on pressurized water reactor performance and safety

This work investigates nuclear reactor performance and safety characteristics of UO₂ with high thermal conductivity Mo insert structures by using multiphysics modeling techniques. The purpose of this study is to use scoping analyses to quantify the impact of using Mo inserts from neutronic and heat transfer standpoints. Attention is given to reactor performance parameters, such as cycle length, maximum fuel temperature, temperature gradients in the fuel, and stored energy in the fuel. The finite-element code BISON and the Monte Carlo particle transport code Serpent were used to perform sensitivity analyses on the Mo insert geometry to optimize the insert design and inform larger scale modeling that required the homogenization of the UO₂ and Mo. Although BISON is often used as a fuel performance analysis tool, it is used in this context for heat transfer analysis only. Fuel performance optimization is outside the scope of the current study, but would be important for future work focused on this concept. The results showed that the insert had little impact on neutronic performance and that homogenizing the UO₂ and Mo was acceptable for reactor physics calculations. Reactivity temperature coefficients calculated using homogeneous UO₂-Mo were shown to be relatively similar to UO₂, but higher Mo content and ²³⁵U enrichment can reduce the worth of soluble boron and control rods. The effect of insert geometry on heat transfer was much greater, and an approximately 15–20% difference in maximum fuel temperature was predicted between the best and worst performing heat transfer geometries. Thermal conductivity calibration based on the finite element analysis results was performed to improve the accuracy of temperature predictions in reactor analysis models that homogenized the UO₂-Mo fuel. Compared with UO₂ in a pressurized water reactor (PWR), the optimized UO₂-Mo design increased the margin to fuel melt by 13–32% across the fuel cycle, but it requires the ²³⁵U enrichment to exceed 5% to match the cycle length of conventional UO₂.