Impact of Short-Range Order (SRO) on Thermodynamic Properties of Point Defects

U-Mo alloys, particularly with Mo ranging around 10 wt.% (U-10Mo), are crucial as high-density, low-enrichment nuclear fuels in high-performance research and test reactors (HPRRs) and fast reactors. A significant challenge with U-Mo fuel is its unpredictable and excessive swelling under high-temperature irradiation, primarily due to fission-product gas accumulation in bubbles resulting from point defect formation, diffusion, and subsequent microstructural evolution and property degradation. Consequently, understanding the fundamental properties of point defects is essential to addressing these issues and improving fuel performance. However, the thermodynamic and kinetic properties of point defects depend on chemical ordering, i.e., the distribution of Mo atoms, which may not necessarily be random. This study systematically calculates the equilibrium ordering of Mo in U-10Mo in the temperature range from 300 K to 1500 K using hybrid molecular dynamics and Monte Carlo (MDMC) simulations and identifies non-random, short-range ordering (SRO) of Mo, which alters the thermodynamic and kinetic properties of point defects. Formation energies, diffusivities, and migration energies are determined as a function of SRO parameters and temperatures. Importantly, our findings reveal that interstitials play a significant role in self-diffusion at thermal conditions, which is abnormal in metals. The results provide more accurate atomistic data for basic defect parameters to upper-scale modeling, thereby facilitating a more comprehensive understanding of the thermal properties of point defects and the impact of SRO in U-Mo alloys.