Numerical Simulation and Experimental Characterization of Pore Evolution in U?Si? Fuel Pellets under Irradiation

Abstract: As one of the core candidate materials for accident-tolerant fuel (ATF), U₃Si₂ is a preferred alternative to conventional UO₂ fuel in light-water reactors owing to its high uranium density, high thermal conductivity, and excellent irradiation-induced swelling resistance. During reactor operation, the generation, migration, and agglomeration of fission gases drive pore evolution inside the fuel, which further affects the irradiation swelling behavior, fuel-cladding compatibility, and structural integrity of the fuel element. To clarify the irradiation performance of U₃Si₂ fuel pellets and provide theoretical and experimental support for the engineering application of ATF, this study systematically investigates the effects of primary knock-on atom (PKA) energy, irradiation temperature, and burnup on the pore evolution behavior of U₃Si₂ pellets using LAMMPS molecular dynamics (MD) simulations combined with optical microscopy (OM) and scanning electron microscopy (SEM) characterization in hot cells. The simulation results show that increasing PKA energy and temperature significantly raise the number of point defects in U₃Si₂ fuel pellets, and the defects gradually aggregate and interconnect to form uniformly distributed vacancy clusters.The experimental results indicate that, after eliminating the influences of inherent fabrication pores, cascade collision defect response, and pore maturation behavior, the average pore size and porosity of U₃Si₂ fuel pellets are both positively correlated with burnup, and the trends of simulation and experimental results are highly consistent. Both simulation and experiments confirm that the pore morphology of U₃Si₂ fuel pellets is irregular but uniformly distributed, and the pore evolution follows the subgrain growth mechanism. No significant irradiation swelling occurs under low-temperature and low-burnup conditions.