First-Principles Study of Irradiation Defects in γ′-U₂Mo

Understanding defect behavior and fission gas transport in uranium-molybdenum (U-Mo) fuels is key to explaining their swelling during reactor operation. In this study, we employed density functional theory (DFT) to systematically investigate the point defect structures and self-diffusion mechanisms in U₂Mo, with particular emphasis on the diffusion behavior of fission gas atoms Xe. Among intrinsic defects, vacancies and substitutional defects are the most stable, combining low formation energies with relatively small migration barriers; as a result, they largely control defect-mediated processes. Further analysis shows that self-diffusion in U₂Mo is strongly element-dependent, as U atoms migrate predominantly through vacancy-mediated mechanisms, while Mo atoms diffuse primarily via substitutional pathways. In addition, Xe atoms migrate through two distinct pathways: by combining with vacancies to form stable complexes and diffusing via vacancy-assisted migration, or by migrating as interstitial species along the Tetrahedral → Octahedral → Tetrahedral path between interstitial sites, eventually moving outward along defect channels and leading to gas release. Self-diffusion and fission gas transport in U-Mo fuels are governed by point defects, linking defect behavior to the swelling resistance of advanced nuclear materials.