Effects of Compressive Strain on Defect Behaviors in Uranium Dioxide and Uranium Mononitride: A Comparative Study

TRISO (Tristructural Isotropic) fuel particles exhibit excellent applicability across multiple reactor types, but their unique layered structure imposes strong confinement effects on the fuel kernel, which strongly influences their swelling behavior. To investigate the irradiation damage mechanisms of TRISO fuel under the constrained conditions, we systematically studied the effects of isotropic compressive strain on defect behavior in UO2 and UN using first-principles calculations based on Hubbard-corrected density functional theory (DFT+U). We analyzed the evolution of stability and defect interaction energetics of intrinsic point defects and fission gas Xe atoms in both UO2 and UN under compressive strains ranging from 0 to 5%. The results show that compressive strains generally lower vacancy formation energies while increasing interstitial defect formation energies in both materials with the exception of oxygen interstitials in UO2. Xe atoms preferentially occupy uranium vacancies sites in both systems, and their incorporation energies increase monotonically with strain. Binding energy analyses indicate that Xe–defect interactions in UO2 are more strongly affected by strain, exhibiting energy variations of 2–3.5 eV, compared to ∼1 eV in UN. These findings highlight the different defect behaviors in TRISO fuel under a constrained environment, which provide a theoretical foundation for understanding irradiation damage in nuclear fuel materials.