Oxygen substitution-induced structural, mechanical, and electronic evolution in UC1-xOx: A first-principles study for TRISO UCO fuel applications

Uranium oxycarbide (UCO) TRISO fuel kernels undergo irradiation-driven redistribution of carbon and oxygen, leading to the formation of oxygen-substituted carbide domains (U C 1 - x O x ). The intrinsic properties of these domains influence dimensional stability and fission-product behaviour. In this work, density functional theory calculations within the GGA + U framework were combined with special quasi-random structures (SQS) to investigate structurally disordered U C 1 - x O x with oxygen concentrations of 0–16 at.% on the anion sublattice. Occupation matrix control was applied to ensure consistent convergence of the uranium 5f states, and a 1 K-type antiferromagnetic configuration was used to examine magnetic behaviour. Oxygen substitution preserves the rock salt structure up to 16 at.% O and produces a non-monotonic lattice response, with an initial lattice contraction at low oxygen content followed by lattice expansion at higher concentrations. The formation enthalpy becomes increasingly negative with oxygen addition, indicating enhanced thermodynamic stability of the carbide lattice. Simultaneously, uranium magnetic moments increase, and the density of states at the Fermi level decreases, reflecting stronger 5f localization and enhanced U-O bonding interactions. Elastic calculations show that oxygen incorporation increases C 44 and shear-related moduli, while C 11 remains nearly unchanged, accompanied by a reduction in Poisson’s ratio. Overall, these results establish clear composition structure property relationships for U C 1 - x O x in the low-oxygen composition range relevant to irradiation conditions in TRISO UCO fuels.