Carbon-induced magnetoelastic coupling and elastic anisotropy evolution in dilute oxide-rich fluorite UO2 − x C x for TRISO fuels

Understanding how low levels of dissolved carbon modify the mechanical response of oxide-rich tristructural isotropic (TRISO) fuel kernels is important for predicting irradiation-induced stress evolution. In the present work, density functional theory is employed to investigate the structural, electronic, magnetic, and elastic properties of fluorite UO2−xCx containing 2–8 at. % substitutional carbon on the oxygen sublattice, considered here as a representative low-carbon (dilute) regime. Carbon incorporation preserves the fluorite-derived structure while producing non-monotonic variations in volume and uranium magnetic moments, indicating the perturbation of the correlated U-5f states. A progressive bandgap reduction is observed due to increasing C-2p/U-5f hybridization. All investigated compositions satisfy the adopted Born elastic stability criteria. Carbon selectively decreases C11 and the bulk modulus while progressively modifying the orientation-dependent elastic response, leading to an overall reduction in elastic anisotropy at higher carbon concentrations. These results establish a mechanistic link between electronic reconstruction, bonding redistribution, and elastic anisotropy evolution in oxide-rich TRISO fuel phases.