Outstanding stability of Gd-doped UO₂ against surface oxidation: First-principles study

An accurate understanding of structural integrity and chemical reactivity of UO₂ disposed in deep underground sites is of importance. Owing to the specific condition of the site location, UO₂ may have substantially different properties from the conventional prediction. In this study, we demonstrate that the oxidation resistivity of UO₂ is considerably modified by gadolinium (Gd), which is the element of neutron absorber and a byproduct of nuclear decay of radioactive U-235. Using density functional theory calculations, we investigate how the oxidation mechanism of UO₂ changes with Gd incorporation in U lattice. Our study indicates that Gd remarkably enhances the thermodynamic stability of pristine UO₂ against surface oxidation via three underlying mechanisms: (i) weakens the chemical bonding of adsorbed oxygen atom (O) with U, (ii) reduces active sites (U) for oxygen adsorption, and (iii) suppresses the subsurface diffusion of adsorbed O delaying the growth of the oxide layers on the UO₂. Electronic and lattice structure analyses for Gd-doped UO₂ indicate that amount of charge transfer from U to O is critically reduced and the lattice of the UO₂ surface is contracted. Our results provide useful information for understanding long-term stability and improving the structural integrity of UO₂ through the chemical doping process.