Defect disorder in UO2

A defect disorder model has been developed to determine equilibrium off-stoichiometry and its spatial variations in UO2 crystals. The model gives the concentrations of atomic defects and electronic carriers as functions of oxygen partial pressure and temperature in the bulk and near crystal surfaces subject to an oxygen environment. Energetic parameters from published density functional theory calculations have been integrated into the defect disorder model for an accurate determination of the defect density and off-stoichiometry. The ionosorption theory has been used to couple the oxygen environment with the defect state in the crystal as we solved for the defect disorder near crystal surfaces. Contrary to the common belief that hyper-stoichiometry of UO2 is dominated by oxygen interstitials, the current model predicts that this regime is rather dominated by uranium vacancies. The model predictions also show that, in the presence of surfaces, the point defect concentrations vary by orders of magnitude in the subsurface region relative to the bulk region. Highlights: • Defect disorder in bulk UO2+x is modeled in terms of temperature and oxygen pressure. • The densities of atomic defects and electronic charge carriers are determined. • The model is extended to study the heterogeneity of defect density near crystal surfaces. • The surface effect is modeled using ionosorption theory. • The dominant defect type and off-stoichiometry profile near surface are found