Impact of Hydrogen on the Intermediate Oxygen Clusters and Diffusion in Fluorite Structured UO_{2+ x}

Uranium dioxide is the most prevalent nuclear fuel. Defect clusters are known to be present in significant concentrations in hyperstoichoimetric uranium oxide, UO_{2+ x}, and have a significant impact on the corrosion of the material. A detailed understanding of the defect clusters that form is required for accurate diffusion models in UO_{2+ x}. Using ab initio calculations, we show that at low excess oxygen concentration, where defects are mostly isolated oxygen interstitials, hydrogen stabilizes the initial clustering. The simplest cluster at this low excess oxygen stoichiometry consists of a pair of oxygen ions bound to an oxygen vacancy, namely the split mono-interstital, which resembles larger split interstitials clusters in UO_{2+ x}. Our data shows that, depending on local hydrogen concertation, the presence of hydrogen stabilizes this cluster over isolated oxygen interstitials.