Density-functional theory investigation of oxidative corrosion of UO2

Corrosion and weathering of uranium dioxide (UO2) is a serious concern in a broad range of technological and environmental systems. Oxidation of UO2 can compromise the integrity of nuclear fuel rods, as well as result in the bioavailability of uranium in contaminated ground water at mines, mills, and nuclear waste storage facilities. How the oxidation proceeds, however, is not well understood. In this work, density-functional theory and ab initio thermodynamics are used to delineate the initial stages of surface and subsurface oxidation of UO2 at the (1 1 1) surface as a function of temperature and oxygen pressure. Initially, chemisorption of oxygen on the clean stoichiometric surface results in formation of highly stable triple-bonded uranyl groups and oxidation of the topmost uranium atoms to U6+ at a minimal p(O2) near 0 K. Once the surface is saturated with uranyl groups and the oxygen chemical potential increases above −1.0 eV, subsurface oxidation becomes thermodynamically favored. The degree of oxidation of the subsurface uranium atoms is determined by quantifying the transfer of electrons from the localized U 5f bands to those dominated by the delocalized O 2p bands as oxygen atoms occupy octahedral interstitial sites in the UO2 lattice. Occupation of the octahedral site nearest the surface results in an expansion of the lattice, whereas movement of the oxygen interstitial to deeper sites results in a net contraction.