Uraninite (UO2) is the most abundant uranium ore mineral, the product of proposed bioremediation strategies for uranium-contaminated soils and aquifers, and its synthetic analog is the primary constituent of most nuclear fuels. It incorporates interstitial oxygen up to a stoichiometry of UO2.25 without disruption of the uranium lattice, but the structural details of the process are the subject of ongoing study and debate. Because the solubility and dissolution kinetics of uraninite depend heavily on the oxidation state of uranium, understanding the mechanims of UO2 surface oxidation and corrosion is essential to predicting its stability in the environment throughout the nuclear fuel cycle. To date, however, no study has addressed this process at the molecular scale at atmospheric pressure and room temperature.We present crystal truncation rod (CTR) x-ray diffraction studies of pristine and oxidized UO2 (111) and (100) surfaces. The clean (111) surface shows minimal contraction of the uppermost atomic layers and a layer of oxygen or hydroxyl group adatoms above the vacuum-terminated surface. Upon exposure to dry O2 gas, an oxidation front proceeds into the crystal, interstitial oxygen atoms penetrate to depths of 30 � or more, surface-normal layer distances contract (consistent with bulk uraninite oxidation), and an ordered superlattice forms, commensurate with the underlying bulk. Similar O2 surface penetration and layer contraction are observed upon oxidation of the (100) surface. These results demonstrate that the solid state diffusion of oxygen into UO2 and UO2+x surfaces is facile and that ordering kinetics are relatively rapid, even at room temperature.Ab initio thermodynamics, which combines density-functional theory calculations with macroscopic thermodynamics, provides insight into the energetics, bonding, and oxidation processes that occur as oxygen reacts with the surfaces and diffuses into the solid. Subsurface oxidation is predicted to contract surface-normal layers consistent with experimental observations.

Oxidative corrosion of uraninite (UO<sub>2</sub>) surfaces

246th National Meeting (2013)

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Oxidative corrosion of uraninite (UO2) surfaces

Oxidative corrosion of uraninite (UO₂) surfaces