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 mechanisms 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 ambient pressure and 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 with three-layer periodicity forms, laterally commensurate with the underlying bulk. Oxidation under liquid water results in dissolution and roughening of the surface, which is enhanced in the presence of dissolved CO2. Similar oxygen surface penetration and layer contraction are observed upon oxidation of the (100) surface. The structures determined here contrast with previously published structures for bulk UO2+x and U4O9, and highlight the importance of surface structures in controlling surface-mediated processes such as corrosion and dissolution.

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

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.

Oxidative corrosion of uraninite (UO2) surfaces

Oxidative corrosion of uraninite (UO₂) surfaces