Weeks Lecture – Sep 19th, 2014, Friday, Noon and 3:30 PM

Weeks Lecture

stubbs

Joanne Stubbs
University of Chicago

Sep 19th, 2014, Friday, Noon, Room A320 – Weeks Hall
Oxidative corrosion of uraninite (UO2) by non-classical solid-state diffusion

Uraninite and synthetic UO2 play central roles in nearly every stage of the nuclear fuel cycle, from mining through fuel fabrication and use, to environmental remediation. The solubility and dissolution kinetics of uraninite depend on the oxidation state of uranium, therefore understanding the mechanisms of UO2 oxidative corrosion is essential to predicting its chemical stability. We present evidence from crystal truncation rod (CTR) x-ray diffraction of the incorporation of oxygen interstitial atoms below the UO2 (111) surface at room temperature and atmospheric pressure. Oxygen enters discrete planes parallel to the surface with three-layer periodicity, resulting in a complex, oscillatory oxidation front. Using first principles computational methods we show this non-classical diffusion is driven by electron transfer from multiple uranium atoms to each interstitial oxygen. X-ray photoelectron spectroscopy shows U(IV) is oxidized to both U(V) and U(VI). 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.

Sep 19th, 2014, Friday, 3:30 PM Room AB20 – Weeks Hall
Synchrotron Tools for Geoscientists

GeoSoilEnviroCARS (GSECARS) is a national synchrotron x-ray user facility at the Advanced Photon Source (APS) dedicated to forefront research on earth materials and open to the entire scientific community. This presentation will focus primarily on synchrotron-based surface and interface science tools available at GSECARS. Examples of recent research in this field ranging from environmental fate and transport of contaminant metals to the chemistry of the early solar system will be discussed. Other capabilities will also be briefly reviewed, including high pressure research with the laser-heated diamond anvil cell and multi-anvil press, x-ray absorption fine structure spectroscopy, x-ray fluorescence microprobe, microtomography and in-situ powder diffraction.