T95 Mineralogy Writ Small: A Tribute to the Distinguished Career of David R. Veblen

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 317

SURFACE DIFFRACTION STUDIES OF UO2 – ATOMIC SCALE INSIGHTS INTO NANO-DIMENSIONED CORROSION RINDS

Paper number: T95-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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 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.

Using crystal truncation rod (CTR) x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) we have recently shown that when the UO2 (111) surface is exposed to O2, oxygen interstitials order into a nanoscale superlattice with three-layer periodicity and uranium in three oxidation states: IV, V, and VI. This oscillatory diffusion profile is driven by the nature of the electron transfer process [1]. CTR measurements of O2-exposed UO2 (001) surfaces reveal two possible models for oxygen diffusion profiles. The first is relatively smooth and suggests a highly oxidized surface, a layer with few if any interstitials, and deeper layers with roughly equal interstitial occupation. The second is strongly oscillatory, and suggests preferential interstitial occupation of every other layer below the surface, which is geometrically consistent with the interstitial network that forms below the oxidized (111) surface. The structures determined here contrast with previously published structures for bulk UO2+x and U4O9 as well as classical diffusion profiles, and highlight the importance of surface structures in controlling surface-mediated processes such as corrosion and dissolution.

[1] Stubbs et al. (2015), Phys. Rev. Lett. 114, 246103.