Oxidative corrosion of uraninite (UO2) surfaces
EMSL Project ID
49380
Abstract
Uraninite (UO2) is the most abundant uranium ore mineral, its synthetic analog is the primary constituent of most nuclear fuels, it is the desired product of bioremediation strategies for uranium-contaminated soils and groundwaters, and it is of fundamental interest in basic and applied actinide science. The solubility and dissolution kinetics of uraninite depend heavily on the oxidation state of uranium, therefore understanding the mechanisms of UO2 oxidative corrosion is essential to predicting its chemical stability throughout the nuclear fuel cycle. Despite decades of research, a full molecular-scale understanding of uraninite corrosion is lacking due to a dearth of knowledge regarding the atomic and electronic structures of UO2 surfaces under both pristine and corroded conditions. This proposal extends previous work done under Proposal 48555 which we have recently published in Physical Review Letters (1). In recognition of its importance and potential impact, this publication has been selected as a science highlight by PNNL (2), EMSL (3), the Advanced Photon Source (4), and the journal Science (5). The paper is a combined crystal truncation rod (CTR) x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) study of the oxidation of the UO-2 (111) surface at ambient pressure and temperature. In it, we used CTR to measure the surface and near-surface atomic structures of UO2 single crystal surfaces, and showed that upon exposure to dry oxygen gas, oxidation fronts proceed into the crystals, with interstitial oxygen atoms penetrating to depths of 30 A or more. A complex diffusion profile arises, with interstitial oxygen atoms occupying every third layer beneath the surface. We showed with XPS conducted at RadEMSL that uranium at such surfaces occurs in three oxidation states: (IV), (V), and (VI). The experimental results are consistent with the theoretical calculations conducted by our collaborator at PNNL, Anne Chaka.
The CTR method, while exemplary for determining the arrangement of atoms at surfaces and interfaces, is insensitive to oxidation states. Since the oxidation state of uranium is key in determining its fate, full understanding requires quantitative determination of this parameter. X-ray photoelectron spectroscopy is ideally suited to this problem because it probes depths comparable to the oxidation fronts observed in our CTR experiments, and because it can be used to unambiguously distinguish between U(IV), U(V), and U(VI). We propose here to use XPS measurements conducted at RadEMSL to complete the picture of early-stage oxidation mechanism on UO2 surfaces. Specifically, we will extend our earlier work to the (100) surface, investigate reversibility of the oxidation reaction, and explore the influence of water on the corrosion process.
This research has broad relevance to energy, materials, biogeochemical, and environmental sciences and will generate knowledge that allows for the refinement and validation of a wide variety of predictive models, covering subjects from material failure to nuclear waste disposal to environmental remediation to fundamental science.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2016-10-01
End Date
2018-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
Related Publications
Ilton ES, Y Du, J Stubbs, P Eng, AM Chaka, JR Bargar, CJ Nelin, and PS Bagus. 2017. "Quantifying Small Changes in Uranium Oxidation States Using XPS of a Shallow Core Level." Physical Chemistry Chemical Physics. PCCP 19:30473-30480. doi:10.1039/c7cp05805e
Stubbs J, CA Biwer, AM Chaka, ES Ilton, Y Du, J Bargar, and P Eng. 2017. "Oxidative Corrosion of the UO2 (001) Surface by Nonclassical Diffusion." Langmuir 33(46):13189-13196. doi:10.1021/acs.langmuir.7b02800