XPS Investigation on Changes in UO 2 Speciation following Exposure to Humidity
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XPS Investigation on Changes in UO2 Speciation following Exposure to Humidity Scott B. Donald, M. Lee Davisson and Art J. Nelson MRS Advances / FirstView Article / July 2016, pp 1 - 5 DOI: 10.1557/adv.2016.286, Published online: 27 April 2016
Link to this article: http://journals.cambridge.org/abstract_S2059852116002863 How to cite this article: Scott B. Donald, M. Lee Davisson and Art J. Nelson XPS Investigation on Changes in UO2 Speciation following Exposure to Humidity. MRS Advances, Available on CJO 2016 doi:10.1557/adv.2016.286 Request Permissions : Click here
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MRS Advances © 2016 Materials Research Society DOI: 10.1557/adv.2016.286
XPS Investigation on Changes in UO2 Speciation following Exposure to Humidity Scott B. Donald, M. Lee Davisson, Art J. Nelson Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. ABSTRACT High purity UO2 powder samples were subjected to accelerated aging under controlled conditions with relative humidity ranging from 34% to 98%. Characterization of the chemical speciation of the products was accomplished using X-ray photoelectron spectroscopy (XPS). A shift to higher uranium oxidation states was found to be directly correlated to increased relative humidity exposure. Additionally, the relative abundance of O2-, OH-, and H2O was found to vary with exposure time. Thus, it is expected that uranium oxide materials exposed to high relative humidity conditions during processing and storage would display a similar increase in average uranium valence. INTRODUCTION Under ambient conditions, U3O8 is the most stable compound in the U-O system [1]. The initial step in the oxidation of uranium dioxide (UO2) in a wet environment is the adsorption of water on the surface of the material. While most DFT calculations suggest water nondissociatively adsorbs on low-index surfaces, high defect surfaces are predicted to adsorb water in a dissociative manner, yielding H+, OH-, and O2- [2-7]. Experimentally, even at low temperatures, UO2 oxidizes when exposed to air [8,9], initially yielding U4O9 and U3O7 through incorporation of adventitious oxygen in the fluorite structure. Oxidation in this regime is diffusion-controlled, yielding an oxidation front with a characteristic oxygen penetration rate. The subsequent formation of U3O8 in this oxidized region entails a more extensive structural rearrangement and involves nucleation and growth kinetics, yielding a comparatively sluggish transformation. In the presence of significant quantities of water (i.e., a high relative humidity) various hydrates, hydroxides, and carbonates can form during the oxidation process [9]. Due to the kinetics of oxygen adsorption and penetration into UO2 surfaces, oxidation is typically confined to the near-surface region. X-ray photoelectron spectroscopy (XPS) has been used previously to identify th
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