The determination of electrochemical reactivity and sustainability on individual hyper-stoichiometric UO 2+x grains by Raman microspectroscopy and scanning electrochemical microscopy Heming He, Zhifeng Ding, David W. Shoesmith * Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 article info Article history: Received 24 June 2009 Received in revised form 7 July 2009 Accepted 7 July 2009 Available online 9 July 2009 Keywords: Hyper-stoichiometric uranium dioxide Scanning electrochemical microscopy Raman microspectroscopy Electrochemical reactivity Sustainability Oxygen excess defect structure abstract Using scanning electrochemical microscopy and Raman microspectroscopy, we have successfully observed four distinct defect structures in hyper-stoichiometric UO 2+x and demonstrated the relation- ships between the defect structures and their ability to sustain cathodic reduction processes. When only random point defects are present, the initially inert surface is enhanced by oxidation of the UO 2+x . How- ever, when the UO 2+x is already extensively oxidized and cuboctahedral clusters are present, further oxi- dation reduces the surface reactivity. At intermediate levels of stoichiometry corresponding to Willis clusters the surface appears to be reversibly oxidizable. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The release of radionuclides from spent nuclear fuel under per- manent disposal conditions is determined primarily by corrosion of the fuel, and a mixed-potential model has been developed to study the factors that control this process [1]. One factor is the de- gree of non-stoichiometry, which could influence both the anodic and cathodic kinetics of corrosion. Our primary goal is to determine the influence of non-stoichi- ometry, present as an anion-excess defect structure, on the local reactivity of a UO 2+x surface. The cathodic reduction rates at indi- vidual UO 2+x grains have been determined using scanning electro- chemical microscopy (SECM) in the feedback mode [2], and quantitatively demonstrate that local corrosion kinetics vary with the degree of non-stoichiometry of the grains. Here, we report a novel approach to determine whether or not the corrosion process on the non-stoichiometric grains is sustainable over significant time periods. Previous studies [1] indicated that cathodic reactions, specifically O 2 and H 2 O 2 reduction, are catalyzed on adjacent U IV / U V atoms, which act as donor–acceptor sites. 2. Experimental 2.1. Fuel specimens and reagents A hyper-stoichiometric UO 2+x specimen fabricated by Atomic Energy of Canada Limited at Chalk River Laboratories (Ontario, Canada) to produce an overall O/U ratio of 2.1 was used. 2.2. SEM and EDX analysis A Hitachi S-4500 (Hitachi, Japan) field emission scanning elec- tron microscopy (SEM) with an energy dispersive X-ray analysis (EDX) system was used. An electron beam with a 20 keV electron acceleration voltage allowed concurrent EDX analysis on some individual spots to determine the uranium/oxygen elemental compositions. 2.3. Raman spot analysis Measurements were carried out with a Renishaw 1000 confocal Raman spectrometer with a beam size of 2 lm and a He–Ne laser (633 nm) operating at low power to avoid oxidation of the sam- ple surface during the measurements. 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.07.013 * Corresponding author. Tel.: +1 519 661 2111x86366; fax: +1 519 661 3022. E-mail address: dwshoesm@uwo.ca (D.W. Shoesmith). Electrochemistry Communications 11 (2009) 1724–1727 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom