David Shoesmith1

1, University of Western Ontario, London, Ontario, Canada

An understanding of the surface redox chemistry of uranium dioxide is essential to develop models for the release of radionuclides from spent nuclear fuel inside a failed nuclear waste container emplaced in a deep geologic repository. Slightly non-stoichiometric UO2+x and stoichiometric UO2 doped with rare-earth fission products are electrically conducting. Consequently, this mechanistic understanding can be developed electrochemically on custom-fabricated uranium oxide pellets with known stoichiometry and/or controlled levels of rare-earth dopants. This requires the application of a combination of electrochemical and surface analytical techniques, such as scanning electrochemical microscopy, electrochemical impedance spectroscopy, microRaman and X-ray photoelectron spectroscopies, and current sensing atomic force microscopy.
Using this combination of techniques the evolution of the chemical state of the UO2 has been determined both as a function of applied potential and also as a function of the corrosion (open-circuit) potential developed in the presence of oxidants (H2O2, O2) and reductants (H2) expected to be produced by water radiolysis, the key source of oxidants within a failed container. This surface state has a controlling effect on the kinetics of dissolution of U (as UO22+) a process which is also very dependent on the influence of anions anticipated in groundwaters, especially bicarbonate/carbonate which form strong soluble complexes with the uranyl ion.
Depending on the chemical state of the surface and their concentration, the various radiolytically-produced redox agents (H2O2, O2, H2) can react differently, with O2 requiring catalysis by donor-acceptor states (UIV/UV) present in the oxide surface, while H2O2 can create its own donor-acceptor states which are then utilized partially in the decomposition of the H2O2 to the less reactive O2 and H2O. The balance between H2O2 decomposition and its consumption by fuel corrosion is inextricably linked to peroxide concentration and the chemical state of the surface. The ability of H2 to suppress UO2 corrosion depends predominantly on the number density of noble metal particles formed by fission products unstable as oxides in the fuel matrix. These particles activate the H2 by catalyzing its decomposition into H radicals which can then suppress the anodic oxidation of UO2 by galvanic coupling to the UO2 matrix or by consuming H2O2 by reaction to produce H2O.