Reaction of hydrogen peroxide with uranium zirconium oxide solid solution - Zirconium hinders oxidative uranium dissolution

We studied oxidative dissolution of uranium and zirconium oxide [(U,Zr)O-2] in aqueous H2O2 solution to estimate (U,Zr)O-2 stability to interfacial reactions with H2O2. Studies on the interfacial reactions are essential for anticipating how a (U,Zr)O-2-based molten fuel may chemically degrade after a severe accident. The fuel's high radioactivity induces water radiolysis and continuous H2O2 generation. Subsequent reaction of the fuel with H2O2 may oxidize the fuel surface and facilitate U dissolution. We conducted our experiments with (U,Zr)O-2 powder (comprising Zr:U mole ratios of 25:75, 40:60, and 50:50) and quantitated the H2O2 reaction via dissolved U and H2O2 concentrations. Although (U,Zr)O-2 reacted more quickly than UO2, the dissolution yield relative to H2O2 consumption was far less for (U,Zr)O-2 compared to that of UO2. The reaction kinetics indicates that most of the H2O2 catalytically decomposed to O-2 at the surface of (U,Zr)O-2. We confirmed the H2O2 catalytic decomposition via O-2 production (quantitative stoichiometric agreement). In addition, post-reaction Raman scattering spectra of the undissolved (U,Zr)O-2 showed no additional peaks (indicating a lack of secondary phase formation). The (U,Zr)O-2 matrix is much more stable than UO2 against H2O2-induced oxidative dissolution. Our findings will improve understanding on the molten fuels and provide an insight into decommissioning activities after a severe accident. (C) 2017 Elsevier B.V. All rights reserved.