Uranium dissolution and uranyl peroxide formation by immersion of simulated fuel debris in aqueous H2O2 solution

The fuel debris generated in the accident at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station has been subject to water and ionizing radiation for years and will be until retrieval. Therefore, we investigated the potential degradation of this fuel debris caused by H2O2, which is the oxidant of major impact from water radiolysis. We performed leaching experiments on different kinds of simulated debris comprising U, Fe, Cr, Ni, and Zr in an aqueous H2O2 solution. Chemical analysis of the leaching solution showed that U dissolution was induced by H2O2. Raman analysis after the leaching revealed that uranyl peroxides were formed on the surface of the simulated debris. The U dissolution and uranyl peroxide formation were observed in most of the simulated debris. These results demonstrate that uranyl peroxides are possible alteration products of fuel debris from H2O2 reaction. However, the sample in which the main U-containing phase was a U-Zr oxide solid solution showed much less U dissolution and no Raman signal of uranyl peroxides. Comparison of these results indicates that formation of an oxide solid solution of Zr with UO2 improves the stability of fuel debris against H2O2 reaction.