Speciation on the reaction of uranium and zirconium oxides treated under oxidizing and reducing atmospheres

Abstract Characterization of the fuel debris has been required for decommissioning Fukushima Daiichi Nuclear Power Station. To understand the reaction mechanisms involved for mixed UO2 – ZrO2 compounds, these materials were treated at high temperatures (1473 to 1873 K) under oxidizing, inert, and reducing atmospheres. The reaction products were analysed by a combination of powder X-ray diffraction (PXRD) and X-ray absorption spectroscopic measurements of the U LIII- and Zr K-edges. Principal component analysis of the X-ray absorption near edge structure and extended X-ray absorption fine structure of U LIII- and Zr K-edges provided a breakdown of the composition of each species within the products, these results were further supported by PXRD. Under an oxidizing atmosphere, the formation of U3O8 and U2Zr5O15 was observed in equilibrium with UO2, monoclinic-ZrO2, and tetragonal-ZrO2. However, when O2 gas was purged through the reaction tube during the cooling process to room temperature, pentavalent U in ZrU2O7 was produced by the oxidation of solid solution UO2 formed at > 1774 K during the temperature dropped at 1673 K due to a low concentration of O2 impurity with the Ar gas. Although the oxidized UO2 was able to form in such a system, tetravalent UO2 and its solid solution were instead present throughout the whole temperature range examined under a reducing atmosphere (H2 gas). This study can pave the way for understanding the interaction between the nuclear fuels and the cladding materials in damaged reactors enabling further simulation of possible decontamination procedures.