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

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 L_III- and Zr K-edges. Principal component analysis of the X-ray absorption near edge structure and extended X-ray absorption fine structure of U L_III- 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 < 1473 K. Under the inert atmosphere, mixed oxides of uranium were found to form 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.