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

Abstract

• Mixed UO 2 - ZrO 2 compounds were treated at temperatures from 1473 to 1873 K under oxidizing, inert, and reducing atmospheres. • Formation products were analysed by both powder X-ray diffraction and X-ray absorption spectroscopy. • U 2 Zr 5 O 15 and ZrU 2 O 7 of pentavalent U were formed depending on the oxidizing conditions. Characterization of the fuel debris has been required for decommissioning Fukushima Daiichi Nuclear Power Station. To understand the reaction mechanisms involved for mixed UO 2 – ZrO 2 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 U 3 O 8 and U 2 Zr 5 O 15 was observed in equilibrium with UO 2 , monoclinic-ZrO 2 , and tetragonal-ZrO 2 . However, when O 2 gas was purged through the reaction tube during the cooling process to room temperature, pentavalent U in ZrU 2 O 7 was produced by the oxidation of solid solution UO 2 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 O 2 impurity with the Ar gas. Although the oxidized UO 2 was able to form in such a system, tetravalent UO 2 and its solid solution were instead present throughout the whole temperature range examined under a reducing atmosphere (H 2 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.