Reaction of uranium- and zirconium oxides under reducing and oxidizing atmospheres: X-ray absorption and X-ray diffraction studies

Chemical and redox reaction between uranium (U) and zirconium (Zr) at high temperature under a reducing or oxidizing atmosphere were investigated to simulate fuel debris formed by Fukushima nuclear power plant accident. X-ray absorption spectroscopy (XAS) at U LIII-edge and Zr K edge and powder X-ray diffraction (XRD) of the mixed UO2/ZrO2 materials treated at high temperature from 1473 to 1873 K under reducing or oxidizing atmosphere were performed. For UO2-ZrO2 samples from1473 to 1873 K under the oxidizing atmosphere (Ar + 2% O2, 1h), it was found that the compounds were primarily consisted of five species including U3O8, UO2, U2Zr5O15, monoclinic-ZrO2, and tetragonal-ZrO2, whose fractions were calculated by principal component analysis (PCA) on both the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) collected at both U LIII edge and Zr K edge. Here, the formation of U2Zr5O15, a pentavalent uranium compound, was confirmed by the U LIII edge-XANES between the temperature range from 1473 to 1573 K. These results were also supported by powder-XRD analysis. Under the reducing atmosphere (Ar + 10% H2, 1h), UO2 remained at the whole temperature range without reacting with ZrO2, whereas monoclinic- and tetragonal ZrO2 were formed at 1573 K (60- and 40%, respectively). This study can pave the way for understanding the interaction between nuclear fuels and cladding materials in damaged reactors, enabling to simulate possible decontamination procedures.