Phase analysis of uranium oxides after reaction with stainless steel components and ZrO ₂ at high temperature by XRD, XAFS, and SEM/EDX

In the Fukushima Daiichi nuclear power station accident in March 2011, fuel debris was formed when fuel materials reacted with various structural materials in reactor core. Fuel debris retrieval is expected to start in 2021 for decommissioning of the damaged plants. To perform this activity safely, it is necessary to know the properties of the fuel debris. In this study, we investigated the reaction of UO ₂ , stainless steel (SS) components, and ZrO ₂ at high temperature under oxidizing and reducing conditions, and determined the valence state of uranium in the products, such as CrUO ₄ and (Fe _x , Cr _1−x )UO ₄ , by X-ray absorption near edge structure (XANES) spectroscopy. Mixed powders of UO ₂ and SS, Fe, and/or Cr were heated in Ar + 2% O ₂ (oxidizing condition) or Ar + 10% H ₂ (reducing condition) at a flow rate of 20 mL/min for 2 h at 1473 or 1673 K. After heat treatment, the phase relation of the products was analyzed by powder X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Under reducing conditions, UO ₂ did not react with SS, Fe, Cr, or ZrO ₂ . In contrast, under oxidizing conditions, UO ₂ reacted with SS and Cr to form CrUO ₄ or (Fe _x , Cr _1−x )UO ₄ at 1473 and 1673 K. When the UO ₂ and Fe mixture was heated under oxidizing conditions, the Fe ₂ O ₃ phase coexisted with U ₃ O ₈ at 1473 K, whereas FeUO ₄ formed at 1673 K. When the UO ₂ , Fe, and Cr mixtures were heated at 1473 K under the oxidizing condition, the molar ratio of Fe/Cr in the (Fe _x , Cr _1−x )UO ₄ phase corresponded to the initial molar ratio in the sample. As the iron content of these samples increased, all three lattice parameters of (Fe _x , Cr _1−x )UO ₄ approached those of FeUO ₄ . XANES spectra revealed that the oxidation state of uranium in CrUO ₄ and (Fe _x , Cr _1−x )UO ₄ is pentavalent.