Phase analysis of uranium oxides after reaction with stainless steel components and ZrO₂ at high temperature by XRD, XAFS, and SEM/EDX

Highlights: • UO₂ reacted with chromium and SS at 1473 and 1673 K under oxidizing conditions, forming CrUO₄ and (Fe_x, Cr_1−x)UO₄. • UO₂ reacted with iron at 1673 K under oxidizing conditions, forming FeUO₄. • Chromium promoted the formation of (Fe_x, Cr_1−x)UO₄ in the UO₂–SS system. • When zirconium was coexisted with the UO₂-SS, formation of (Fe_x, Cr_1−x)UO₄ was suppressed. • The oxidation state of uranium in CrUO₄ and (Fe_x, Cr_1−x)UO₄ was pentavalent. - Abstract: 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 UO₂, stainless steel (SS) components, and ZrO₂ at high temperature under oxidizing and reducing conditions, and determined the valence state of uranium in the products, such as CrUO₄ and (Fe_x, Cr_1−x)UO₄, by X-ray absorption near edge structure (XANES) spectroscopy. Mixed powders of UO₂ and SS, Fe, and/or Cr were heated in Ar + 2% O₂ (oxidizing condition) or Ar + 10% H₂ (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, UO₂ did not react with SS, Fe, Cr, or ZrO₂. In contrast, under oxidizing conditions, UO₂ reacted with SS and Cr to form CrUO₄ or (Fe_x, Cr_1−x)UO₄ at 1473 and 1673 K. When the UO₂ and Fe mixture was heated under oxidizing conditions, the Fe₂O₃ phase coexisted with U₃O₈ at 1473 K, whereas FeUO₄ formed at 1673 K. When the UO₂, 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)UO₄ 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)UO₄ approached those of FeUO₄. XANES spectra revealed that the oxidation state of uranium in CrUO₄ and (Fe_x, Cr_1−x)UO₄ is pentavalent.