Studtite is known to exist at the back-end of the nuclear fuel cycle as an intermediate phase formed in the reprocessing of spent nuclear fuel. In the thermal decomposition of studtite, an amorphous phase is obtained at calcination temperatures between 200 and 500 °C. This amorphous compound, referred to elsewhere in the literature as U2O7, has been characterised by analytical spectroscopic methods. The local structure of the amorphous compound has been found to contain uranyl bonding by X-ray absorption near edge (XANES), Fourier transform infrared and Raman spectroscopy. Changes in bond distances in the uranyl group are discussed with respect to studtite calcination temperature. The reaction of the amorphous compound with water to form metaschoepite is also discussed and compared with the structure of schoepite and metaschoepite by X-ray diffraction. A novel schematic reaction mechanism for the thermal decomposition of studtite is proposed.
The structure of amorphous U2O7 has been examined by extended x-ray absorption near edge spectroscopy (EXAFS) and modelled using density functional theory (DFT) simulations. A hybridised metastudtite-UO3 structure is proposed, consisting of peroxide bonds (O-Operoxo), uranyl units (U=Oyl) and U-O bonding. Experimental and simulated X-ray diffraction (XRD) is used to confirm the proposed structure.
Uranyl oxalate (UO2C2O4·xH2O) may exist at the back-end of the nuclear fuel cycle (NFC) as an intermediate in spent fuel reprocessing. The conditions used in aqueous reprocessing and thermal treatment can affect the physical and chemical properties of the material. Furthermore, trace impurities, such as Fe, may incorporate into the structure of these materials. In nuclear forensics, understanding relationships between processing variables aids in determination of provenance and processing history. In this study, the thermal decomposition of UO2C2O4·3H2O and phase analysis of its thermal products are examined. Their morphologies are discussed with respect to a matrix of solution processing conditions.
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