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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.

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.

At the back-end of the nuclear fuel cycle, spent nuclear fuel (SNF) may be processed in solution to form a precipitate of uranyl peroxide tetrahydrate, also known as studtite ([(UO2)(O2)(H2O)2]•2H2O). The physical and chemical properties of studtite are influenced by the solution processing conditions employed, thus, the processing history of the precipitate may be elucidated from these characteristics. This is useful in the field of nuclear forensics, where the provenance of intercepted illicit nuclear material is of interest. Studtite is often calcined to ≥500 °C to form alpha uranium trioxide (α-UO3), an oxide prevalent in waste storage. In this study, the crystal structure of UO3 is analysed, with a discussion on the α/α’-UO3 C2mm symmetry. The morphological and structural signatures of both studtite and UO3 are investigated, with respect to a matrix of solution processing conditions and the thermal treatment of studtite.

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.