Search

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.

Actinide chlorides, such as those arising from pyrochemical reprocessing operations can be problematic to immobilise, as the high chloride content often makes their solubilities in melts very low, and even in small quantities can seriously affect the properties of the waste-form. Rather than attempt to immobilise the chlorine, one potential approach is to utilise the chloride salt as a reaction medium from which the An(III) cations can be extracted and immobilised. To this end, lanthanide titanate pyrochlores have been prepared by molten salt synthesis in CaCl 2 :MgCl 2 , CaCl 2 :NaCl and MgCl 2 :NaCl eutectics. Single-phase pyrochlore is found to be formed at temperatures as low as 650 °C in the CaCl 2 :NaCl system, whereas in the CaCl 2 :MgCl 2 and MgCl 2 :NaCl eutectics reaction with Mg produces a magnesium titanate secondary phase. Compositions of Yb 2 Ti 2 O 7 doped with Sm 3+ as an actinide surrogate have been synthesised, and cold-pressing and sintering at 1500 °C yields fully dense pellets.

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.

Substituting Pu(IV) onto the Ca site and using Fe(III) as a charge balancing species, a series of 6 compositions of 239Pu doped zirconolite (of general formula (Ca1−xPux)Zr(Ti2-2xFe2x)O7) have been fabricated, where x = 0.1, 0.2, 0.3, 0.4, 0.6 and 0.7. Sintered pellets from each of these compositions have been characterised by XRD, SEM, EDX and Raman techniques in order to analyse the change in microstructure and determine the polytype boundaries as the doping level increases, showing that the zirconolite is stable in the natural 2M polytype up until x = 0.3, then adopting the 3T polytype from x = 0.3 to x = 0.4, but from there onwards an increasing degree of phase separation occurs.