Secondary Uranium Phases of Spent Nuclear Fuel. Coffinite, USiO₄, and Studtite, UO₄.4H₂O. Synthesis, Characterization, and Investigations Regarding Phase Stability

The miscibility behavior of the USiO₄ - ThSiO₄ system was investigated. The end members and ten solid solutions U_xTh_(1-x)SiO₄ with x = 0.12 - 0.92 were successfully synthesized, without formation of other secondary uranium or thorium phases. Lattice parameters of the solid solutions evidently follow Vegard's Law. Investigation of the local structure with EXAFS reveals small differences between U and Th environment attributed to different atomic radii of the metal atoms but no implications for a miscibility gap. The data provided confirms complete miscibility for the system USiO₄ - ThSiO₄. The structure of the end members was studied in detail with XRD and discussed with special regard to the oxygen positions and the often neglected Si-O bond length. USiO₄ could be obtained without UO₂ impurities and the lattice parameters derived from Rietveld refinement as c = 6.2606(3) Aa and a = 6.9841(3) Aa. The Si-O distance in USiO₄ appears to be 1.64 Aa, which is more reasonable than earlier reported values. Synchrotron X-ray powder diffraction pattern and Raman spectra of synthetic coffinite, USiO₄, were obtained for pressures up to 35 GPa and 18 GPa, respectively. From the changes in the diffraction pattern it can be concluded that USiO₄ undergoes a first order phase transition from zircon-type (space group I 4₁/amd) to scheelite-type structure (space group I 4₁/a) at ∼ 15 GPa and room-temperature. Contrary to earlier reports, the data indicates that this transition is completely reversible upon pressure release. Pressure dependencies of the Raman modes for the zircon structured phase are larger than those reported for hafnon, HfSiO₄, and zircon, ZrSiO₄, indicating that coffinite, USiO₄, is more compressible than these orthosilicates. Bulk moduli fitted from the p-V data for the zircon-type and scheelite-type USiO₄ phase are compared to those known to literature for other MSiO₄ (M = U, Hf, Zr) compounds. The bulk modulus for zircon-type USiO₄ is 180(7) GPa and hence lower than those of ZrSiO₄ (205 GPa) as expected from the larger unit cell. The pressure dependence of the Raman modes of USiO₄ was studied up to 18 GPa, yet no abrupt changes of peaks or in the peak shifts appear. Furthermore it could be established, that the B_1g- and the A_1g-modes of the SiO^4-₄-tetrahedron in the Raman spectrum are very close and overlap at ambient conditions. Structural investigations employing synchrotron X-ray and neutron powder diffraction allowed for a better determination of hydrogen positions in the studtite structure. Through EXAFS measurements the U-O distances of the first coordination sphere and the U-U distance were determined for studtite and metastudtite and compared to those reported in literature and derived from powder diffraction data. While the derived atomic distances reassured the studtite structure and were in good accordance with diffraction data, the spectrum of metastudtite showed an interesting feature in the range of the U-U distance. The structure model from Weck et al. for metastudtite is generally supported by the diffraction data. Through the results derived from NPDF it was possible to amend the model, which ended up in a different atomic position for one uranyl O atom. However, the derived new model seems plausible and encourages further discussion on the topic. Furthermore, this is the first comprehensive synchrotron and neutron powder diffraction study reported on studtite and metastudtite so far. The Raman spectra show no differences in the ν(O-O_peroxo)^sym stretching mode, implying that this distance is the same in studtite as in metastudtite. From IR spectra it can be observed, that the modes associated to water are strictly confined in metastudtite. The ν(U=O)^asym is split in metastudtite, a sign for two different U=O-distances. For studtite the presence of crystal water is observed in the strong ν(OH) mode in the IR spectrum, and confirmed by the observance of the librational modes of water in the IINS spectra. The IINS spectrum of metastudtite completely lacks any features of the librations of water, instead, the δ(HOH) vibrations give a strong signal. Suggesting, that metastudtite only contains directly bound water with strong O.. H coordination. The morphology of studtite and metastudtite was investigated with SEM and TEM. Both materials appeared as small elongated platelets and consist of numerous nano-scaled grains, with 5 - 10 nm in dimension. DSC-TG measurements showed that the dehydration of studtite to metastudtite takes place at 80 C and that the amorphization happens at ∝ 215 C. During the in-situ X-ray diffraction study, the onset of the studtite → metastudtite transition was observed at 39 C and the reaction was completed at 56 C. Further amorphization was observed at 212 C. The thermal expansion of the lattice parameters of studtite and metastudtite were investigated, showing that the axes perpendicular to the [UO₈]-polyhedra are least influenced by temperature. The pressure dependence of the ν(U=O)^sym and ν(O-O_peroxo)^sym stretching vibrations were derived in a high pressure Raman study. Judging from the appearance of the ν(U-O-U-O) long range mode, metastudtite undergoes amorphization to UO₃ at 14 GPa. Studtite is less stable and already decomposes at 1.6 GPa. A pressure induced transformation from studtite to metastudtite could not be observed.