The thermodynamic and mechanical properties of rutherfordine, a uranyl carbonate mineral, were studied by means of first principles calculations based on density functional theory. Thermodynamic properties, including enthalpy, free energy, entropy, heat capacity, and Debye temperature, were evaluated as a function of temperature and compared with experimental data in the 300–700 K range. Our calculations show very good agreement with experimental data, and based on them, the knowledge of these properties is extended to the temperature range from 0 to 1000 K, including the full range of thermal stability (0–700 K). The computed values of the heat capacity, entropy, and free energy at 298 K deviate from the experimental values by about 8, 0.3, and 0.3%, respectively. At 700 K, the corresponding differences remain very small, 3.9, 2.3, and 1.3%, respectively. The equation of state and mechanical properties were also computed. The crystalline structure is seen to be mechanically and dynamically stable. Rutherfordine is shown to be a highly anisotropic and brittle material with a very large compressibility along the direction perpendicular to the sheets characterizing its structure. The computed bulk modulus is very small, B ≈ 20 GPa, in comparison to the values obtained in previous calculations.

A rutherfordine mineral was studied by means of Raman spectroscopy combined with first principle calculations based on the density functional theory (DFT) method. The pseudopotential of a uranium atom was generated and its performance was evaluated for a series of uranium-containing minerals. The structure of rutherfordine was determined for two symmetries (Pmmn and Imm2) and the resulting lattice parameters, bond lengths, bond angles, and X-ray powder diffractogram were found to be in very good agreement with experimental values. The Raman spectrum was experimentally determined in the range 0-1700 cm(-1) and calculated using density functional perturbation theory. The non-scaled theoretical wavenumbers also agreed with the experimental values, and therefore a detailed interpretation of the theoretical spectra allowed us to assign the Raman bands found in the experimental spectrum.

Uranyl silicate mineral soddyite, (UO2)2(SiO4)•2(H2O), is a fundamental component of the paragenetic sequence of secondary phases that arises from the weathering of uraninite ore deposits and corrosion of spent nuclear fuel. In this work, soddyite was studied by first principle calculations based on the density functional theory. As far as we know, this is the first time that soddyite structure is determined theoretically. The computed structure of

Gamma uranium trioxide, γ-UO3, is one of the most important polymorphs in uranium trioxide system which is common throughout the nuclear fuel cycle and used industrially in the reprocessing of nuclear fuel and uranium enrichment. In this work, a detailed theoretical solid-state density functional theory study of this material was carried out. The computed lattice parameters, bond lengths, bond angles and X-Ray powder pattern were found in very good agreement with their experimental counterparts determined by X-Ray diffraction. The equation of state of γ-UO3 was obtained and, therefore, the values of the bulk modulus and its derivatives, for which there are not experimental data to compare with, were predicted. The computed bulk modulus differs from that of a previous density functional theory calculation by only 4.4%. The thermodynamic properties of this material, including heat capacity, entropy, enthalpy, free energy and Debye temperature were also determined as a function of temperature in the range 0-1000 K. The computed low-and high-temperature thermodynamic functions are in excellent agreement with the experimental ones determined from calorimetric measurements. At ambient temperature, the computed values of heat capacity, entropy, enthalpy and free energy differ from the experimental values by 5.3, 3.3, 3.9 and 2.6%, respectively. Finally, the Raman spectrum was determined and compared with the experimental one and was found to be in good agreement. A normal mode analysis of the theoretical spectra was carried out and used in order to resolve the uncertainty of the assignment in the observed Raman bands. The assignment permits to attribute the different bands to vibrations localized in the different distorted octahedra associated to the two non-equivalent uranium atom types present in the structure of γ-UO3.

The design of a safe spent nuclear fuel repository requires the knowledge of the stability of the secondary phases which precipitate when water reaches the fuel surface. Studtite is recognized as one of the secondary phases that play a key-role in the mobilization of the radionuclides contained in the spent fuel. Thereby, it has been identified as a product formed under oxidation conditions at the surface of the fuel, and recently found as a corrosion product in the Fukushima-Daiichi nuclear plant accident. Thermal stability is one of the properties that should be determined due to the high temperature of the fuel. In this work we report a detailed analysis of the structure and thermal stability of studtite. The structure has been studied both by experimental techniques (SEM, TGA, XRD and Raman spectroscopy) and theoretical DFT electronic structure and spectroscopic calculations. The comparison of the results allows us to perform for the first time the Raman bands assignment of the whole spectrum. The thermal stability of studtite has been analyzed by in situ Raman spectroscopy, with the aim of studying the effect of the heating rate and the presence of water. For this purpose, a new cell has been designed. The results show that studtite is stable under dry conditions only at temperatures below 30°C, in contrast with the higher temperatures published up to date (~130°C). Opposite behaviour has been found when studtite is in contact with water; under these conditions studtite is stable up to 90°C, what is consistent with the encounter of this phase after the Fukushima-Daiichi accident.