To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U 1−y M y O 2±x materials is essential. These compounds show complex chemical properties, originating from the fact that actinides and rare earths may occur with different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce changes in the oxygen stoichiometry, with dramatic effects on the properties of the fuel. First studies on U 1−y Am y O 2±x indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of these compounds, over a wide compositional domain, by combining XRD, XAS and Raman spectroscopy. We provide evidences of the coexistence of four different cations (U 4+ , U 5+ , Am 3+ , Am 4+) in U 1−y M y O 2±x compounds, which nevertheless maintain the fluorite structure. Indeed, we show that the cationic sublattice is basically unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice.

Solubility and diffusion of boron and oxygen in the γ-TiAl intermetallic compound are herein investigated by firstprinciple calculations. On the basis of the space group of the γ phase, the accommodation of the light atoms in the various interstitial positions is studied by density functional theory. Diffusion coefficients are also obtained from ab initio calculations. Barrier energies for the boron and oxygen diffusion among the different stable interstitial sites are computed using the Nudged Elastic Band method and atomic jumping rates are obtained from the Transitional State Theory. Diffusion coefficients are obtained from the solution of the transport equation in the infinite time limit, using the analytical Multi-State Diffusion method. The methodology here applied is validated by the good agreement between the computed diffusion coefficient of boron and the experimental data available in the literature for this species.

In the frame of minor actinide transmutation, americium can be diluted in UO and (U, Pu)O fuels burned in fast neutron reactors. The first mandatory step to foresee the influence of Am on the in-reactor behavior of transmutation targets or fuel is to have fundamental knowledge of the Am-O binary system and, in particular, of the AmO phase. In this study, we coupled HT-XRD (high-temperature X-ray diffraction) experiments with CALPHAD thermodynamic modeling to provide new insights into the structural properties and phase equilibria in the AmO-AmO-AmO domain. Because of this approach, we were able for the first time to assess the relationships between temperature, lattice parameter, and hypostoichiometry for fcc AmO. We showed the presence of a hyperstoichiometric existence domain for the bcc AmO phase and the absence of a miscibility gap in the fcc AmO phase, contrary to previous representations of the phase diagram. Finally, with the new experimental data, a new CALPHAD thermodynamic model of the Am-O system was developed, and an improved version of the phase diagram is presented.

Most materials expand with temperature because of the anharmonicity of lattice vibration, and only a few shrink with increasing temperature. UO2, whose thermal properties are of significant importance for the safe use of nuclear energy, was considered for a long time to belong to the first group. This view was challenged by recent in situ synchrotron X-ray diffraction measurements, showing an unusual thermal decrease of the U–O distances. This thermal shrinkage was interpreted as a consequence of the splitting of the U–O distances due to a change in the U local order from Fm3̅m to Pa3̅. In contrast to these previous investigations and using an element-specific synchrotron-based spectroscopic method, we show here that the U sublattice remains locally of the fluorite type from 50 to 1265 K, and that the decrease of the first U–O bond lengths is associated with an increase of the disorder.

In the frame of minor actinide recycling, (U,Am)O2 are promising transmutation targets. To assess the thermodynamic properties of the U-Am-O system, it is essential to have a thorough knowledge of the binary phase diagrams, which is difficult due to the lack of thermodynamic data on the Am-O system. Nevertheless, an Am-O phase diagram modelling has been recently proposed by Gotcu. Here, we show a recent investigation of the Am-O system using in-situ High Temperature X-ray Diffraction under controlled atmosphere. By coupling our experimental results with the thermodynamic calculations based on the Gotcu model, we propose for the first time a relation between the lattice parameter and the departure from stoichiometry.

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