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International audience; The capabilities of the Thermodynamic of Advanced Fuels – International Database (TAF-ID) for reproducing the chemical behaviour of irradiated nuclear fuel in severe accident conditions were studied, by comparing calculation results to experimental observations. SIMFUELs samples containing UO2 + 11 elements (Ba, Ce, La, Mo, Nd, Pd, Rh, Ru, Sr, Y and Zr, in concentration representative of a 76 GWd•tU-1 Burn-up) were submitted to 1327°C in oxidizing conditions, and to 1800, 2000, and 2100°C under reducing ones. Samples were characterized by Electron Probe Micro-Analysis (EPMA), High-Resolution XRD, and X-ray Absorption Spectroscopy (XAS). Thermodynamic calculations reproduced accurately the solubility of fission products in the UO2 matrix, and the type and composition of minor oxide and most metallic phases. Calculations also explain metallic phases microstructure as a result of the progressive solidification of liquids. However, some features such as the U-Pd-Rh association could not be reproduced, due to the lack of a thermodynamic model for this ternary system.

International audience; The UMo/Al dispersion fuel in plate form is considered for the conversion of high-performance research reactors in Europe. In the framework of the UMo fuel qualification program, the adequate margin of safety performance by considering several technological solutions associated with the fabrication parameters, such as heat-treatment of the UMo particles, coating with a diffusion barrier material and powder size distribution as some examples. All these parameters, along with the effect of the hot-rolling process were evaluated by means of image processing and detailed microstructural characterizations for fresh samples i.e. prior to irradiation tests. Principle macroscopic features of powder batches include the size and shape distributions and coating surface examinations. Microscale investigations explored both the coating and kernel microstructures as well as the interface layer between them. Finally, nanoscale analyses examined the UMo–coating interface. The extensive stresses associated with the hot-rolling process have a significant impact on the deformation of the UMo kernels and the degradation of the coated film. The UMo kernels mostly lost their spherical shape for faceted and elongated shapes whereas three types of film degradations were identified including cracks, chippings and delamination.

International audience; Neutrons and high energy ions create displacement cascades in materials, which have been simulated using Molecular Dynamics, for many decades now. The breakthrough of this work is to explore in a large statistics of more than 70 0 0 cascades the relation between early cascade morphology and the final primary damage using a multivariate multiple regression analysis. For two energies in Fe and W, the total number of defects, the number of SIA and vacancy clusters and their full size distributions have been characterized using a multivariate multiple linear regression analysis based on 7 descriptors of the primary damage and 3 morphology descriptors. We find that the combination of the volume and the sphericity is significant. This analysis highlights several cascade properties, among them, that the large and spherical cascades create less defects and in particular, less mono defects than small and fragmented ones. 13 interatomic potentials differing either by their equilibrium part or the way they were hardened have been included in this study and the multivariate analysis shows that the choice of potential has a limited influence on the total number of defects but a large one on the number of mono vacancies. On average, soft potentials create cascades of larger volume, smaller sphericity and producing more defects than hard potentials. Finally, the formation of vacancy clusters is different in Fe than in W. In Fe, the fraction of vacancies in clusters is larger than that of SIAs and larger vacancy clusters are created than SIA clusters. In W, it is the opposite. The reasons are the differences of stopping power and threshold displacement energies, which result in different spatial distributions of open volumes that form during the expansion stage of the cascade. ?

International audience; The primary damage in metallic alloys, i.e. the point defect distribution resulting from the interaction between an energetic particle and a metallic matrix has been investigated for more than 60 years using atomistic simulations. The defect distribution produced in cascades is sensitive to the equilibrium part of the potential as well as its hardened part. An analysis based on large statistics of molecular dynamics simulations and comparison with different embedded atom method potentials in Fe and W allows to rationalize the potential behavior. Correlations between static non-equilibrium properties (quasi static drag (QSD)), threshold displacement energies (TDE), replacement collision sequences (RCS) along the < 110 > direction have been revealed. Along this direction, the lower the TDE, the lower the QSD and the more energy is transmitted along the < 110 > direction during the RCS, i.e. the softest potentials are the ones for which the most energy is transmitted from the PKA to the first head-on atom in the direction of the RCS sequence.