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Etude du rapport O/M dans des nouveaux combustibles oxydes à base d'U et Pu : élaboration et caractérisation de matériaux modèles U1-y PuyO2-x

Abstract : Uranium-plutonium mixed oxides are considered within the scope of the development of nuclear fuel for the next generation of nuclear reactors (Sodium-cooled fast reactors). Because of some technological choices and safety constraints, the mixed oxide fuel will exhibit an oxygen hypostoichiometry, i.e. its Oxygen/Metal ratio (noted O/M) will be lower than 2.00. The control of this deviation from stoichiometry is essential as the O/M ratio influences numerous of the fuel properties irradiation (thermal conductivity, melting temperature, dilatation, etc.) which in turn strongly affect the behavior under. First, a special attention was paid to the fabrication of mixed oxide pellets U1-yPuyO2-x with different plutonium contents (y = 0.15 ; 0.28 and 0.45) by powder metallurgy. The two main goals were to obtain: o A homogeneous U-Pu distribution in order to have suitable materials for a thermodynamic study. o A high density of the resulting pellets in order to determine oxygen chemical diffusion coefficients within the three compounds by thermogravimetric analysis and oxygen self-diffusion coefficients by secondary ion mass spectrometry (SIMS). The second part of this study was focused on associating the O/M ratio values to the micro- and crystallographic structures of the fabricated samples. Beforehand, the qualification of the used gravimetric and thermogravimetric experimental devices dedicated to the O/M ratio measurements was performed. A reliable experimental method was then proposed for the determination of the oxygen stoichiometry of uranium-plutonium mixed oxides taking into account the presence of americium within the samples generated by natural decay of plutonium. With the aim of controlling the O/M ratio of U1-yPuyO2-x during fabrication, the influence of the cooling rate on the oxygen stoichiometry during sintering was investigated. Particularly, the crystallographic and microstructural effects of a variation in the O/M ratio during cooling were studied both at high and room-temperatures. Moreover, these effects made it possible to obtain new data on the kinetics and mechanisms of the phase separation occurring in the hypostoichiometric mixed oxides at high Pu content. Finally, the stability of U1-yPuyO2-x at room-temperature during standard storage conditions was investigated by thermogravimetry, X-ray absorption spectroscopy and X-ray diffraction. Finally, an experimental thermodynamic study of U1-yPuyO2-x was performed by thermogravimetric analysis and high-temperature X-ray diffraction as a function of temperature and oxygen partial pressure. The main factor allowing the establishment of the thermodynamic equilibrium being the oxygen diffusion, the associated chemical and self-diffusion coefficients were determined by thermogravimetry and SIMS after 16O – 18O isotopic exchange. These innovative results will allow a better understanding of the U-Pu mixed oxide properties on the basis of the point defect chemistry.
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Romain Vauchy. Etude du rapport O/M dans des nouveaux combustibles oxydes à base d'U et Pu : élaboration et caractérisation de matériaux modèles U1-y PuyO2-x. Autre. Université de Grenoble, 2014. Français. ⟨NNT : 2014GRENI053⟩. ⟨tel-01635286⟩

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