ETUDE DU PLUTONIUM EN PHASE ET DE SES ALLIAGES AVEC LES ELEMENTS DE LA COLONNE IIIB

PARIS-BIUSJ-Physique recherche (751052113) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocSudocFranceF

Etude du plutonium et de ses alliages avec les éléments de la colonne IIIB

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Etude du comportement du produit de fission césium dans le dioxyde d uranium par méthode ab initio

La connaissance du comportement des produits de fission dans le combustible nucléaire est importante pour des questions de sûreté et pour comprendre l évolution des propriétés du combustible irradié. L objectif principal de la thèse est l étude par calculs ab initio du comportement du césium dans UO2 : la solubilité de Cs dans les défauts de la matrice, sa diffusion, la stabilité relative du Cs dissous dans le combustible vis à vis de la précipitation de phases solides ont été traités. Le rôle des corrélations électroniques entre électrons f de l uranium sur ces différentes propriétés et sur la description du cristal parfait a été mis en évidence. Les énergies de formation des principaux défauts ponctuels dans UO2 ont été calculées, l évolution de leur concentration en fonction de la stœchiométrie du combustible et de la température a été estimée. Les barrières énergétiques et les chemins de migration pour l auto-diffusion des lacunes et interstitiels d oxygène et des lacunes d uranium dans UO2 sont discutés. La solubilité de Cs dans UO2 est trouvée très faible, en accord avec les mesures expérimentales. Les sites de piégeage les plus favorables ont été déterminés dans les différents domaines de stœchiométrie du combustible. Nous montrons que la diffusion de Cs à partir de son site d incorporation le plus favorable dans UO2+x est limitée par la diffusion des lacunes d uranium. La formation des phases solides issues de l oxydation de Cs (Cs2O, Cs2O2, CsO2) et de son interaction avec le combustible (Cs2UO4), le molybdène (Cs2MoO4) et le zirconium (Cs2ZrO3) a été considérée car l existence de ces phases, leur solubilité et leur interdépendance affectent le relâchement de Cs.The knowledge of the behaviour of fission products in the nuclear fuel is very important for safety considerations and for understanding the evolution of the fuel properties under irradiation. In this work, we focussed mainly on the behaviour of caesium in UO2 through ab initio studies of its solubility at point defects in the matrix, its diffusion and its contribution to the formation of solid phases in the fuel. The role of electronic correlation effects of the f electrons of uranium on these properties and on the description of the defect free crystal, is assessed. The formation energies of the main point defects are calculated and their concentration as a function of fuel stoichiometry and temperature is estimated. The migration barriers and migration paths for the self-diffusion of oxygen and uranium vacancies and oxygen interstitials in UO2 are discussed. The solubility of Cs is found to be very low in UO2 in agreement with experimental findings. The most favourable trapping sites are determined as a function of oxygen concentration in the fuel. Our results show that in the hyper-stoichiometric regime, the diffusion of Cs from its most favourable trapping site is limited by the uranium vacancy diffusion mechanism. We also considered the formation of the main solid phases of caesium resulting from its oxidation (Cs2O, Cs2O2, CsO2) and from its interaction with the fuel (Cs2UO4), with molybdenum (Cs2MoO4) and with the zirconium of the clad (Cs2ZrO3), since the formation of such phases, their solubility and their interdependence will affect the release of caesium.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Atomic-scale structural signature of dynamic heterogeneities in metallic liquids

International audienceWith sufficiently high cooling rates, liquids will cross their equilibrium melting temperatures and can be maintained in a metastable undercooled state before solidifying. Studies of undercooled liquids reveal several intriguing dynamic phenomena and because explicit connections between liquid structure and liquids dynamics are difficult to identify, it remains a major challenge to capture the underlying structural link to these phenomena. Ab initio molecular dynamics (AIMD) simulations are yet especially powerful in providing atomic-scale details otherwise not accessible in experiments. Through the AIMD-based study of Cr additions in Al-based liquids, we evidence for the first time a close relationship between the decoupling of component diffusion and the emergence of dynamic heterogeneities in the undercooling regime. In addition, we demonstrate that the origin of both phenomena is related to a structural heterogeneity caused by a strong interplay between chemical short-range order (CSRO) and local fivefold topology (ISRO) at the short-range scale in the liquid phase that develops into an icosahedral-based medium-range order (IMRO) upon undercooling. Finally, our findings reveal that this structural signature is also captured in the temperature dependence of partial pair-distribution functions which opens up the route to more elaborated experimental studies

On the partial enthalpy of mixing of Nb in liquid Al

International audienc

On the partial enthalpy of mixing of Nb in liquid Al

International audienc

On the heat of formation of ye'elimite Ca 4 Al 6 O 12 .SO 4 using density functional theory

International audienceThe heat of formation of ye'elimite Ca 4 Al 6 O 12 .SO 4 was calculated using density functional theory (DFT). The crystallographic ground state of ye'elimite is the orthorhombic Pcc2 modification in agreement with experimental information. Three different many-body interaction functionals were used for DFT calculations: the generalised gradient approximation (GGA), the Armiento/Mattsson 2005 functional (AM05) and the strongly conditioned and appropriately normed functional (Scan). The best results were obtained with the Scan functional and a heat of formation at 298 K of −35·5 kJ/mol referred to pure calcium oxide (CaO), aluminium oxide (Al 2 O 3) and calcium sulfate (CaSO 4) was found. All other reported modifications of ye'elimite (cubic, tetragonal, monoclinic) are unstable with endothermic heat of transformations ranging from +18 to +78 kJ/mol