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Réactivité de l’eau à la surface des oxydes d’actinide. Modifications surfaciques et radiolyse / Reactivity of water on the surface of actinide oxides. Surface modifications and radiolysisDeroche, Arnaud 26 November 2018 (has links)
Les oxydes d’actinides sont des matériaux hygroscopiques. L’adsorption de l’eau à leurs surfaces est susceptible d’entrainer des modifications quant à la nature ou à son état. Dans le cas des oxydes à fort débit de dose, vient s’ajouter les effets de radiolyse de l’eau, entrainant sa décomposition et générant du dihydrogène. Ces deux aspects, étude de surface et radiolyse de l’eau, ont été étudiés ici. L’étude de la génération de dihydrogène par radiolyse de l’eau adsorbée à la surface a montré que cette génération linéaire dans les premiers temps atteint une concentration stable au bout de plusieurs heures. Cet état stationnaire a été très peu observé, et est absent dans le cas d’humidité importante. Un conditionnement dans une atmosphère contenant du dihydrogène a permis de mettre en lumière une réaction de consommation du dihydrogène par le matériau. Ces expériences ont permis de faire émerger un modèle cinétique basé sur deux réactions de production et de consommation de dihydrogène. La première correspond à la décomposition de l’eau sous l’effet du rayonnement, et pour la seconde il est suspecté une réduction partielle de la surface avec la formation d’une phase sous-stœchiométrique en surface. Cependant, aucune technique d’analyse de surface n’a permis de mettre en évidence formellement cette phase. La chromatographie gazeuse inverse est une technique peu intrusive vis-à-vis des couches d’eau adsorbée du fait des températures et des pressions mis en jeux et de l’absence de dépôt d’énergie. Cette technique a été employée sur des oxydes de thorium et d’uranium. Sur oxyde de thorium, il en résulte un impact de la température de calcination, avec un maximum d’énergie de surface pour une calcination à 650°C. Par ailleurs, il a été montré que la préparation du dioxyde de thorium pouvait impacter l’état de sa surface. En effet, il a été observé une déshydratation de l’oxalate de thorium au fil du temps, impactant la structure de ce dernier. Cette modification se répercute sur la surface de l’oxyde final par une chute de l’énergie de surface et par une modification sur la répartition des sites d’adsorption en surface. Néanmoins un traitement chimique de l’oxalate permet de retrouver la réactivité de surface et une distribution des sites d’adsorption. L’hydratation de la surface montre une augmentation de l’énergie de surface, mais cette augmentation n’est observée que pour des hydratations de longues durées. / Actinide oxides are hygroscopic materials. The adsorption of water on their surfaces is likely to cause changes in the nature or condition. In the case of oxides with a high dose rate, the effects of radiolysis of the water causes the decomposition of water and generates hydrogen. These two aspects: surface study and radiolysis of water have been studied here.The study of the generation of dihydrogen by radiolysis of water adsorbed on the surface has shown that this linear generation in the early stages reaches a stable concentration after several hours. This stationary state has been very little observed, and is absent in the case of significant humidity. Conditioning in a dihydrogen-containing atmosphere made it possible to highlight a reaction of consumption of dihydrogen by the material. These experiments led to the emergence of a kinetic model based on two reactions of production and consumption of dihydrogen. The first corresponds to the decomposition of the water under the effect of the radiation, and for the second it is suspected a partial reduction of the surface with the formation of a sub-stoichiometric phase on the surface, however no technique of analysis of surface has not formally highlighted this phase.Inverse gas chromatography is a technique that is not very intrusive with respect to the adsorbed water layers because of the temperatures and pressures involved and the absence of energy deposition. This technique has been used on oxides of thorium and uranium. On thorium oxide, this results in an impact of the calcination temperature, with a maximum of surface energy for calcination at 650 ° C. In addition, it has been shown that the preparation of thorium dioxide can impact the state of its surface. Indeed, it has been observed dehydration of thorium oxalate over time, impacting the structure of the latter. This modification affects the surface of the final oxide by a drop-in surface energy and a change in the distribution of surface adsorption sites. Nevertheless, a chemical treatment of oxalate makes it possible to recover the surface reactivity and a distribution of the adsorption sites. The hydration of the surface shows an increase in surface energy, but this increase is observed only for hydrations of long duration.Keywords: water sorption, radiolysis, plutonium, inverse gas chromatography, thorium, uranium
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Defect Clustering in Irradiated Thorium Dioxide and alpha-UraniumSanjoy Kumar Mazumder (16634130) 07 August 2023 (has links)
<p>Thorium dioxide (ThO<sub>2</sub>) and metallic uranium (alpha-U) represent important alternative nuclear fuels. Investigating the behavior of defects introduced into these materials in an irradiation environment is critical for understanding microstructure evolution and property changes. The objective of this dissertation is to investigate the clustering of point defects in ThO<sub>2</sub> and alpha-U under irradiation, into voids and prismatic dislocation loops as a function of irradiation dose rate and temperature. To achieve this, we have developed a mean-field cluster dynamics (CD) model based on reaction rate theory to predict the evolution of self-interstitial atom (SIA) and vacancy loops in neutron-irradiated alpha-U. Detailed atomistic simulations have been carried out using molecular dynamics (MD) to study the configuration of such loops and compute their energetics, which are essential parameters of the CD model. Bond-boost hyper-MD simulations have been performed to compute the diffusivity of uranium SIA and vacancies, which govern the kinetics of the clustering phenomenon. Another CD model has been demonstrated for proton-irradiated ThO<sub>2</sub>, considering the clustering of Th and O SIA and vacancies into SIA loops and voids, respectively, with varying sizes and stoichiometry. The compositions of all SIA loops and voids dictated by crystallography of ThO<sub>2</sub> in its fluorite structure have been presented in their respective cluster composition space (CCS). The CD model solves the density evolution of off-stoichiometric loops and voids, with irradiation, in their respective CCS. MD simulations have been performed to compute the energetics of different clusters in their CCS, as parameters of the CD model. Temperature-accelerated MD simulations have been performed to compute the diffusivity of Th and O point defects, that dictates the kinetics of defect clustering on irradiation. In alpha-U, the CD predictions show an accumulation of small sized vacancy loops and the growth of SIA loops with irradiation dose, which closely fits the reported size distribution of loops in neutron-irradiated alpha-U by Hudson and coworkers. The CD predicted density of defect clusters in proton-irradiated ThO<sub>2</sub>, shows the evolution of near-stoichiometric SIA loops in their CCS. The size distribution of SIA loops at high irradiation doses closely corresponds to the transmission electron microscopy (TEM) observations reported in the literature. Also, the CD model did not predict the growth of voids and vacancy clusters, which is consistent with findings in literature. The model was further used to predict the density of sub-nanometric defect clusters and point defects, on low-dose irradiation, that significantly impairs the thermal conductivity of ThO<sub>2</sub>. An extensive TEM and CD investigation has also been carried out to study the growth and coarsening of SIA loop and voids during post-irradiation isochronal annealing of ThO<sub>2</sub> at high temperatures.</p>
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