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Our study is focused on the behavior of grain boundaries in uranium dioxide system under irradiation conditions. The research can be seen as two parts: the study of interaction of the grain boundary and the damage cascade, and the calculation of Kapitza resistance of grain boundaries. The connection between these two parts lies in that damage cascades bring in changes in the structure and other properties of grain boundaries, and inevitably the Kapitza resistance of the grain boundary changes as well. For the first part, we studied interactions of grain boundaries and damage cascades in uranium dioxide system by simulating two types of bombardments: one direct bombardment into a grain boundary leading to ballistic-collision-mediated interface mixing; the other bombardment is in the close vicinity of a grain boundary causing interface biased defect migration. We found that more defects are trapped by the grain boundary followed by the first type of bombardment, resulting in enhanced grain boundary energy. By comparing with the second type of bombardment, we are able to reveal the mechanisms of the interaction between defects and grain boundaries. For the second part, we employed the non-equilibrium molecular dynamics method to calculate the Kapitza resistance of different coincident site lattice boundaries with or without defects loaded, and later we found that a universal positive correlation between the Kapitza resistance and the grain boundary energy can be well established, regardless of the cause of boundary energy changes. Our study provides a deeper understanding of the Kapitza resistance of the grain boundary and its evolutions under irradiation, which benefits multi-scale modeling of uranium dioxide thermal properties under extreme radiation conditions as well as experimental studies of fuel material thermal properties.

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abstract: The United States Department of Energy (DOE) has always held the safety and reliability of the nation's nuclear reactor fleet as a top priority. Continual improvements and advancements in nuclear fuels have been instrumental in maximizing energy generation from nuclear power plants and minimizing waste. One aspect of the DOE Fuel Cycle Research and Development Advanced Fuels Campaign is to improve the mechanical properties of uranium dioxide (UO2) for nuclear fuel applications. In an effort to improve the performance of UO2, by increasing the fracture toughness and ductility, small quantities of oxide materials have been added to samples to act as dopants. The different dopants used in this study are: titanium dioxide, yttrium oxide, aluminum oxide, silicon dioxide, and chromium oxide. The effects of the individual dopants and some dopant combinations on the microstructure and mechanical properties are determined using indentation fracture experiments in tandem with scanning electron microscopy. Indentation fracture experiments are carried out at room temperature and at temperatures between 450 °C and 1160 °C. The results of this work find that doping with aluminosilicate produces the largest favorable change in the mechanical properties of UO2. This sample exhibits an increase in fracture toughness at room temperature without showing a change in yield strength at elevated temperatures. The results also show that doping with Al2O3 and TiO2 produce stronger samples and it is hypothesized that this is a result of the sample containing dopant-rich secondary phase particles. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2014

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Made available in DSpace on 2014-10-09T12:44:35Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:39Z (GMT). No. of bitstreams: 0 / A incorporação do gadolínio diretamente no combustível de reatores nucleares para geração de eletricidade é importante para compensação da reatividade e para o ajuste da distribuição da densidade de potência, permitindo ciclos de queima mais longos, com intervalo de recarga de 18 meses, otimizando-se a utilização do combustível. A incorporação do Gd2O3 sob a forma de pó homogeneizado a seco diretamente com o pó de UO2 é o método comercialmente mais atraente devido à sua simplicidade . Contudo, este método de incorporação conduz a dificuldades na obtenção de corpos sinterizados com a densidade niínima especificada, devido a um bloqueio no processo de sinterização. Pouca informação existe na literatura específica sobre o possível mecanismo deste bloqueio, restrita principalmente à hipótese da formação de uma fase (U,Gd)O2 rica em gadolínio com baixa difusividade. Este trabalho tem como objetivo a investigação do mecanismo de bloqueio da sinterização neste sistema, contribuindo para o esclarecimento da causa deste bloqueio e na elaboração de possíveis soluções tecnológicas. Foi comprovado experimentalmente que o mecanismo responsável pelo bloqueio é baseado na formação de poros estáveis devido ao efeito Kirkendall, originados por ocasião da formação da solução sólida durante a etapa intermediária da sinterização, sendo difícil a sua eliminação posterior, nas etapas finais do processo de sinterização. Com base no conhecimento deste mecanismo, possíveis propostas são apresentadas na direção da solução tecnológica do problema de densificação característico do sistema UO2-Gd203. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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Une étude cinétique de l’hydrofluoration du dioxyde d’uranium a été menée entre 375 et 475°C sous des pressions partielles d’HF comprises entre 42 et 720 mbar. La réaction a été suivie par thermogravimétrie isotherme et isobare. Les données cinétiques obtenues, couplées à une caractérisation de la poudre avant, pendant et après réaction par MEB, EDS, DRX et BET, ont montré que les grains de poudre d’UO2 se transforment selon un modèle de germination instantané, croissance anisotropre et développement interne. L’étape limitant la vitesse de croissance est la diffusion d’HF dans la couche d’UF4. Un mécanisme de croissance de la couche d’UF4 a été proposé. Dans le domaine de température et de pression étudié, la réaction est d’ordre 1 par rapport à HF et suit une loi d’Arrhénius. Une équation de vitesse a été déterminée et a servi à effectuer des simulations cinétiques qui ont toutes montré une très bonne corrélation avec l’expérience.Le couplage de cette équation de vitesse avec les phénomènes de transport de chaleur et de matière a permis d’effectuer des simulations à l’échelle d’un agglomérat de poudre. Elles ont montré que certaines structures d’agglomérats influencent la vitesse de diffusion des gaz dans le milieu poreux et par conséquent influencent la vitesse de réaction. Enfin des simulations de vitesse de transformation de tas de poudre et de pastilles ont été réalisées et comparées aux vitesses expérimentales. Les courbes cinétiques expérimentales et simulées ont même allure, mais des améliorations dans les simulations sont nécessaires pour pouvoir prédire avec précision des vitesses : le couplage entre les trois échelles (grain, agglomérat, four) en est un exemple. / A kinetic study of hydrofluorination of uranium dioxide was performed between 375 and 475°C under partial pressures of HF between 42 and 720 mbar. The reaction was followed by thermogravimetry in isothermal and isobaric conditions. The kinetic data obtained coupled with a characterization of the powder before, during and after reaction by SEM, EDS, BET and XRD showed that the powder grains of UO2 are transformed according a model of instantaneous germination, anisotropic growth and internal development. The rate limiting step of the growth process is the diffusion of HF in the UF4 layer. A mechanism of growth of the UF4 layer has been proposed. In the temperature and pressure range studied, the reaction is of first order with respect to HF and follows an Arrhenius law. A rate equation was determined and used to perform kinetic simulations which have shown a very good correlation with experience. Coupling of this rate equation with heat and mass transport phenomena allowed to perform simulations at the scale of a powder’s agglomerate. They have shown that some structures of agglomerates influence the rate of diffusion of the gases in the porous medium and thereby influence the reaction rate. Finally kinetic simulations on powder’s beds and pellets were carried out and compared with experimental rates. The experimental and simulated kinetic curves have the same paces, but improvements in the simulations are needed to accurately predict rates: the coupling between the three scales (grain, agglomerate, oven) would be a good example.