Experimental approach of the two-grain model : first stage of UO2+x and UO2+x/CeO2 microspheres sintering ; Approche expérimentale du modèle à deux grains : premier stade du frittage de microsphères UO2+x et UO2+x/CeO2

Sintering is a key step during the fabrication of UOx and MOx nuclear fuels used in the PWR reactors. The first stage of this process that is characterized by the formation of necks between the grains, leading to the mechanical consolidation of the sample, is particularly crucial. It is usually described through mathematical models based on two spherical grains in contact. In this frame, an experimental approach of the first stage of sintering for ceramic materials of interest for the nuclear fuel cycle, mainly based on in situ high-temperature environmental scanning electron microscopy observations, is developed in this PhD work. An original synthesis route towards size-controlled UO2+x and (U,Ce)O2+x microspheres was first developed. Then, experimental observations of the first stage of UO2+x sintering have been carried out under different operating conditions by varying the oxygen partial pressure and temperature. Quantitative data obtained through image processing led to determine prevailing diffusion mechanisms and associated activation energies for different hyper-stoichiometric uranium dioxides as well as for U3O8. Under reducing conditions, complementary ex situ experiments and high-temperature transmission electron microscopy observations has allowed identifying a two-step mechanism based on the growth of crystallites in the grains, then on the formation and growth of a neck between the grains. Finally, a preliminary study on (UO2 – CeO2) heterogeneous systems revealed a drastic modification of the morphological evolution of the two grains, induced by the increasing complexity of the involved diffusion mechanisms (including the preferential diffusion of uranium in CeO2). Conversely, the first sintering tests carried out on homogeneous (U,Ce)O2 solid solutions evidenced the classical behavior described by the models. The stoichiometry of the samples involved in the sintering of actinide dioxides, but also their homogeneity, must be carefully taken into account in the aim to build predictive numerical ...