Abstract
After its passage through a reactor, the spent nuclear fuel presents a variety of chemical composition and microstructural properties. The presence of fission products in the different constituent phases of this spent fuel is likely to influence their dissolution in a nitric medium, the first stage in the reprocessing process. In order to improve our ability to anticipate the behaviour of the spent fuel during reprocessing, the main objective of this thesis work was to study the dissolution mechanisms and kinetics of model compounds based on uranium dioxide and lanthanide elements, and to identify the main influencing factors. In this context, this work focused especially on understanding and highlighting the effects of incorporating trivalent lanthanide elements into the UO2 structure and their impact on the sinterability and chemical durability in a nitric medium of these materials.The approach adopted was based firstly on the preparation of a panel of U1-xLnxO2±y model solid solutions of varying compositions and microstructures. These mixed oxides were obtained from hydroxide precursors synthesised by wet route, followed by their conversion into oxides by heat treatment. Following a shaping stage, dense pellets with physicochemical and microstructural properties of interest were prepared using a high-temperature sintering process. Two sintering atmospheres were studied in order to vary the uranium speciation and the oxygen stoichiometry within the sintered pellets, as well as their microstructural properties (density, porosity, grain size). Particular attention was paid to the detailed characterisation of the various compounds obtained at the end of each stage (synthesis, conversion, sintering).Secondly, the dissolution of powdered or sintered samples was carried out in a nitric medium using a multiparametric study. The impact of neodymium composition and sample preparation conditions, particularly temperature and sintering atmosphere, on dissolution kinetics and mechanism was studied. The incorporation of lanthanide elements led to a moderate increase in the dissolution rate compared with the reference UO2 sample. In addition, the U(V) fraction showed little impact on the dissolution kinetics, while the microstructure had a moderate but noticeable effect, and the presence of structural defects could adversely affect the chemical durability of these materials. Finally, operando monitoring of the microstructural evolution of the solid/solution interface during dissolution was carried out using ESEM image acquisition. The information collected on a microscopic scale showed a preferential dissolution localised at the grain boundaries in the case of pellets sintered under reducing atmosphere, whereas the dissolution of pellets sintered under inert atmosphere appeared to be much more homogeneous.