The context of this study is about increasing the performances of nuclear fuel used in existing nuclear power plants. These nuclear fuels are made by sintering process of UO2 powders. The aim of our study consist in correlating powders characteristics to the sintered pellet microstructure, and relating microstructure parameters to their mechanical behaviour at high temperatures which are representative to reactor conditions.In order to study the sintering process, we define controlled UO2 model powders with reproducible and defined characteristics. Those powders, with a more simple granular packing than the industrial UO2 powder ones, are made whether by milling process (de agglomeration) or by sequences of oxidative, reductive and milling treatments. The powder sintering was processed in reducing atmosphere. The dilatometric monitoring of model powders and the microstructure characterization of resulting sintered pellets enabled to determine the important role of: a powder characteristic - the fraction of fine particles - and a sintering process parameter – the rinsing temperature.Mechanical tests with strain rate and stress controlled were made on sintered pellets made from industrial powders and simplified (de agglomerated) UO2 powders. The tests conditions have a dominant effect on the visco-plastic deformation mechanisms. Those mechanisms were identified as well as their dominant area according to the stress (or the strain rate), the microstructure grain size and the temperature. The corresponding area limits were determined.The characterization of the deformed microstructures (macroscopic observations, optical microscopy, SEM) highlighted the fact that working on the curve resulting from the mechanical compression tests is not enough to analyze the high temperature mechanical behavior of UO2 pellets. A significant damage of the microstructures was observed. Its initiation and evolution depend on the deformation rate and the studied microstructure.
Nuclear UO2 fuel microstructure with controlled properties : understanding their elaboration mechanisms and high temperature mechanical behaviour