Abstract
Sintering is a key step in the elaboration of UOx and MOx (U/Pu mixed oxide) nuclear fuels pellets used in the pressurized water reactors. The first step of this process, which consists in the elaboration of a neck between the grains and led to the consolidation of the material, is generally described through numerical simulation. The models used for the theoretical description of this step are generally constituted by two spherical grains in contact. In order to perform the first experimental observations of the initial stage of sintering of ceramics materials of interest for electronuclear fuel cycle and to complement the numerical approaches, samples of lanthanide (CeO2) and actinides (ThO2 and UO2) dioxides with controlled morphology were examined by environmental scanning electron microscopy during heat treatment at high temperature (HT-ESEM).First, the protocols leading to the synthesis of lanthanides and actinides oxides microspheres were developed, and the powders obtained characterized. It was thus possible to obtain, for all the compounds studied, systems similar to those generally modeled. HT-ESEM was then used as the main investigation tool for the in situ study of the first stage of sintering of these compounds. The study of the morphological modifications occurring in isolated microspheres first confirmed their polycristalline character. Indeed, heat treatment led to a progressive decrease of the crystallites number included inside the grains through different mechanisms (oriented attachment, diffusion), whose activation energy was evaluated. For the systems constituted by two CeO2 or ThO2 microspheres in contact, the ESEM micrographs allowed to observe the evolution of several parameters during heat treatment, such as neck size and grain size as well as distance between the grains center. Images processing methods using custom software were then applied in order to determine the quantitative kinetic data. The mechanisms involved, such as the rearrangement of crystalline planes and the matter diffusion, and the corresponding activation energies, were also identified. Furthermore, the law of neck growth, which allows one to describe the evolution of sintering degree, was used to determine the prevailing diffusion mechanism during heat treatment. The influence of various parameters on the sintering degree was also highlighted. For example, the influence of grains polycristallinity on sintering mechanisms and kinetics was particularly investigated study by working in parallel with polycristalline and single crystal grains, then by comparing the experimental results with data coming from modeling. Finally, the methodology developed for the study of CeO2 and ThO2 was transposed to the compound of interest UO2. In this case, the data previously described were complemented by a first approach of the influence of atmosphere used during the heat treatment.