Résumé
The sintering of hydrothermally-prepared U0.8Ce0.2O2+δ powders was investigated as a model system for future MOX nuclear fuels. Dilatometric analyses evidenced a two-step process, with dehydration, redox reactions and partial removal of residual carbon at low temperature, followed by the onset of sintering around 800°C. The powders can be directly sintered at 1700°C, yielding pellets with relative densities up to ∼98%. Nevertheless, maintaining temperature for several hours led to cracking and de-densification. A sintering map (1300–1700°C, 1–15 h) revealed two regimes associated with densification and grain growth, separated near 91% theoretical density, with a unique densification trajectory. Ex situ analyses confirmed the persistence of a fluorite structure and progressive reduction toward oxygen-stoichiometric dioxide. Residual carbon, still detected at high temperature, is proposed to contribute to mechanical degradation. These results highlight the potential of hydrothermal conversion to produce homogeneous, directly sinterable mixed oxides, while emphasizing the need to optimize carbon removal.