Investigation of the Sintering Mechanisms for (U,Am)O2 Pellets Obtained by CRMP Process

The use of CRMP (Calcined Resin Microsphere Pelletization) process for AmBB (Americium Bearing Blankets) fabrication is today a key research axis in americium transmutation domain, where its very high activity requires minimization of powder dissemination. In this aim, the use of oxide microspheres as compaction precursors is a promising clean alternative to powder metallurgy. Understanding the different steps of densification during CRMP pellet sintering thus appears as fundamental to obtain final materials with the specific features required for AmBB. The densification curve recorded in dynamic conditions shows different sintering steps. A first decrease of shrinkage rate happens at low temperature, around 1100 K. This phenomenon is not normally observed in the sintering of conventional powders. Chemical and microstructural studies were performed on (U,Am)O2 and also on (Ce,Gd)O2 surrogate compound to highlight the causes of this low-temperature sintering step. Multiscale reorganization finally appears as the sole explanation, through the sintering of nanometric aggregate present in the green pellet and related to the morphology of the starting microspheres employed as pelletization precursors.