Characterization and property evaluation of (Th–3.75U)O2+x fuel pellets fabricated by impregnated agglomerate pelletization (IAP) process

Impregnated agglomerate pelletization (IAP) process has been developed by Advanced Fuel Fabrication Facility (AFFF), Tarapur for the fabrication of (Th–3.75U)O2+x mixed oxide fuel for Indian advanced heavy water reactors (AHWR). In this process, ThO2 spheroids were impregnated with uranyl nitrate solution and the resultant mixture was compacted to form green pellets which were sintered in the oxidizing atmosphere to obtain high density (Th–3.75%U)O2+x pellets. An attempt has been made in this paper to characterize the pellets fabricated by IAP route. The characterization of the sintered IAP pellets was made by X-ray diffraction, EPMA, alpha autoradiography, chemical analysis, O/M ratio, immersion density and optical microscopy. The characteristics of (Th–U)O2 pellets fabricated by impregnated agglomerate pelletization (IAP) were also compared with those fabricated by coated agglomerate pelletization (CAP), and conventional powder pellet (POP) processes. In case of pellets fabricated by IAP route, XRD data showed the presence of single fluorite phase. The uranium concentration and grain size distribution were found to be uniform throughout the pellet. These phase characteristics were also found to be more uniform in pellets made by both IAP and POP processes as compared to pellets fabricated using the CAP process.

► Characterization of (Th–3.75%U)O2+x pellet fabricated by a new technique i.e., impregnated agglomerate pelletization (IAP).
► Comparison with pellets fabricated by powder to pellet (POP) and coated agglomerate pelletization processes.
► Solid solution was more complete in POP and IAP pellets.
► Uranium distribution was less uniform in the pellets fabricated by the CAP route.