Thermal study of sintered (Th-U)O2 MOX pellet by a commercial thermo-gravimetric analyzer coupled with an evolved gas analyzer

Full text: Fabrication of (Th-U)O2 MOX pellets by the impregnation agglomerate pelletization (lAP) process is being explored in Advanced Fuel Fabrication Facility, BARC, Tarapur for the forthcoming Advanced Heavy Water Reactor (AHWR). High temperature thermal study of this fuel is important in order to understand the behaviour of the fuel under the operational temperature of the reactor. In this study, fabrication of ThO2-3%UO2 was carried out by impregnation agglomerate pelletization process and subsequently sintered in reducing or air atmosphere. The degassed pellets were broken into small pieces and subjected to high temperature (1050 deg C-1250 deg C) heating under high pure argon gas in a commercial thermal analyzer. Subsequently the evolved gases were qualitatively analyzed by a quadrupole mass analyzer. The pellet sintered in reducing atmosphere (IAP-R) shows an increase in weight after the analysis where as the pellet sintered in oxidizing atmosphere (IAP-O) shows a decrease in final weight. The IAP-R pellet may become slightly hyper-stoichiometric on heating due to the presence of small amount of oxygen in the high pure argon gas. This is further supported by the mass spectrum at m/z 32(O2+) that shows a decrease in the signal intensity as the temperature of analysis increases. The sharp decrease of the signal intensity at m/z 32(O2+) started at 920 deg C temperature may be attributed to the formation of SO2 (m/z=64) and CO2 (m/z=44) gases. On the other hand the IAP-O pellet being hyper stoichiometric initially may lose its weight to form water on reaction with the excess oxygen on heating due to the presence ( small amount of hydrogen in the high pure argon gas. This is being supported by the appearance of small peak at m/z 18 (H2O+) in the mass spectrum. The formation of SO2 and CO2 gases started at higher temperature in case of IAP-O pellet as compared to that of IAP-R pellet. This may be due to the higher density achieved in case of IAP-O pellet which hindered the formation of SO2 and CO2 gases at lower temperature. The presence of carbon and sulphur in these pellets were also quantitatively determined by a commercially available determinator and found to contain around 55 ppm of carbon and 25 ppm of sulphur in both the cases