Effect of sintering atmosphere on the performance of MOX fuel pellet

[Background] The mixed oxide fuel of uranium and plutonium (MOX fuel) is mainly used in fast reactor (FBR) and pressurized water reactor (PWR), and a small amount of MOX fuel is used in boiling water reactor (BWR). The MOX fuel manufacturing is the key link of 'nuclear fuel closed cycle' whilst the key process of MOX fuel manufacturing is to sinter the fully mixed uranium plutonium oxide powder into the sintering pellets which meet the performance requirements. Sintering temperature, insulation time and sintering atmosphere are the key factors that affect the densification, the oxygen metal ratio and the microstructure of sintering pellet, etc. [Purpose] This study aims to investigate the influence of sintering atmosphere on the properties of pellets. [Methods] PuO₂ powder was pretreated and mixed with UO₂ powder in a certain proportion. The mixed powder was ground and granulated to form green body by two-way pressing. The same batch of green samples obtained by the same treatment process were fired in a high-temperature sintering furnace at 1700℃ for 6 h, and then slowly reduced to room temperature. The microstructure of MOX fuel pellet was observed and analyzed by metallurgical microscope. The effect of the different water content on the properties of pellets in hydrogen argon mixture sintering atmosphere was studied experimentally. [Results] Experimental results show that the compactness and the O/M ratio of MOX sintered pellets increases with the increasing of water content in sintering atmosphere when the sintering temperature and holding time remain unchanged. When the water content in sintering atmosphere is more than 1600 ppm, the obtained stoichiometric MOX sintered pellets have uniform microstructure without microcracks. [Conclusions] The sintering process conforms to the theory of vacancy diffusion mechanism. (authors)