Effect of Particle Size Distribution of Hyper-Stoichiometric UO₂ Powder on UO₂ Pellet Density

The sintering behavior of monomodal particle size distribution (PSD) and trimodal PSD of UO_2+x compacted pellets in 2% H₂-N₂ in CO₂ and N₂ atmosphere was studied. The monomodal PSD was in the range of 5 to 125 μm while the trimodal PSD was in the range of 2.75 to 837 μm. The reductive sintering process of uranium dioxide (UO₂) compacted pellets was conducted in 2% H₂-N₂ atmosphere at 1700°C for 3 h. The CO₂ and N₂ atmosphere was used to accelerate the sintering process, allowing it to be conducted at a lower temperature than conventional reductive sintering. The oxidative sintering process needed to be followed by a process using reducing atmosphere to obtain near-stoichiometric UO₂ pellets. The oxidative-reductive sintering process was carried out at 1200°C for 1 h of each atmosphere. CO₂ and N₂ were used as oxidative atmosphere. The results show that densification in CO₂ and N₂ atmosphere at 1200°C is considerably larger than 2% H₂-N₂ atmosphere at 1700°C. The highest density in reductive sintering and in oxidative-reductive sintering is obtained from monomodal PSD compacted UO_2+x powder because of the smallest mean particle diameter and highest fraction of finer particles with a 2.34 oxygen-to-uranium (O/U) ratio. The density of sintered pellets in CO₂-reductive atmosphere has a reverse trend with the N₂-reductive sintered pellet density. The highest sintered pellet density in N₂-reductive sintering was given by the highest finer fraction and smaller coarse fraction of trimodal PSD UO_2+x compacted powder with a 2.49 O/U ratio. From this study, it is concluded that there is a combined effect between the PSD and the O/U ratio of UO_2+x compacted powder in affecting the sintering behavior in each atmosphere.