Sintering behavior of UO2–Gd2O3 fuel: Pore formation mechanism

The incorporation of Gd2O3 has a harmful effect on traditional UO2 sintering behavior. Above 1200°C, the sintering rate decreases and the final sintered density is significantly lower. Some effort has been made to investigate the mechanism that could explain this abnormal sintering behavior of UO2–Gd2O3 mixed fuel. A previous work concluded that the sintering difficulties of UO2–Gd2O3 fuel cannot be explained by the formation of Gd-rich (U,Gd)O2 phases with low diffusivity (diffusion barrier). This work investigates a new mechanism based on stable pore formation. Experimental observations show that gadolinium from Gd2O3 agglomerates preferentially diffuse into the UO2 phase. The UO2 matrix expands to receive extra gadolinium cations and a void is generated at the original Gd2O3 agglomerate site. Pores are generated when solid solution occurs in the intermediate sintering stage, making their removal more difficult in the final sintering stage. The new pores remain in the pellets after sintering.