Effect of sintering temperature and doping content on phase evolution and microstructure of doped UO2 ceramics

ZrO2-doped UO2 ceramics have been considered as a promising high performance nuclear fuel in LWR due to its desirable properties. ZrO2-doped UO2 ceramic fuel samples with different contents of ZrO2 (20 wt%, 25 wt% and 30 wt% ZrO2, equivalent to 35.4 mol%, 42.3 mol% and 48.4 mol%, respectively) were prepared by pressureless sintering at various temperatures (1550 degrees C, 1650 degrees C and 1750 degrees C, respectively) for 4 h in a hydrogen reducing environment. Phase evolution and microstructure of as-sintered ZrO2-doped UO2 ceramic samples were investigated by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), respectively. Results show that there exists coexistence of uranium-rich U1-xZrxO2 phase with facecentered cubic (FCC) structure and zirconium-rich Zr1-yUyO2 phase with tetragonal structure in the 20 wt% ZrO2-doped UO2 ceramic sample when the sintering temperature is lower than 1650 degrees C, while the UO2 and ZrO2 will form a single U1-xZrxO2 solid solution after sintering at 1750 degrees C for 4 h with the ZrO2-doped UO2 ceramic samples containing 25 or 30 wt% ZrO2, respectively. The significant differences of phase structure and ionic radius between ZrO2 and UO2 and the high atomic diffusion barrier are the main reasons that it is difficult to form a single solid solution for UO2-ZrO2 ceramic samples sintered at low temperature.