Densification and grain growth of UO2 and MnO-UO2 during pressureless sintering.

Large-grain UO 2 has attracted much attention due to its advantage of suppressing fission gas release by reducing the specific surface area of grain boundaries. In this study, a single grain growth additive MnO is selected to modify UO 2 by conventional pressureless sintering method. The effect of MnO modification on UO 2 densification and grain growth at temperatures between 1200 and 1700 °C was investigated in detail. 0.5 wt%MnO-UO 2 pellets can reach nearly 95% T.D. when sintered at 1200 °C for 24 h. An average grain size greater than 300 µm can be obtained at 1700 °C for 24 h. A series of sintering experiments were designed and conducted to verify the specific mechanism of MnO on the grain growth of UO 2. The classical grain growth kinetics indicates that the grain growth exponent of MnO modified UO 2 in the early stage is 3, which corresponds to the lattice diffusion under pore control, while that of UO 2 is 2 and conforms to the grain boundary migration control. The SEM results show that there is solute segregation of MnO at the grain boundaries, followed by nucleation of lamellar structure in the form of piles and plates on the boundaries after high temperature treatment and traces of liquid phase in some region. Diffusion sintering experiments show that the mechanism of MnO-promoted UO 2 grain boundary migration is based on solid phase diffusion. The XRD results of UO 2 modified with different amounts of MnO demonstrate that the growth degree of UO 2 grains can be related to the variation of lattice parameters resulted from the MnO modification. This is due to the enhanced mobility of UO 2 grain boundaries because of the large distortion of surrounding lattice caused by Mn modification. These results can provide new ideas for the manufacture of UO 2 ceramic fuels. [ABSTRACT FROM AUTHOR]