Effects of Nb2O5 addition on grain growth and densification in UO2 pellets under reducing and/or oxidizing atmospheres

Grain growth and densification in Nb2O5-doped UO2 pellets have been investigated under reducing and/or oxidizing atmospheres. Pellets are sintered at 1300 C in oxidizing (CO2) atmosphere or at 1700 C in reducing (H2) atmosphere and then annealed for 4 to 40 hours in reducing or oxidizing atmosphere. The addition of Nb2O5 causes the formation of large pores in microstructure, and the resultant decrease in density is smaller in reducing atmosphere than in oxidizing. Grain growth in Nb2O5-doped UO2 is much more significant in reducing atmosphere than in oxidizing. Thus the enhancement of uranium diffusion due to Nb2O5 addition is thought to be larger in reducting atmosphere. When an atmosphere is changes from oxidizing to reducing or vice versa, the effect of Nb2O5 addition is accordingly enhanced or reduced. Possible formation of defects due to Nb2O5 addition is discussed in oxidizing and/or reducing atmospheres. (orig.)