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

Abstract Grain growth and densification in Nb 2 O 5 -doped UO 2 pellets have been investigated under reducing and/or oxidizing atmospheres. Pellets are sintered at 1300°C in oxidizing (CO 2 ) atmosphere or at 1700°C in reducing (H 2 ) atmosphere and then annealed for 4 to 40 hours in reducing or oxidizing atmosphere. The addition of Nb 2 O 5 causes the formation of large pores in mirostructure, and the resultant decrease in density is smaller in reducing atmosphere than in oxidizing. Grain growth in Nb 2 O 5 -doped UO 2 is much more significant in reducing atmosphere than in oxidizing. Thus the enhancement of uranium diffusion due to Nb 2 O 5 addition is thought to be larger in reducing atmosphere. When an atmosphere is changed from oxidizing to reducing or vice versa, the effect of Nb 2 O 5 addition is accordingly enhanced or reduced. Possible formation of defects due to Nb 2 O 5 addition is discussed in oxidizing and/or reducing atmospheres.