A mechanism for the sintered density decrease of UO2–Gd2O3 pellets under an oxidizing atmosphere

A mixture of UO2 and Gd2O3 powders was pressed into compacts and sintered under various atmospheres ranging from reducing to oxidizing gases. The sintered density of UO2–10 wt% Gd2O3 pellets decreases with increasing oxygen potential of the sintering atmosphere. Dilatometry and X-ray diffraction studies indicate that the delay of densification takes place between 1300°C and 1500°C, along with the formation of (U,Gd) O2. A very large solubility of Gd2O3 in UO2 relative to the reverse solubility might cause Gd ions to diffuse into UO2 so directionally that new pores are produced at the places of Gd2O3 particles. The new pores may be difficult to shrink and thus lead to the density decrease under an oxidizing atmosphere but not under a reducing atmosphere, because a driving force for the shrinkage of new pores may be smaller under an oxidizing atmosphere than under a reducing atmosphere.