U 3O 8 powder made by oxidation of defective UO 2 pellets was mixed with UO 2 powder, and Nb 2O 5 was then added. Powder mixtures including various U 3O 8 contents were pressed into compacts and sintered at 1680°C in a reducing atmosphere. Without Nb 2O 5 addition, the U 3O 8 content of a mixed UO 2–U 3O 8 compact is allowed only within about 15 wt%, since higher U 3O 8 contents make the UO 2 pellet density unacceptable. Using more than 0.3 wt% Nb 2O 5, however, the UO 2 pellet density is higher than 94% TD under any U 3O 8 content of the mixed compact. The grain size of the UO 2 pellets increases with the Nb 2O 5 content of the mixed compact but is not influenced by the U 3O 8 content. Therefore, the U 3O 8 powder can be reused without any limit in the UO 2 pellet fabrication by sintering with Nb 2O 5. It is supposed that NbO 2, a reduced product of Nb 2O 5, is dissolved in UO 2 to form uranium vacancies. The enhancement of densification and grain growth may be attributed to the increase of the uranium vacancy concentration.