Surface Modification of Uranium Dioxide Powder by Low Temperature Pre-oxidation

The pre-oxidation process of low temperature sintering of UO₂ powder was studied. UO₂ powder exposed to flowing air is heated from room temperature to 900 ℃ with 10 or 20 ℃/min, respectively. The result shows that the first exothermic peak temperature is 224.6 and 239.7 ℃, the second exothermic peak temperature is 367.0 and 381.7 ℃. And then UO₂ powder was oxidized at 240, 382 and 815 ℃ for 8 h, respectively. The phases of oxidized powders were analyzed by XRD. It demonstrates that the UO₂ powder loses planar water between 100 and 120 ℃ approximately. A small quantity of U₃O₇ forms when the UO₂ powder is kept in furnace at 240 ℃ for 8 h, while U₃O₈ forms when the UO₂ powder is kept in furnace at 382 or 815 ℃ respectively for 8 h. In order to obtain 2.25 O/U atom ratio that is suitable to low temperature sintering of UO₂ pellet, the temperature for pre-oxidation process of UO₂ powder should be limited between 240 and 370 ℃.