Grain size control of UO2 pellets by adding heat-treated U3O8 particles to UO2 powder

A method of controlling the grain size of UO2 pellets was investigated by adding heat-treated U3O8 particles to UO2 powder. The recycled U3O8 powder, which was made by oxidizing defective UO2 pellets in air, was heat-treated at 1200–1500 °C in air and then divided into individual crystals. The U3O8 seed – product – consists of one or several crystals. The U3O8 seed was added to UO2 powder, and the powder mixture was pressed and then sintered at 1700 °C in hydrogen gas. The grain size of seeded UO2 pellets is dependent on the amount and size of the U3O8 seed. A seeded UO2 pellet can have a grain size of 14 μm, while a conventional UO2 pellet has a grain size of 8 μm. A mechanism for grain growth in seeded UO2 is proposed and discussed.