Variation in the surface morphology of polycrystalline UO2 powder induced by helium precipitation

This report addresses the precipitation of helium in polycrystalline UO2, which deforms the morphology of the grains and their surfaces. The formation of intragranular gas bubbles by the diffused helium can adversely affect the sintering process of ceramic-type nuclear fuel pellets. Helium was injected into pulverized UO2 particles at 1473 K by hot isostatic pressing (HIP). The specific surface area measured by volumetric gas adsorption instrument implied that small pores should exist on the as-helium-treated sample surface. Field-emission scanning electron microscopy observations showed that numerous shallow basins (approximately 500 nm in radius) with hexagonal fringe were formed on the surface. The basin resembles a ruptured blister whose lid has been forced open. SEM observations showed a uniform polygonal-shaped section of the gas bubble on the fracture surface; this implies that precipitated helium forms a negative crystal in the grain. These interesting results suggest the possibility that the rupture of the negative crystal formed in the vicinity of the surface is related to the formation of the basin with a hexagonal fringe.