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

Serizawa, Hiroyuki  ; Matsunaga, Junji*; Shirasu, Noriko  ; Nakajima, Kunihisa  ; Kashibe, Shinji*; Kaji, Yoshiyuki  

This report addresses the precipitation of helium in polycrystalline UO, which deforms the morphology of the grains and their surfaces Helium was injected into pulverized UO 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.