Effect of irradiation on the microstructure of swaged UO₂

A study was made of the microstructure of swaged UO₂, irradiated to a burnup of 3,000 MWD/T-U at an estimated center-line temperature of 1,400℃. In the peripheral zones of the specimen near the cladding, evidence was observed of sintering of the UO₂ particles by irradiation even at comparatively low temperatures (about 300° ∼ 400℃). Tracks of fission product fragments were observed at the inner wall surface of cracks in the specimen. The width of the tracks is seen to be larger than observed on thin films and on sintered UO₂. Mixed phases of UO₂ and fission products precipitated in the interior of crystal grains, observed in the form of hillocks left unetched, were surmounted by several sprouts of substances still more chemically stable. Mixed phases of UO₂ and fission products were precipitated on the inner wall surface of the cracks and appeared upon etching as stepped protuberances of irregular contours. A comparison between the microstructures of swaged UO₂ and sintered UO₂ irradiated under the similar conditions is discussed. (author)