An ion beam simulation of the swelling of U3Si

Uranium intermetallics are under consideration as possible low-enrichment reactor fuels. These materials divide into two classes with regard to dimensional stability during their service lifetime: those which suffer extreme dimensional growth and those which do not. It has been suggested that the rapid-swelling materials are those that become glassy under irradiation while the low-swelling materials are those that remain crystalline. The structural and dimensional stabilities of U3Si (depleted uranium) have been investigated as a function of temperature during Kr irradiation by in situ HVEM observations. Below 5500K, this material becomes glassy during 1.0 MeV Kr irradiation. Prolonged irradiation at 4750K also leads to rapid darkening of the TEM bright field image of the specimen, growth of the initial perforation at rates which far exceed those due to sputtering, and formation of additional thin areas and holes. Irradiation at 6150K does not result in observable image darkening or rapid growth of the initial perforation. After 2 x 1020 Kr m-2, the crystalline material irradiated at 6150K is stabilized against subsequent ion-irradiation-induced amorphization and growth at 4750K. Similarly, after 2 x 1020 Kr m-2 at 4750K, the glassy phase persists at 6200K under additional Kr irradiation, and the rapid growth continues. The mechanism of the irradiation-induced growth of the glassy material does not involve gas precipitation but rather may involve deformation by viscous flow assisted by the defects generated during irradiation. 12 refs., 3 figs