An Ion Beam Simulation of the Swelling of U 3 Si
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AN ION BEAM SIMULATION OF THE SWELLING OF U3 Sil" Charles W. Allen*, Robert C. Birtcher*, Lynn E. Rehn* and Gerard L. Hofman** *Materials Science Division ** Materials
and Components Technology Division
Argonne National Laboratory, Argonne, IL 60439,
USA
ABSTRACT 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 duning Kr irradiation by in situj. HVEM observations. Below 550 K, this material becomes glassy during 1.0 MeV Kr irradiation. Prolonged irradiation at 475 K 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 615 K does not result in observable image darkening or rapid growth of the initial perforation. After 2x1 020 Kr m-2 , the crystalline material irradiated at 615 K is stabilized against subsequent ion-irradiation-induced amorphization and growth at 475 K. Similarly, after 2x1 020 Kr in- 2 at 475 K, the glassy phase persists at 620 K 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. INTRODUCTION Development of uranium-base intermetallics as low enrichment reactor fuel
materials is underway [1,2]. In certain of the candidate alloys, such
as U3 Si
and
U6 Fe, extraordinarily large voids form at low fuel burnup, which is manifested inacroscopically as the phenomenon of anomalous, rapid swelling. In other cases, such as U3 Si2 , UAI 3 and UAI4 , a fine and stable dispersion of fission-gas bubbles forms, and catastrophic swelling does not occur. Unfortunately these latter materials contain much smaller concentrations of uranium so that higher levels of fuel enrichment would be required. While no convincing explanation of this dramatic difference in behavior has been put forward, for some time it has been presumed that the swelling is associated with fission-gas bubble migration and coalescence. However, the total void-volume fraction associated with anomalous, rapid swelling is orders of magnitude greater than one can account for on the basis of existing fission-gas concentrations. In addition, there is limited correlation between the occurrence of anomalous, rapid swelling and irradiation-induced amorphization of the intermetallics. The correlation with amorphization tendency, however, also suggests a tota
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