Release of ion-implanted Kr from (Th, U)O 2 — effect of matrix oxidation

Abstract

The effect of matrix oxidation on diffusion and trapping of gas atoms in (Th, U)O 2 was studied. Krypton was ion-beam implanted into the (Th, U)O 2 and its release in air was measured. Trapping of gas atoms during migration had been previously observed in pure ThO 2 at the implantation doses selected. Before implantation, the specimens were annealed at 1400°C in either air or hydrogen to remove polishing damage. Kr release was measured in air at temperatures increasing from 700°C to 1500°C. The H 2-annealed (Th, U)O 2, stoichiometric at the time of implantation, became oxidized early in the heat treatments. The Kr release from these oxidized specimens showed only mild or weak trapping. That is, the fractional release was significantly larger than that from ThO 2 which had shown much stronger trapping at the corresponding doses. This enhanced release is in line with theoretical predictions of a lowered solution energy of noble gases in the anion-excess oxide. Specimens oxidized before implantation suffered uranium depletion at the surface by preferential vaporization. As a result, the release from the pre-oxidized specimens showed a behavior of strong trapping.