Dose Effect on Release of Ion-Implanted Noble Gases From Oxide Fuel

Abstract

Release of ion-beam implanted noble gases from ThO₂ was studied. Discs of ThO₂ were ion-beam implanted with 40 keV Xe or Kr at various doses ranging from 9x10¹⁵ to ~2x10²⁰ ions/m². Isochronal release measurements for Xe and Kr, and isothermal measurements for Kr were made. It has been widely assumed that trapping effect on noble-gas diffusion increases to a saturation maximum with increasing irradiation or implantation dose. Contrary to this assumption, the isochronal release data showed that the trapping effect is a complicated function of dose. The trapping effect at a given temperature reached a maximum at ~2x10¹⁷ ions/m² and a minimum at ~5x10¹⁸ ions/m² . Isothermal Kr release was analyzed by a weak trapping model modified for the initial condition of ion-implanted gas atoms. The model accounts for probabilities of trapping and re-solution in the diffusion process of gas atoms. The re-solution probability was appreciable (10^-6~10^-5s^-1) even at 1000°C, and became maximum at a dose of ~2x10¹⁸ ions/m². Since the implantation dose of 2-5x10¹⁸ ions/m² gives gas concentration at ~1at% burnup, the above observation would be of practical significance in analyzing gas-atom behavior in nuclear fuel.