Dependency of irradiation damage density on tritium migration behaviors in Li2TiO3

Abstract

Tritium migration behaviors in Li2TiO3 with the increase of irradiation damage density were investigated by means of electron spin resonance and thermal desorption spectroscopy. The irradiation damages of F+-centers and O−-centers were formed by neutron irradiation, and their damage densities were increased with increasing neutron fluence. Tritium release temperature was clearly shifted toward higher temperature side with increasing neutron fluence, i.e. increasing damage density. The rate determining process for tritium release was also clearly changed depending on the damage density. Tritium release was mainly controlled by tritium diffusion process in crystalline grain of Li2TiO3 at lower neutron fluence. The apparent tritium diffusivity was reduced as the damage density in Li2TiO3 increased due to the introduction of tritium trapping/detrapping sites for diffusing tritium. Then, tritium trapping/detrapping processes began to control the overall tritium release with further damage introductions as the amount of tritium trapping sites increased enough to trap most of tritium in Li2TiO3. The effects of water vapor in purge gas on tritium release behaviors were also investigated. It was considered that hydrogen isotopes in purge gas would be dissociated and adsorbed on the surface of Li2TiO3. Then, hydrogen isotopes diffused inward Li2TiO3 would occupy the tritium trapping sites before diffusing tritium reaches to these sites, promoting apparent tritium diffusion consequently. Kinetics analysis of tritium release for highly damaged Li2TiO3 showed that the rate determining process of tritium release was the detrapping process of tritium formed as hydroxyl groups. The rate of tritium detrapping as hydroxyl groups was determined by the kinetic analysis, and was comparable to tritium release kinetics for Li2O, LiOH and Li4TiO4. The dangling oxygen atoms (O−-centers) formed by neutron irradiation would contribute strongly on the formation of hydroxyl groups. The efficiency of tritium trapping/detrapping by the dangling oxygen atoms was clearly increased with increasing damage density due to the stabilization of damages by neighboring irradiation damages and/or the lithium burn-up which produces lithium vacancy acting as a pass way of tritium to the dangling oxygen atoms.