Study of hydrogen trapping at precipitates in view of fusion reactor materials

In fusion devices, controlling the hydrogen retention process in the first wall is crucial in order to get the temperature and density of the plasma high enough to sustain the fusion reaction. The trapping of hydrogen also affects the safety of the reactors, as tritium inside the breeder blanket may migrate through the walls causing radiation hazards. In the present study ion-beam techniques were used to evaluate some potential fusion materials. Especially hydrogen retention at helium and sodium precipitates was studied. The studies concentrated on the release of trapped hydrogen, the recovery of the associated traps, and the effects of ion irradiation. The results were discussed both according to their relevance to basic physics research and to the first wall of fusion devices. Annealing studies made it possible to deduce recovery temperatures of precipitate associated traps for several metals. It was found that in a given metal the studied precipitates have the same release temperature of trapped hydrogen. Measurement of hydrogen in helium and sodium implanted samples showed that the basic mechanism that makes hydrogen remain in the samples is in all studied materials and structures the same: trapping by implantation or ion-irradiation-induced vacancies at the precipitate. The electronic stopping power of Ta, Nb, W and AISI 316L stainless steel for helium was measured. Correct stopping powers are needed in simulating erosion and implantation profiles. It was found out that in order to simulate the experimental He-ranges, the electronic stopping power for helium predicted by the ZBL theory [The Stopping Powers and Ranges of Ions in Matter (Pergamon, New York, 1985), Vol. 1] has to be modified.