Peculiarities of helium porosity formation in the surface layer of the structural materials used for the first wall of fusion reactor

Transmission electron microscopy is used to study the formation of helium porosity in the nearsurface layer of ferritic–martensitic steels and vanadium irradiated by 40-keV He⁺ ions at a temperature of 923 K up to fluence of 5 × 10²⁰ He⁺/m² and, then, by 7.5-MeV Ni²⁺ ions at 923 K up to dose of 100 dpa. Large gas bubbles are found to form in the zone with the maximum concentration of radiation vacancies during He⁺ ion irradiation. Moreover, small bubbles form in some grains at the depths that are larger than the He+ ion range in the irradiated material. Sequential irradiation by He⁺ and Ni²⁺ ions leads to the nucleation of helium bubbles at still larger depths due to helium atom transport via recoil and/or ion mixing. The precipitation hardening of the steels by Y₂O₃ oxide nanoparticles is found to suppress helium swelling substantially.