The transport and fate of helium in nanostructured ferritic alloys at fusion relevant He/dpa ratios and dpa rates

A novel in situ 59Ni(n,α) reaction He-implanter technique is described, that can be used to characterize the effect of the He/dpa ratio on microstructural evolution and changes in the flow properties of various materials at fusion relevant conditions. Irradiations in the high flux isotope reactor resulted in α-implantation from 1 to 5 μm thick NiAl coatings on TEM discs producing a uniform He concentration of 5–50 appm He/dpa to a depth of 5–8 μm. Initial post-irradiation TEM observations of MA957 irradiated to 9 dpa and 380 appm He at 500 °C provide strong evidence for the existence of a high density of small ⩽1 nm size bubbles trapped on nano-scale YTiO clusters. In contrast, similar irradiations of Eurofer 97, described in a companion paper, produce a lower number density of larger cavities, which may be a mixture of bubbles and voids.