Helium and deuterium retention in Eurofer97 under sequential irradiation at low fluxes

Abstract

The helium (He) retention and the influence of He-induced defects on the deuterium (D) retention and migration in Eurofer97 were investigated by in-situ and ex-situ thermal desorption spectroscopy technique. Samples were pre-irradiated with 3 keV He ions in the fluence range of 1019–1022 He/m2, then irradiation with 2 keV D3+ ions (667 eV/D) with a fluence of 1021 D/m2 was performed. Experiments were carried out at the sample temperature of 300 K. It was shown that the D retention in Eurofer97 increases and decreases with pre-irradiation with He fluences below and above 1021 He/m2, respectively. However, in all cases of He pre-irradiation, the D retention is higher than that in Eurofer97 without He pre-irradiation. The effect of the reduction of the D retention at He fluences above 1021 He/m2 can be connected with either the formation of open porosity due to interconnected bubbles or the strain field induced by high pressurized He bubbles that preventing D penetration into the bulk of a metal. Both effects reduce the D diffusion towards the bulk of a metal and, therefore, lead to a decrease in the D retention. Our data of in-situ and ex-situ experiments revealed that HemVn complexes in the temperature range of 600–800 K are unstable and transformed in stable HemVn complexes in the temperature range of 900–1100 K after air exposure. Due to a formation of oxide layer during a contact with the atmosphere, ex-situ TDS of D leads to a shift of the D release temperature in the high-temperature range compared to in-situ TDS in both cases with and without He pre-irradiation. Finally, the presence of He in a metal lattice will have the dominant influence on the D retention in the future fusion devices together with neutron irradiation and high temperature gradient. In the case of He saturation, the similar D retention was measured in different bcc metals and their alloys.