Helium and tritium behavior in neutron irradiated beryllium

The efficiency of the beryllium application as a plasma-facing material and a neutron multiplier in a solid breeder blanket will depend on helium-induced swelling and tritium and helium release from this metal. The effect of a neutron irradiation on helium and tritium mobility and swelling for three beryllium grades fabricated by VNIINM is described in this paper. The beryllium blocks were irradiated with a neutron fluence (E >0.1 MeV) (2.6 - 3.4) . 1021 cm-2 (1.3 - 1.8 dpa) at 550 C, 620 C and 790 C. Mass-spectrometry techniques was used to simultaneously monitoring of gas release during isothermal multi-stage annealing over 500 - 1300 C temperature range. It is shown that the first signs of the helium release have been detected at temperature about 700 C, while the intense tritium release has occurred at all stages of annealing. Based on the data obtained, the diffusion parameters (Do, E) for both the gases in beryllium were calculated. The total amount of helium accumulated in irradiated beryllium varied from 240 appm to 620 appm. The tritium mobility increases significantly when swelling increases, while that for helium changes very slightly. With swelling increase from 0.5 to 1.8% the ratio of helium to tritium retentions alternates approximately from 4 : 1 to 10 : 1. The tritium and helium retentions and beryllium swelling are presented as a functions of a distance from the irradiated surface. The experimental data are also discussed in comparison with calculations. (orig.)