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-indu ced 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 bery llium blocks were irradiated with a neutron fluence (E > 0.1 MeV) (2.6 - 3. 4) 10(21) cm(-2)(1.3 - 1.8 dpa) at 550 degreesC, 620 degreesC and 790 degre esC. Mass-spectrometry techniques was used to simultaneously monitoring of gas release during isothermal multi-stage annealing over 500 - 1300 degrees C temperature range. It is shown that the first signs of the helium release have been detected a t temperature about 700 degreesC, while the intense tritium release has occ urred at all stages of annealing. Based on the data obtained, the diffusion parameters( D-o, E) for both the gases in beryllium were calculated. The t otal amount of helium accumulated in irradiated beryllium varied from 240 a ppm to 620 appm. The tritium mobility increases significantly when swelling increases, while that for helium changes very slightly. With swelling incr ease from 0.5 to 1.8% the ratio of helium to tritium retentions changes app roximately from 4:1 to 10:1. The tritium and helium retentions and berylliu m swelling are presented as functions of the distance from the irradiated s urface. The experimental data are also discussed in comparison with calcula tions.