Helium and tritium retention and migration in beryllium

Efficiency of the application of beryllium for making such elements of ITER as neutron multiplicator, first wall coating and divertor plates depends on the helium and tritium mobility in this metal. Release parameters of helium and tritium from two grades of isotropic beryllium have been investigated after neutron irradiation at 550, 620, 780°C with fluence of 2.6·10²¹ cm^-2 3.5-10²¹ cm^-2 and 2.6·10²¹ cm^-2 (E>0.1 MeV), correspondingly. Isothermic multi-stage annealings in the temperature range from 400 to 1300°C with the simultaneous monitoring of release gases was fulfilled by the mass-spectrometric method. It was shown that an intensive release of tritium took place at 500°C, while the first signs of helium release were revealed above 600°C. On the base of data obtained the diffusion parameters (D₀, E) both gases in beryllium were calculated. Tritium mobility was found approximately three orders of magnitude as high as that of helium. The total amount of helium accumulated in the irradiated beryllium varied from 0.59 to 1.48 ntp cm³·g^-1. Helium to tritium ratio was estimated as about 4. Beryllium swelling as a function of the quantity of retained helium is presented