Desorption of tritium and helium from high dose neutron irradiated beryllium

The effect of high dose neutron irradiation on tritium and helium desorption in beryllium is described. Beryllium samples were irradiated in the SM and BOR-60 reactors to a neutron fluences (E > 0.1 MeV) of (5–16) × 1022 cm−2 at 70–100 °C and 380–420 °C. A mass-spectrometry technique was used in out of pile tritium release experiments during stepped annealing in the 250–1300 °C temperature range. The total amount of helium accumulated in irradiated beryllium samples varied from 6000 to 7200 appm. The first signs of tritium and helium release were detected at temperature of 312–445 °C and 500–740 °C, respectively. It is shown that most tritium (∼82%) from sample irradiated at 70–100 °C releases in temperature range of 312–700 °C before the beginning of helium release (740 °C). In the case of beryllium sample irradiated at 380–420 °C, tritium release starts at a higher temperature (Ts > Tann = 445 °C) and most of the tritium (∼99.8%) is released concurrently with helium which could be considered as evidence of co-existence of partial amounts of tritium and helium in common bubbles. Both the Be samples differ little in the upper temperatures of gas release: 745 and 775 °C for tritium; 1140 and 1160 °C for helium. Swelling of beryllium starts to play a key role in accelerating tritium release at Tann > 600 °C and in helium release – at Tann > 750 °C.