Tritium and helium release from neutron irradiated beryllium pebbles from the EXOTIC-8 irradiation

For the Helium Cooled Pebble Bed Blanket, which is one of the two reference concepts studied within the European Fusion Technology Programme, the neutron multiplier presently consists of a mixed bed of about 2 and 0.1–0.2 mm diameter beryllium pebbles. Beryllium pebbles with a diameter of 0.1–0.2 mm have been irradiated for 200 days in the High Flux Reactor (HFR) in Petten at temperatures ranging from 500 to 600 °C with a fast (>1 MeV) neutron fluence of about 5×1024 m−2. Tritium inventories as well as release kinetics of both tritium and helium from the irradiated pebbles were investigated by out-of-pile annealing experiments, heating the samples with 7 °C/min up to 1000 °C and purging them with either He+0.1 vol.% H2 or Ar+1 vol.% H2. According to previous studies, the tritium release from the pebbles was found to be slow and the maximum release rate occurred at a temperature of about 900 °C. Although tritium is released, no helium release is observed at temperatures lower than 800 °C. However, at 900 °C the samples showed a concurrent release of tritium and helium leading to the conclusion that T and He partially reside in common bubbles in the irradiated material.