The role of grain boundaries and denuded zones for tritium retention in high-dose neutron irradiated beryllium

Neutron irradiation of beryllium leads to a lasting material damage. Besides the displacement damage, large amounts of helium and tritium are produced by neutron induced transmutation. In future fusion reactors the accumulation of large amounts of tritium in beryllium must be prevented due to safety concerns and potential waste disposal problems. It is therefore essential to understand where tritium is trapped and how it is released from the sample during irradiation. Using a simple self-developed theoretical model we show that all tritium which is produced and already released during irradiation originates from the area of later bubble denuded zones (DZ) around grain boundaries (GB). In these areas no or only very few bubbles can be observed since grain boundaries are strong sinks for point defects. The formation of precipitation-bubble-pairs in the immediate vicinity of these regions presumably prevents the release of larger amounts at temperatures ≤ 500 °C, as they act as additional tritium traps. The presented model can be used in the future to estimate the tritium retention in irradiated samples using a small number of easily determined experimental parameters, providing important insights for the blanket safety of future fusion reactors.