Modelling of swelling and tritium release from irradiated beryllium

Beryllium is one of the best neutron multipliers for the blanket of a fusion reactor, thus several studies have been initiated to evaluate this material behaviour under irradiation both for typical operating and accidental conditions. The most important effects of radiation damage in beryllium are swelling, embrittlement and tritium retention. The helium produced mainly by the reaction 9Be(n, 2n)2 4He is the dominant cause of beryllium swelling which represents the major lifetime limiting factor for the material. Furthermore, the tritium inventory in beryllium, produced by simultaneous transmutation reactions, constitutes a safety hazard in case of accidental temperature excursions in the reactor. It was therefore decided to develop a computer code capable of describing the helium and tritium behaviour in beryllium. The approach used was to modify an existing code available for the modelling for gas behaviour in fuel elements irradiated in fission reactors. The relevant effects occurring in irradiated beryllium under steady-state or transient temperature conditions have been considered from a microscopic (lattice and subgranular volume elements), structural (metallographic features of the material) and geometrical (specimen design parameters) point of view. A new model describing the trapping effects on tritium due to chemical reactions with beryllium oxyde and capture in helium bubbles has been included in the code. The resulting computer code ANFIBE (ANalysis of Fusion Irradiated BEryllium) allows the calculation of gas distribution, induced swelling and helium and tritium release from beryllium. The performance of the code was assessed by comparison with the available swelling and tritium release experiments. Good agreement between calculated and experimental data was found. Finally, analyses for the European DEMO BOT Ceramic Breeder Blanket have been performed. (orig.)