Several authors have reviewed the properties of neutron-irradiated beryllium for fusion applications in the past.139-141 Neutron irradiation leads to com­plex changes in the microstructure, such as the radiation-induced change of volume in beryllium, which is dominated by the nucleation and growth of He bubbles.

There are two important pathways for gas produc­tion. One is the (n, 2n) reaction in which the 9Be is reduced to 8Be, which then splits into two 4He atoms. The second is the (n, a) reaction where the 9Be absorbs a neutron and then splits to form a 4He and a 6He. The 6He rapidly undergoes a p decay to become 6Li. The 6Li then reacts with a thermal neutron to produce 4He and 3H. These processes have been incorporated into the inventory code FISPACT,142 which is used (see, e. g., Forty et a/.143) to estimate the generation rates of gas and other reaction products in a tokamak.

Helium generation has significant effects on the properties of materials, especially at elevated tempera­tures. Helium is initially trapped within the beryllium lattice in submicroscopic clusters. At higher neutron fluence massive helium-bubble-induced swelling occurs, especially at elevated irradiation or postanneal temperatures. Because of the atomistic nature of the helium bubble nucleation and growth, porous beryl­lium microstructures, such as from powder metallurgy or plasma spray technology, were not found to be effective in releasing significant amounts of helium under fusion reactor conditions.2

The maximum neutron-induced damage and helium production expected in Be for ITER first — wall applications (fluence of 0.5MWam~) are ~1.4-1.7dpa and ~1500appm, respectively and the expected irradiation temperatures are in the range of 200-600 °C. The maximum temperature is on the surface of beryllium tile and depends on thick­ness and heat flux. Tritium production in beryllium is expected to be about 16 appm. Recently, Barabash et a/.144 have analyzed the specific effects of neutron — induced material property changes on ITER PFCs foreseen during ITER operation.

Typically, property changes induced by neutron irradiation are investigated by exposing samples/ mock-ups in fission reactors. However, the differ­ences between the fission and fusion neutron spectra are important to interpret and predict the effects. The key difference is transmutation production, which needs to be considered for the correct predic­tion of the material performance.145 During irradia­tion in fission reactors, for example, the typical value of the ratio (appm He per dpa) is 100-250, whereas for a fusion neutron spectrum this value is ~1000. Depending on operational temperature, the dpa or He transmutation must be used as a reference neu­tron damage parameter. For beryllium, during low — temperature irradiation (<~300 °C) the dpa value must be considered. For high-temperature irradiation (more than ~500 °C), the He generation must be taken as the reference parameter.

A detailed discussion on this subject is beyond the scope ofthis review. We summarize only some ofthe main findings with emphasis on results for ITER relevant grades. Considerations of the effects of neu­tron irradiation of duplex Be/Cu alloy mock-ups are provided in Section 4.19.5.