Microstructure of long-term annealed highly irradiated beryllium

The Material Testing Reactor BR2 of SCK.CEN has a beryllium matrix which has been replaced in 1995. Pieces of this matrix, which were irradiated over a period of 15 yr at about ±50 deg. C up to a fast neutron fluence (E > 1 MeV) of 4.67 x 1022 n/cm2, have been retrieved for creep and microstructure studies. An amount of helium resulting from transmutation has accumulated up to approximately 2.2 at.% which is comparable to the expected He content of a fusion reactor blanket at its end of life. Post-irradiation anneals have been performed at 500, 750 and 900 deg. C, with an annealing time ranging between 50 hours and three months. Microstructural analyses show a clear evolution in bubble growth and He diffusion towards the grain boundaries where the bubbles coalesce. Complete release of 4He is observed at temperatures >750 deg. C, resulting in a stagnation of the beryllium swelling. The microstructural changes are mainly temperature driven. Only at high annealing temperature, an influence of the annealing time is observed