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 degrees C up to a fast neutron fluence (E > 1 MeV) of 4.67 x 10(22) n/cm(2), 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 degrees 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 4 He is observed at temperatures >750 degrees 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. (C) 2007 Elsevier B.V. All rights reserved.