Study of helium bubble evolution in highly neutron-irradiated beryllium by using x-ray micro-tomography and metallography methods

Beryllium will be used as a plasma-facing material and a neutron multiplier in fusion reactors. The processes of helium accumulation in fusion and fission reactors are similar. Therefore, it is possible to extrapolate the data obtained on beryllium irradiated in a fission reactor to a fusion one. Fragments of the beryllium matrix of the BR2 reactor irradiated at 323 K up to a fast neutron fluence of 4.67×1022 cm-2 (E>1 MeV) and helium accumulation of about 22 500 appm with additional annealing at 1123 and 1273 K for 0.5, 1. 5 and 10 h were used for the investigation of helium porosity by optical metallography and x-ray micro-tomography methods. The possibility of using the XRT method for investigations of helium porosity evolution in irradiated and annealed beryllium was demonstrated. The non-uniformity of helium bubble formation in the material leads to a non-homogeneous swelling observed on the cross-section of the beryllium samples. The reason for high swelling in the bulk of the beryllium sample could be the formation of large stable helium bubbles, which are not mobile and not mutually connected.