Abstract Beryllium, irradiated in the SM and BR2 research nuclear reactors at 323–343 K up to neutron fluences of (0.4–14.4) × 10 22 cm −2 ( E > 0.1 MeV), was investigated. The dependences of beryllium swelling, brittle strength and microhardness on fast neutron fluence are presented. Three intervals can be outlined where the swelling has the various rates of increase with the growth of the neutron fluence that is connected with accumulation of radiogenic helium in beryllium and evolution of grain boundaries pores to cracks with the formation of a volume-connected network of cracks. The key points for the crack formation and propagation in beryllium under irradiation are the presence of beryllium oxide particles on boundaries and compression of the hexagonal grains along parameter “ c” owing to radiation growth. The annealing at 1123 K for 5 h results in an increase of the helium amount on grain boundaries and an evolution of the pores to gas bubbles.