Mechanical compression tests of beryllium pebbles after neutron irradiation up to 3000 appm helium production

•Compression tests of highly neutron irradiated beryllium pebbles have been performed.•Irradiation hardening of beryllium pebbles decreases the steady-state strain-rates.•The steady-state strain-rates of irradiated beryllium pebbles exceed their swelling rates.

Resultsof mechanical compression tests of irradiated and non-irradiated beryllium pebbles with diameters of 1 and 2 mm are presented. The neutron irradiation was performed in the HFR in Petten, The Netherlands at 686–968 K up to 1890–2950 appm helium production. The irradiation at 686 and 753 K cause irradiation hardening due to the gas bubble formation in beryllium. The irradiation-induced hardening leads to decrease of steady-state strain-rates of irradiated beryllium pebbles compared to non-irradiated ones. In contrary, after irradiation at higher temperatures of 861 and 968 K, the steady-state strain-rates of the pebbles increase because annealing of irradiation defects and softening of the material take place. It was shown that the steady-state strain-rates of irradiated beryllium pebbles always exceed their swelling rates.