Tritium release and retention properties of highly neutron-irradiated beryllium pebbles from HIDOBE-01 experiment

Abstract

The current helium cooled pebble bed (HCPB) tritium breeding blanket concept for fusion reactors includes a bed of 1mm diameter beryllium pebbles to act as a neutron multiplier. Beryllium pebbles, fabricated by the rotating electrode method, were neutron irradiated in the HFR in Petten within the HIDOBE-01 experiment. This study presents tritium release and retention properties and data on microstructure evolution of beryllium pebbles irradiated at 630, 740, 873, 948K up to a damage dose of 18dpa, corresponding to a helium accumulation of about 3000appm. The measured cumulative released activity from the beryllium pebbles irradiated at 948K was found to be significantly lower than the calculated value. After irradiation at 873 and 948K scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed large pores or bubbles in the bulk and oxide films with a thickness of up to 8μm at the surface of the beryllium pebbles. The radiation-enhanced diffusion of tritium and the formation of open porosity networks accelerate the tritium release from the beryllium pebbles during the high-flux neutron irradiation.