Helium and tritium kinetics in irradiated beryllium pebbles

High-precision measurements of the release rate of helium and tritium from weakly irradiated beryllium pebbles were carried out by means of a Knudsen-cell technique. The samples were heated in a high vacuum to temperatures above the melting point and the evolution of gas release was measured by a mass spectrometer. Different gas diffusion and release stages were recorded and the ruling mechanisms were analysed up to complete exhaustion of the gas inventory. A minor part of the helium content was released by atomic diffusion to free surfaces, starting at about 700 K with a temperature ramp of 10 K/min and about 850 K with 30 K/min. Most of the helium precipitated into bubbles and was released only during abrupt pore venting events, which started at approximately 1500 K. Tritium release was mostly diffusion-dominated. A small fraction of tritium was trapped by helium bubbles during their nucleation phase and was consequently released corresponding to the appearance of the helium peak.