Tritium distribution and chemical forms in the irradiated beryllium pebbles before and after thermoannealing

Beryllium pebbles are foreseen as a neutron multiplier in the tritium breeding blanket of the future fusion devices. Tritium inventory in the beryllium as a result of neutron-induced transmutations is a significant safety and technological issue for the operation of the breeding blanket. In this study, beryllium pebbles from 3 different irradiation experiments: BERYLLIUM, EXOTIC 8/3-13 and PBA, performed at High Flux Reactor HFR have been investigated. The distribution of tritium in the bulk of the pebbles and the abundance ratios of chemical forms of tritium T⁰, T⁺ and T₂ have been analysed before and after the different thermo-annealing experiments. In order to determine the abundance ratios of chemical forms, the method of chemical scavengers has been used. The structure analysis has been done by scanning electron microscopy. The main chemical form of the tritium localized in the irradiated beryllium pebbles is T₂, especially in the pebbles from PBA experiment. As a result of thermo-annealing, the abundances of the T₂ and T⁰ change, T⁺ stays unaffected up to relatively high temperatures. The distribution of the tritium in the bulk is uneven - it increases rapidly at the centre of the pebble. Changes of pebble structure as a result of thermal treatment have been observed.