Influence of ionizing radiation on the electrical behavior of a Be pebble bed

Solid breeder blankets with lithium ceramics as breeder and reduced activation ferritic martensitic (RAFM) steel as structural material require beryllium (Be) to increase the tritium breeding ratio (TBR) performance. In the event of plasma disruptions the values and distribution of induced currents in the breeding blanket will depend on the electrical resistivity of the adopted materials. The electrical conductivity of the insulating Be oxide layer on the pebbles surface could be significantly altered in the presence of a γ field as well as the electrical conductivity of the purging gas present inside the Be pebble bed assembly. In this paper we report the electrical properties of Be pebble beds under an irradiation environment with a γ radiation field ranging from 5×104 to 1.3×106 Gy/h at the Portuguese Research Reactor. Some Be pebbles were accommodated in a container where a typical purging gas of He+0.1% H2, as foreseen in the blanket design, was introduced. The results show a decrease of the electrical resistivity values from 0.061 to 0.055 Ωm during the exposure in a γ field of 1.3×106 Gy/h for the beds containing the purging gas mixture. The conductivity of the pebble bed without purging gas was not affected by the radiation. The results suggest that the radiation induced conductivity (RIC) was due to the ionization of the gas present in the bed.