Experimental investigations on the thermal and mechanical behaviour of a binary beryllium pebble bed

For the next generation fusion reactors with a ceramic breeder blanket, the use, as a neutron multiplier, of a binary bed of large (≈2 mm) and small (≈0.1–0.2 mm) beryllium pebbles is foreseen. The heat transfer parameters of such a binary pebble bed, namely the thermal conductivity and the heat transfer coefficient to the containing wall, have been investigated previously in the experimental device PEHTRA available at FZK. The experiments measured the effect of the bed temperature and of the constraint exerted by the containing walls. The constraint is defined by the bed interference, i.e. the difference in the radial expansion between bed and the constraining walls related to the bed thickness (Δl/l). However, with PEHTRA experiments it was only possible to achieve a Δl/l value of 0.1%. A new experimental rig (SUPER-PEHTRA) has been constructed at FZK, which allows Δl/l values of 0.3% to be achieved and to measure the pressure of the expanding bed on the containing walls. The present paper reports on the experiments performed with SUPER-PEHTRA for a binary beryllium pebble bed.