For the next generation fusion reactors with a ceramic breeder blanket the use, as a neutron multiplier, of either a binary bed of large (≈ 2 mm) and small (≈ 0.1–0.2 mm) beryllium pebbles or a single size bed made of 1 mm or 2 mm 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 allowed to measure the effect of the bed temperature and of 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 the PEHTRA experiments, it was only possible to achieve a Δℓ/ℓ value of 0.1 % .1 A new experimental rig (SUPER-PEHTRA) has been constructed at FZK, which allows to achieve Δℓ/ℓ values of 0.3 % and to measure the pressure of the expanding bed on the containing walls. First experiments with a binary bed have been performed.2 The present paper reports on further experiments with binary beds and the establishing of equations correlating the data obtained for the present binary beds and for the binary bed experiments described in Ref. [2].