Thermal conductivity of compressed binary beryllium pebble beds

For advanced DEMO fusion reactors with ceramic breeder blankets, the use of binary pebble beds both for the ceramic breeder material and the beryllium material is attractive compared to mono-sized pebble beds because of the higher pebble bed density and the larger thermal conductivity which both result in a larger tritium breeding ratio. Measurements of the thermal conductivity k in the bulk of binary beryllium pebble beds are presented using the HECOP facility at the Karlsruhe Institute of Technology, KIT, the former Forschungszentrum Karlsruhe. The pebbles have mean diameters of 2 and 0.15 mm and were manufactured by Materion, the former Brush Wellman company. The measurements of k were carried out as a function of pebble bed deformation (uniaxial strain ε) at temperatures of 280 and 550degC. Similar to the results obtained previously for mono-sized pebble beds, a linear relationship between k and ε was obtained, k=k₀+Aε, where k₀ is the conductivity for non-compressed beds. Compared to mono-sized beds, the slope A is about 2 times larger and k₀ is reasonably predicted by the SBZ model and the modified UCLA model. The temperature dependence of k is negligible within the investigated temperature range. The proposed correlation is k(W/mK)=3.5+17.6ε(%). Although binary pebble beds have distinct advantages compared to mono-sized pebble beds, it must be proven that the thermal-mechanical behaviour for blanket relevant bed dimensions does not deteriorate during temperature cycling. (author)