Thermal conductivity of compressed beryllium pebble beds

For helium cooled pebble bed blankets, the description of the thermal-mechanical interaction between pebble beds and structural material requires the knowledge of the pebble bed thermal conductivity k as a function of temperature T and deformation state (pebble bed strain ɛ).Experimental results for the thermal conductivity of compressed beryllium pebble beds are presented. The pebble beds consisted of 1 mm NGK pebbles and are representative for dense pebble beds (packing factors γ ≈ 63.5%). Measurements were performed in the temperature range between 200 and 650 °C, with maximum pressures of 3.6 MPa and pebble bed deformations up to ɛ ≈ 3.5%. A correlation is proposed which is based primarily on measurements but uses conductivity values for non-deformed pebble beds predicted by the Schlünder Bauer Zehner model. It is argued that the proposed correlation is expected to be applicable also for pebble diameters different from 1 mm and other packing factors than 63.5% as long as densified pebble beds are considered.Finally, a conductivity correlation is presented which is applicable up to the ultimate compaction to a solid body.