Measurements of the thermal conductivity of compressed beryllium pebble beds. EFDA reference: TW2-TTBB-007 a D4

Abstract

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 e). In the frame of the EFDA Technology Work Programme TW2-TTBB-007a-D4, the measurements of thermal and mechanical parameters of beryllium pebble beds have been performed in the HECOP facility in the Forschungszentrum Karlsruhe. This report gives a summary of previous work on the thermal conductivity k of beryllium pebble beds, describes the experimental set-up, and presents the new experimental results. The investigated pebble beds, consisting of 1mm pebbles, are representative for dense pebble beds (vibrated after filling, packing factors of 63.5%). Measurements were performed at bed temperatures between 200 and 650°C and maximum pebble bed deformations up to 3.5%. For this parameter range, two different correlations for the thermal conductivity k as a function of pebble bed deformation e and temperature T are proposed. The first one is primarily based on measurements but makes use of the conductivity values for non-deformed pebble beds predicted by the Schlunder Bauer Zehner (SBZ) model: k(W/(mK)) = 1.81 + 0.0012T(°C) - 5 10 - 7 T(°C) 2 + (9.03 - 1.386 10 - 3 T(°C) -7.6 10 - 6 T(°C) 2 +2.1 10 - 9 T(°C) 3 ) e(%). (1) It is expected that this correlation predicts also satisfactory values for beryllium pebble beds with pebble diameters different from 1mm and other packing factors than 63.5% as long as densified pebble beds are considered. The second correlation connects the a priori unknown contact surface ratio ρ k 2 of the SBZ model with the pebble bed deformation: ρ k 2 (1) = 0.0041 e(%) + 0.0021 e(%) 2 (2) In combination with this correlation, the SBZ model can be also applied for compacted pebble beds consisting of other materials than beryllium. Finally, another type of correlation is presented, to be used if it shows that swelling due to irradiation effects results in much larger pebble bed deformations than mentioned above. With the present data on beryllium pebble bed thermal conductivity, the corresponding data on thermal creep, also obtained in the HECOP facility, and the already existing data for ceramic breeder pebble beds, a complete set of pebble bed data exists now, relevant for the begin of reactor life where irradiation effects are still negligible. Now, calculations of the thermal-mechanical interaction between the pebble beds and the blanket structure in blanket relevant components could be started.