Measurements of the effective thermal conductivity of a Li4SiO4 pebble bed

Li4SiO4 is the reference breeder material for the Helium Cooled Pebble Bed (HCPB) Blanket concept. In the HCPB blanket lithium orthosilicate pebbles in the diameter range 0.25–0.63 mm are placed in a gap of about 10-mm width between two cooling plates. Therefore, in order to study the heat transfer in the blanket, the effective thermal conductivity of pebble beds has to be known. Measurements of the heat transfer parameters of a Li4SiO4 pebble bed have been performed at the Research Centre of Karlsruhe (Germany). In the experimental apparatus the pebble bed is contained in the annular space between two concentric stainless steel tubes. The internal tube is electrically heated and the heat flows through the bed in radial direction. Thermocouples in the tubes and in the pebble bed measure the temperature distribution in axial and in radial direction respectively, thus allowing the calculation of the heat transfer parameters of the bed. Measurements were performed with helium flowing through the bed at very low velocity, so that the heat transfer parameters were not affected by gas convection. In this paper the measured thermal conductivity data are presented and discussed. Experiments with different helium pressures show that, for pressures higher than 1 bar, there is no effect of the pressure variation on the thermal conductivity of the pebble bed (Smoluchowski effect).