Heat transfer in compressed beryllium pebble beds

Beryllium in form of pebbles with diameter of about 1 mm is foreseen as neutron multiplier in the European helium cooled pebble bed (HCPB) blanket for fusion power plants. The pebbles are arranged in beds between flat cooling plates. During operation, the pebble beds and the containing structure have different expansions and it has been estimated that beryllium swells under neutron irradiation up to 10 vol.% during the blanket lifetime. Therefore, large compressive stresses could arise causing considerable plastic deformations of the pebbles and relevant changes in the thermal conductivity of the beds. For the proper thermal mechanical design of the blanket, the thermal conductivity of these beds as a function of deformation and temperature must be known. Measurements of thermal conductivity, heat transfer coefficient to the containing wall, and deformation of strongly deformed pebble beds are being performed at the Research Centre of Karlsruhe (FZK) using the test section HECOP. Beryllium pebble beds are heated up and uniaxially compressed, such that it is possible to adjust independently temperature and strain in the pebble beds. In the paper, the experimental device is described and first results are presented.