Inner structures of compressed pebble beds determined by X-ray tomography

In present ceramic breeder blankets, both the ceramic breeder and beryllium are used in form of pebbles. The thermal–mechanical behaviour of these beds strongly depends on the arrangement of the pebbles in the bed, their contacts with other pebbles and with walls and the related contact surfaces. The knowledge of the contact surfaces is of special importance for beryllium pebble beds because the thermal conductivity is sensitively dependent on these surfaces. Results from compression and microtomography experiments are reported where, first, pebble beds consisting of 3.5 mm aluminium spheres, (simulating the beryllium pebbles) were compressed at different pressures in uniaxial compression tests (UCTs). In a second stage, detailed three-dimensional (3D) microtomography experiments were performed. By post-processing the data, very valuable information was obtained on void fraction distributions, number of contacts between the particles, the corresponding contact surfaces and the angular dependence of these contacts. These data are very important for the interpretation of experiments performed to characterise the thermal–mechanical performances of pebble beds.