Topology of compressed pebble beds determined by X-ray tomography

Abstract

In currently developed ceramic breeder blankets for future nuclear fusion reactors, both the ceramic breeder and beryllium are in form of pebbles. The thermal-mechanical behaviour of pebble 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 quantitative assessment of contact areas is of special importance for beryllium pebble beds because the thermal conductivity is ruled by this quantity. Results from experiments are reported where, first, pebble beds consisting of 3.5mm aluminium spheres, (simulating the 1mm beryllium pebbles) were uniaxially compressed in the Forschungszentrum Karlsruhe at different pressure levels. Then, detailed three-dimensional microtomography experiments were performed at the European Synchrotron Radiation Facility in Grenoble. Three-dimensional views of the pebble arrangements and void fraction distributions were calculated. By further post-processing the data, the number of contacts between the particles, the corresponding contact areas and the angular dependence of these contacts were determined. It has been shown that in uniaxial compression tests the poloidal distribution of contact surfaces is non-homogeneous. This fact should be considered when using these tests as standard tests for the generation of thermal-mechanical pebble bed data.