X-ray tomography investigations on pebble bed structures

The knowledge of pebble bed morphology and topology is important to understand the thermo-mechanical design of present ceramic breeder blankets. At Forschungszentrum Karlsruhe, pebble beds consisting of aluminium spheres with diameters of 2.3 and 5 mm, respectively, (simulating the blanket relevant 1 mm beryllium pebbles), were uniaxially compressed at different pressure levels. High resolution three-dimensional microtomography experiments were subsequently performed at the European Synchrotron Radiation Facility, Grenoble.Radial and axial void fraction distributions were found to be oscillatory next to the walls and non-oscillatory in the bulk. For non-compressed pebble beds, the bulk void fraction is fairly constant; for compressed beds, a characteristic gradient exists along the compression axis because of inner friction.The distribution of pebble contacts on the pebble surface is fairly homogenous in the pebble bed bulk. In the wall region, the pebble layer touching the wall is composed of zones with hexagonal structures, visualised by X-ray microtomography (MT) images and also reflected in the calculated angular contact distribution. With increasing distance from the wall, the regular structure vanishes and the bulk values are approached after the 4th layer away from the wall.Concerning the sum of contact surfaces per pebble, it is shown that the component normal to the compression axis (normal to the heat flow) is approximately independent of the pebble location. For strongly compressed pebble beds, this implies that by extrapolation of the bulk value of the thermal conductivity to the wall a further heat resistance might be neglected.