Topology of compressed pebble beds

Abstract

Thermal stresses occurring during operation of helium-cooled pebble bed blankets will cause pebble deformations which increase the pebble bed thermal conductivity due to an increase of contact surfaces between the pebbles. The dependence of the thermal conductivity from pebble bed strain is generally determined in test set-ups where only global quantities (uniaxial stress, strain and temperature) are measured. For the understanding of heat transfer mechanisms, the knowledge of the topology of the pebble beds (number of pebble contacts with other pebbles or walls, corresponding contact zones, angular distribution of contacts on the pebbles) is of great interest. Experiments were performed where, first, pebble beds were uniaxially compressed to different strain levels. For better measurement accuracy, spherical 3.5 or 5 mm aluminium pebbles were used instead of blanket relevant 1 mm beryllium pebbles. Then, the topological quantities were determined by two methods: A) In the European Synchrotron Radiation Facility (ESRF) Grenoble, a special microtomography experimental set-up was used allowing the computer aided reconstruction of 3D images of pebbles within the pebble beds. By post-processing the data, both radial and axial void fraction distributions were determined as well as the topological quantities. B) In the Forschungszentrum Karlsruhe (FZK), the pebbles were chemically coloured in the compressed state in order to increase the optical contrast. Optical microscopy was then used to determine pebble contact numbers and contact surfaces. Results from both methods complement one another very well due to specific experimental limitations of both techniques.