Non destructive three dimensional analysis of the packing of a binary beryllium pebble bed

In the Helium Cooled Pebble Bed (HCPB) Blanket, studied within the European Fusion Technology Programme, beryllium in form of pebbles is used as neutron multiplier. The thermal–mechanical behaviour of a pebble bed strongly depends on the packing factor of the bed. In a binary pebble bed, in particular, a homogeneous distribution of small pebbles between the larger ones (infiltrated bed) has to be ensured in order to obtain a behaving homogeneously bed. Thus, a detailed non-destructive control of the pebble bed configuration can provide an important help in interpreting the pebble bed thermal mechanical characterisation test results. A three-dimensional (3-D) computer aided microtomography (CMT) experimental setup developed at the European Synchrotron Radiation Facility (ESRF) allowed to reconstruct 3-D images of the attenuation coefficient of a X-ray synchrotron radiation beam within a pebble bed without physically damaging it. By post-processing the acquired data, very useful quantitative informations were obtained (local and average void fraction in the sample, impurities and micro-cracks in the pebbles). In the present work, the micrographic technique and the first results of the analysis are presented and critically discussed in view of a future application for a medium scale HCPB Blanket mock-up.