Gas bubble network formation in irradiated beryllium pebbles monitored by X-ray microtomography

The effective and safe operation of helium cooled ceramic breeder blankets with beryllium as a neutron multiplier requires among others an efficient tritium release. A micrometric resolution computer aided microtomography (CMT) setup located at the European Synchrotron Radiation Facility made possible the 3D reconstruction of interconnected channel networks of helium bubbles in beryllium pebbles, thus enabling the identification of open porosities in the micrometer range. Beryllium pebbles of 2 mm diameter were neutron irradiated at 770 K to a fluence of 1.24 x 10(25) nm(-2), resulting in 480 appm helium and 12 appm tritium. After annealing at 1500 K, CMT was performed on the pebbles at 4.9 and 1.4 mu m spatial resolution. respectively, followed by the post-processing of the reconstructed pebble volumes. Besides a bimodal pore distribution with a smaller population around 10 mu m diameter and a high density of partly interconnected pores around 40 mu m diameter, a swelling of 17% was found. The spatial distribution of the void fraction network will be discussed together with implications on tritium release behaviour. (C) 2009 Elsevier B.V. All rights reserved.