Microstructure of out-of-pile annealed neutron irradiated beryllium studied by X-ray tomography

In fusion reactors, hydrogen isotopes, i.e., tritium and deuterium, will be used as fuel producing energetic neutrons and helium atoms. While deuterium is a stable nuclide frequent in nature, tritium is beta-radioactive. Its half-life time is about 12 years, requiring constant renewal via tritium generation in nuclear reactions. Neutrons produced in fusion reactions should therefore be effectively multiplied. To this end, beryllium is planned to be used in the form of pebble beds in the blanket of a fusion reactor. Unfortunately, helium and tritium are created under neutron irradiation and accumulated in the beryllium matrix, resulting in the formation of gas bubbles, swelling, and, under some circumstances, even in the loss of pebbles' structural integrity. In this work, beryllium reflector fragments irradiated for 15 years in the research reactor BR2 (SCK-CEN, Mol, Belgium) at temperatures below 120 degrees C and containing about 2 at.% helium was vacuum annealed at two temperatures and various annealing times. Gas-induced porosity developed after annealing was investigated using synchrotron X-ray micro-tomography. This technique enables a non-destructive and quantitative analysis of the 3D morphology of the gas-induced porosity providing important insights into the kinetics of the gas bubble growth. Using advanced post-processing of the micro-tomography data it has been possible to determine the volume fraction of gas bubbles and their size distribution, and reveal the formation of 3D bubble clusters. The importance of the data obtained for tritium release and relation between microstructure and tritium retention properties are discussed. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.