Study of stress-induced microstructural changes in nuclear grade graphite using three-dimensional imaging techniques

Graphite is used as a moderating material and as a structural component in the UK’s Advanced Gas-Cooled Reactors and Magnox Reactors, Russian RBMKs and various designs of High Temperature Reactors. During reactor operation graphite components are subjected to considerable stresses and have been shown to deform and, over time, crack. Such changes may compromise the safety and efficiency of the reactor and reduce its operational lifespan, so it is important to have a detailed understanding of graphite’s response to the stresses present in operational nuclear reactors.Samples of reactor grade Gilsocarbon and Pile Grade A graphites were subjected to loading-induced stresses using a deformation rig, and a series of 3D imaging techniques were used to study changes to the microstructure of the samples as they were loaded to progressively greater levels of stress. X-ray tomography was used to image the interior of the samples, with particular attention paid to thecomplex pore structures of the materials which play a significant role in crack propagation and whose responses to stress are poorly understood.Stress-induced changes to the pore structure were quantified in terms of variation of pore volumes and shapes. The observed changes to the microstructure were used to explain the behaviour of the bulk material, and the consequences for the UK’s ageing graphite moderated reactor fleet were discussed.