Studying the build-up of stresses in nuclear grade graphite using X-ray tomography

Graphite is used as a moderating material in British Magnox Nuclear Reactors and Advanced Gas-Cooled Reactors (AGRs). Channels are cut into the graphite components through which coolant, fuel rods and control rods travel during reactor operation. Significant stresses can build up in the graphite components as a result of thermal and irradiation-induced deformation and dimensional change. Large amounts of deformation may inhibit the flow of coolant and the movement of fuel rods and control rods, potentially preventing the reactor from shutting down safely. Hence a detailed understanding of graphite’s response to reactor conditions is needed to guarantee safe and efficient reactor operation. Of particular interest is the variation of the graphite’s pore structure, which can have a significant effect on the strength of moderator components.A series of experiments were performed to investigate how the pore structures of various nuclear grade graphites respond to stresses. Axial compressive loading was used to simulate the build-up of stresses in graphite samples. X-ray tomography was performed on the samples during in-situ application of a range of loads to study variation of the interior microstructure of the samples. Image analysis software was used to define the interior pore structure of the samples and calculate properties including pore volumes, pore shapes and total porosity. The observed changes to the microstructure were used to explain the behaviour of the bulk material, and the consequences for the UK’s aging graphite moderated reactor fleet were discussed.This research is particularly important and timely due to the large numbers of nuclear reactors in the UK that are aging or operating beyond their original lifespans, and due to the ongoing concerns of cracks in graphite components being found during recent inspections.