Studying the Effects of Stress on the Material Properties of Graphite Moderators using Confocal Laser Microscopy

A detailed understanding of the microstructure of graphite is required in order to ensure its safe and continued use as a moderator, and as a structural component, in British nuclear reactors. Considerable stresses are generated in the graphite components during reactor operation, and these stresses can affect the ability of the reactor to cool the fuel and shut down. The effects of these stresses on graphite at the microlevel are poorly understood. Features of particular interest include the pore structure and the local Young’s modulus, due to the current lack of research into how they are affected by stresses, and the importance of both to the integrity of the material. Since graphite can be highly porous, the pore structure has a significant effect on the strength of the material. A confocal laser microscope was used to image samples while they were axially stressed, allowing a three-dimensional surface profile of the material to be produced. By taking a series of images at varying levels of stress, changes to the open pore structure of the material were observed. Digital volume correlation was performed on the micrographs to produce strain maps, from which variation of local Young’s moduli were calculated at the microlevel.