Studying the Stress-Induced Deformation Behaviour of Pile Grade A Nuclear Graphite using Confocal Laser Microscopy

A detailed understanding of the properties 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 graphite components during reactor operation, and these stresses can affect the ability of the reactor to cool the fuel and shut down. Hence being able to reliably predict graphite’s response to such stresses in reactors must be understood. Features of particular interest include the pore structure and Young’s modulus, due to the importance of both to the integrity of the material. Since graphite can be highly porous, particularly after irradiation, the pore structure has a significant effect on the strength and continued reliability of the material. A confocal laser microscope was used to image samples of British Pile Grade A graphite during the application of axial stresses, 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 microstructure and open pore structure of the material were observed; and these changes were quantified through calculations of open pore areas. These observations formed the basis of an explanation of the behaviour of graphite bricks in nuclear reactors in response to loading stresses.