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

Considerable stresses are generated in graphite moderator components during operation of a nuclear reactor, and these stresses can affect the ability of the reactor to cool the fuel and shut down. Thus graphite’s response to stresses in reactors must be understood. The effects of stress on the pore structure and Young’s modulus are of particular interest, due to the importance of both to the strength and integrity of the material.A confocal laser microscope was used to image samples during the application of stresses, allowing the surface profile of the material to be studied. By taking a series of images at varying levels of stress, changes to the open pore structure were observed. Digital volume correlation was performed on the micrographs to produce strain maps, from which local Young’s moduli were calculated. These observations were used to explain the behaviour of graphite bricks in reactors in response to loading stresses.