Keyway Root Crack Arrest in an AGR Graphite Brick

The graphite bricks in an advanced gas-cooled reactor (AGR) core are subject to fast neutron irradiation which leads to significant dimensional and material properties changes. These changes vary across the relatively thick (~100mm) wall section of a typical graphite moderator brick due to the fluence and temperature profiles leading to the generation of internal stresses. In the carbon dioxide cooled environment of an AGR, the graphite also suffers weight loss due to the radiolytic oxidation which further alters the material properties. In early life the brick bore is under tension whilst the periphery is under compression. This state is reversed later in life leading to tensile stresses on the periphery and at the keyways. The keyway near 90° corners cause stress concentrations and are possible locations for crack initiation. Circular methane holes within the brick may provide locations for crack arrest before a crack could reach the brick bore, preventing detection of the crack during a routine inspection. There is a limit put on the number of cracked bricks that may be tolerated as part of the operating safety case. It is therefore desirable to ensure that keyway root cracked bricks are not missed during inspection. In this paper, the possibility of keyway root crack arrest at the methane holes is investigated. Numerical finite element models of different crack propagation scenarios are analysed. The stress levels are then used to predict the likelihood of the arrest of a keyway root crack before reaching the bore of a graphite fuel brick where a crack can be visually detected.