A Strain Energy Approach for Predicting Keyway Root Cracking of an AGR Graphite Brick

The graphite components in an advanced gas-cooled reactor (AGR) core are subjected to fast neutron irradiation, thermal gradients and radiolytic oxidation during the reactor operation. The non-uniform changes in the material properties, irradiation, temperature, and weight loss across a typical graphite moderator brick lead to the generation of significant internal stresses. The internal stresses in a typical moderator brick are tensile at the bore and compressive at the periphery in early life, but later in life the stresses are reversed resulting in tensile stress at the periphery. The presence of sharp near 90° corners (keyway roots) at the brick periphery lead to stress concentrations and possible sites for crack initiation. Unirradiated graphite is a quasi-brittle material that becomes more brittle with increased irradiation. Sharp corners and notches, such as the brick keyways, are stress raisers as the stresses are singular at the notch/corner tip. This is similar to the singularity observed at a crack tip, which needs to be accounted for using fracture mechanics parameters such as the stress intensity factors (SIFs) in finite element assessments. Similarly, notch stress intensity factors (NSIFs) can be used to predict crack initiation at a 90o corner tip; i.e. crack initiation occurs when the NSIF reaches a critical value. In this paper, a local approach based on strain energy is explored and developed to predict the NSIF values at the keyway roots of an AGR moderator brick. The predicted NSIF values provide an alternative local failure criterion at the keyway roots based on fracture mechanics compared with the current criterion which is based on a series of failure stresses derived from feature tests for various loading configurations. One of the main advantages of this new methodology is that the criteria are not load geometry dependent and can be applied at any of the keyways.