Application of notch stress intensity factors for the prediction of keyway root cracking of an AGR graphite brick

The core of an advanced gas-cooled reactor (AGR) consists of a large assembly of graphite bricks that are inter-connected through a keying system. The keying system requires that the geometry of the fuel bricks contains keyways with 90o corners. Therefore, if the fuel brick is stressed, the presence of sharp corners leads to stress concentration at the keyway roots. The fuel bricks are predicted to develop internal stresses due to fast neutron irradiation and thermal gradients. In early life the stresses are tensile at the bore of the brick and compressive at the periphery (keyways). The stresses are reversed as the reactor ages leading to tensile stresses at the keyways. These stresses are intensified at the keyway roots. The stress level at the root would depend on the root radius. The smaller the radius, the higher the stresses will be. When the root radius is zero, the stresses are unbounded, and the problem changes from a stress concentration case to a stress singularity case. The singularity at the corner tip is similar to the singularity at a crack tip where fracture mechanics is typically used for structural integrity assessments. To assess the structural integrity of a component with a stress singularity, fracture parameters such as the stress intensity factors (SIFs) are used to overcome the meaningless stress predictions at the stress singularity. Similarly, V-notch stress intensity factors (NSIFs) could be computed and used to assess the structural integrity of a component with a sharp corner. Failure would occur when the NSIF reaches a critical value. In this talk, an approach to compute the NSIFs at a keyway root based on the strain energy of a local volume will be presented. It will be demonstrated that for small root radii the NSIF values are bounded by the values predicted for a sharp root (root radius is zero). These NSIFs could then be used in a failure criterion to predict keyway cracking. This approach provides an alternative failure criterion to the one based on the stress vs. strength criterion at stress singularity locations. The main advantages of this approach are the NSIF values are bounded at the keyway root unlike the stresses, it can be applied to any keyway and it does not require full size component experiments.