Model experiments to determine the stiffness of joints in graphite reactor cores

The cores of Advanced Gas-Cooled and Magnox nuclear reactors consist of an assembly of graphite bricks. The bricks are normally only loaded by gravitational forces. However, in the event of a seismic event the brick assembly can be subjected to high tangential forces. An important part of the reactor safety case is an assessment of the stability of the core under these tangential forces. Any mechanics based model of the core requires parameters to describe the stiffness of the interface between a pair of bricks in order to determine their deflection. In this work model experiments have been set up to determine the stiffness of typical graphite brick contacts using ultrasonic reflection. Two graphite specimens are pressed together and the reflection signal measured. Simple acoustic wave modelling is used to determine the stiffness from the reflection at each ultrasonic frequency. This method determines the stiffness of the interface in isolation from bulk effects. This data is used in conjunction with a brick tilting model to determine the effect the interface has on the brick deflection behaviour. Results are compared with experiments on the tipping of a full size brick. The interface stiffness approach under-predicts the deflection a brick undergoes. This is thought to be due to the fact that the ultrasonic method gives a stiffness based on small scale roughness and will not account for large scale waviness of the brick surface which may have a large effect on brick tipping behaviour.