Four of the Advanced Gas-cooled Reactors in the UK have a unique design, which includes outer shielding graphite blocks surrounding the graphite core. This design, which is specific to Heysham 2 and Torness power stations, has a number of functions, including directing the carbon-dioxide coolant and shielding the other components outside of the core.
ONR is aware that EDF NGL has recently discovered cracking in a small number of blocks within this shielding outer layer in one of the four reactors. ONR is satisfied that the cracks do not present a challenge to the safety of the graphite core. However, in order to further understand the rationale and long-term consequences, ONR has requested that EDF NGL conduct further investigation and analysis of the issue and carry out inspections of the other three reactors during their next statutory outages.