In March 2018, we were informed that additional cracks had been found by EDF Energy Nuclear Generation Ltd (EDF Energy) at Hunterston Reactor 3 during planned inspections of the graphite bricks that make up the reactor core. This reactor is regarded as the lead reactor for the development of cracks in graphite bricks (known as keyway root cracks) as the reactor core had, in 2018, a higher core burn-up than the rest of the AGR fleet. As a consequence, EDF Energy took the decision to delay return to service of the reactor so that it could undertake further inspection work, and to allow it time to make a safety case for a further period of operation.
In addition, in October 2018, Hunterston B reactor 4 was taken out of service to allow further graphite core inspection work to be undertaken. Although it has operated for a slightly shorter period than Reactor 3, it also showed signs of potentially significant graphite cracking which required a further safety case to be developed.
Since Reactor 3 has been offline, EDF Energy has conducted extensive inspections and completed analysis to support a safety case for a further period of reactor operation. During this period ONR’s specialist inspectors have engaged extensively with EDF Energy in technical discussions to ensure that all significant issues are addressed.
On 20 August 2019, we provided EDF Energy Nuclear Generation Ltd with permission for Reactor 4 at Hunterston B to return to service for the next period of operation. Permission was given for up to 16.025 terawatt days, which is approximately four months operation. We received the safety case to justify a further period of operation for Hunterston B Reactor 3 on 17 June 2019. The safety case will be subject to detailed examination by specialist ONR inspectors and we will publish our decision on our website once we have completed the assessment.