The Office for Nuclear Regulation (ONR) said on its website that the restart safety case is for up to 16.7 terawatt days for Hunterston B-1 and 16.52 terawatt days for Hunterston B-2 – approximately six month’s operation for each reactor.
This will be the facility’s final period of operation before moving into the defueling phase.
The ONR’s assessment for restart focussed on whether cracking observed in the graphite bricks that form the reactor core could compromise the key nuclear safety requirements.
The UK has a long history of using graphite as a moderator, from the early Magnox reactors to advanced gas-cooled reactors (AGRs) such as Hunterston.
The moderator slows down the speed of neutrons produced during nuclear fission and helps to sustain the chain reaction so that the heat can be used for electricity production.
In AGRs, the core is constructed from thousands of interlocking graphite bricks which also form a large number of important channels. These channels contain the nuclear fuel, the reactor control rods and allow the passage of carbon dioxide coolant gas to remove heat from the reactor fuel and core.
The main issue associated with graphite bricks is graphite core ageing, which results in weight-loss and cracking.
“We are satisfied that both reactors at Hunterston B are safe to operate for the specified periods and that they can be safely shutdown, including in a significant seismic event, if require,” the ONR said.
Owner and operator EDF Energy said it had carried out the most detailed investigation ever on this kind of reactor and the approved safety case has been subject to extensive scrutiny and testing, by EDF and independently.
“We have been able to demonstrate that even in the most extreme conditions our reactors will safely shutdown, including in the event of an earthquake larger than ever experienced in the UK,” the company said.
In August 2020, EDF Energy announced that generation at Hunterston B would end by 7 January 2022. The 490-MW Hunterston B-1 began commercial operation in February 1976 and the 495-MW Hunterston B-2 on 31 March 1977.