The Office for Nuclear Regulation (ONR) said permission is for up to 16.025 terawatt days, which is approximately four months operation.
The 495-MW advanced gas-cooled reactor (AGR), which began commercial operation in March 1977, has been offline since October 2018 after routine inspections found cracks in its graphite core and the restart had been delayed several times since.
The ONR said inspectors had completed “an extensive assessment” of the detailed safety case submitted by licensee EDF Energy, focusing specifically on whether cracking observed in the reactor graphite core will compromise its safety.
“Our assessment included extensive scrutiny of the underpinning evidence provided by the licensee and concluded that an adequate safety case has been provided to allow a further period of operation,” a statement said.
EDF Energy, the British arm of French utility EDF, said it had demonstrated that even in the most extreme conditions its reactors operate within large safety margins.
“In particular, all control rods would operate as they are designed to do and will safely shutdown the reactor in all circumstances,” the company said.
“It is important that we continue to monitor the core and so will perform frequent graphite inspections. These will enable us to continue to demonstrate that the reactor operates with large margins to ensure safe operation at all times.”
EDF Energy said the reactor cores of all 14 AGRs in the UK are made up of graphite bricks. Channels run through these bricks for nuclear fuel, and also for control rods which can stop the nuclear reaction if needed.
This graphite was always expected to change over time. How it ages is one factor which will determine how long the UK’s AGRs will operate.
The company said it had spent more than £125m completing the “most extensive investigation of the reactor core that has ever been undertaken” to prove the Hunterston plant’s safety.
In March 2018, EDF Energy told the ONR that additional cracks had been found in reactor B-1 at Hunterston during planned inspections of the graphite bricks that make up the reactor core.
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-2 was taken out of service to allow further graphite core inspection work to be undertaken.
EDF Energy’s application to restart reactor B-1, which was found to have more than 350 hairline cracks in its graphite core, is still pending.