The Office for Nuclear Regulation (ONR) has given consent for Reactors 3 and 4 at Hunterston in Scotland to return to service for a limited period following scrutiny of operator EDF’s safety case.

The advance gas-cooled reactors, which were built in 1976, underwent a shutdown in 2018 for detailed inspections following the discovery of cracks in the graphite bricks used in the reactor cores. At the time, EDF said that the 2mm cracks were within acceptable limits and are “a known phenomenon” for the type of reactor technology given its age and do not pose a safety risk.

Under the ONR agreement, Reactor 3 can operate for a total of 16.7terawatt days and Reactor 4 for 16.52terrawatt days, which equates to around six months for each reactor, after which they will both enter the decommissioning phase. When EDF took ownership of the nuclear reactors, power generation from Reactors 3 and 4 (known as Hunterston B) was expected to end in 2016 but the firm said in 2012 that the date had been extended to 2023, although the end date has now been confirmed as 7 January 2022.

Although the cracks discovered in the cores in 2018 were under the 10mm deemed acceptable within EDF’s internal regulations, the return to operation was subject to review by the ONR.

The ONR has said that focus of the review, published this week, was on whether cracking observed in the graphite bricks that form the reactor core could compromise the key nuclear safety requirements. These requirements included checks that the graphite core will not impede control rod entry, thereby ensuring that the reactors can be safely shutdown and held down, both under normal conditions and during or following fault conditions including seismic events; fuel and core component cooling remains adequate both under normal operating conditions and during or following fault conditions including seismic events; and fuel handling risks due to graphite core cracking remain acceptable.

As well as assessing the evidence provided, the ONR’s assessment included detailed technical discussions with EDF, with specialist inspectors concluding that EDF had adequately demonstrated that the reactors are able to operate safely for a further period of operation.

ONR executive director of operations and deputy chief inspector Donald Urquhart said: Our specialist inspectors scrutinise each safety case in great detail to ensure the safety of workers, local residents and wider public. As the UK’s independent nuclear regulator, we will only allow nuclear facilities to operate if we are satisfied that it is safe to do so.

“I am satisfied that the evidence provided by EDF, which has been assessed by our team of specialist inspectors, demonstrates that Hunterston B’s Reactors 3 and 4 are able to operate and shutdown safely during this period of operation.”

Hunterston B station director Paul Forrest welcomed the decision and added: “We have 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.

“Since Hunterston B came online 45 years ago it has generated enough low carbon electricity to power all of Scotland’s homes for 31 years and has been an integral part of the North Ayrshire community supporting the employment of thousands of people.

“I am looking forward to a safe and successful final period of generation for the station before we move into the defuelling stage of its life.”

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Tagged with: Hunterston B Nuclear

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