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ONR gives permission for Hunterston B Reactors to return to service

The Office for Nuclear Regulation (ONR) has today provided EDF with permission for Reactors 3 and 4 at Hunterston to return to service for a limited period of operation.

Permission for the restart, which follows extensive scrutiny of EDF’s safety case by The ONR’s specialist inspectors, is for up to a total of 16.7 terawatt days for Reactor 3 and 16.52 terawatt days for Reactor 4 – approximately six month’s operation for each reactor.

This will be the power station’s final period of operation before moving into the defueling phase.

The ONR’s assessment focussed on whether cracking observed in the graphite bricks that form the reactor core could compromise the key nuclear safety requirements. Specifically, these are 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 our specialist inspectors concluding that the licensee has adequately demonstrated that the reactors are able to operate safely for a further period of operation.

Through assessment of EDF’s safety justification, which takes into account the expected ageing of the graphite core, our inspectors are satisfied that sufficient safety margins and defence in depth measure remain in place to ensure the safety of Hunterston B’s workers, of local residents and the wider public.

We are satisfied that Reactors 3 and 4 at Hunterston B are safe to operate for the specified periods above, and that they can be safely shutdown (including in a significant seismic event) if required.

Donald Urquhart, ONR’s Executive Director of Operations and Deputy Chief Inspector, 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.”