The UK Office for Nuclear Regulation (ONR) on 13 April gave EDF with permission for reactors 3 and 4 at the Hunterston NPP (also known as B1 and B2) to return to service for a limited period of operation. Permission for the restart follows extensive scrutiny of EDF’s safety case by ONR specialist inspectors, Operation is permitted 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. This will be the final period of operation before the NPP moves to 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; andFuel handling risks due to graphite core cracking remain acceptable.

“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,” said ONR. “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.