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ONR gives permission for return to service at Hunterston B

The Office for Nuclear Regulation (ONR) has provided EDF Energy Nuclear Generation Ltd (EDF NGL) with permission for Reactor 3 at Hunterston B to return to service for a limited period of operation.

The permission is for up to a total of 16.425 Terawatt days, approximately six months’ operation.

The decision to restart the reactor follows an extensive and detailed assessment of EDF NGL’s safety case by specialist ONR inspectors.

ONR’s assessment has focussed on whether cracking observed in the graphite bricks that form the reactor core of Reactor 3 could compromise its fundamental nuclear safety requirements, which are:

  • unimpeded insertion of control rods and unimpeded movement of fuel;
  • ensuring that gas flow will be maintained to ensure cooling of the fuel and core; and
  • that appropriate moderation (slowing neutrons to sustain the nuclear chain reaction) and thermal inertia (reducing the speed of temperature changes) are maintained.

ONR’s assessment included extensive scrutiny of the evidence provided by the licensee, with our specialist inspectors concluding that an adequate safety case for a further limited period has been provided. We are satisfied that Reactor 3 is safe to operate for the next period and could be safely shutdown (including in a significant seismic event) if required.

During the assessment process, specialist inspectors engaged extensively with EDF NGL in technical discussions and sought advice from ONR’s Graphite Technical Advisory Committee to ensure that all significant issues have been addressed.

Through assessment of EDF NGL’s safety justification, which takes into account the expected ageing of the graphite core, our inspectors are satisfied that there are sufficient safety margins and defence in depth measures in place to ensure public and worker safety throughout the further period of operation.

Donald Urquhart, ONR Deputy Chief Inspector, said: “This has been a detailed and complex assessment in which my team of specialist inspectors has rigorously assessed and scrutinised the safety case and underpinning evidence provided by EDF NGL. This work has taken more than two years, and I am grateful to my team for the diligent and professional manner in which this has been done.

“I am satisfied that the detailed safety justification provided by the licensee is sufficient to demonstrate that Reactor 3 can operate safely for this period of operation.We applied stringent national and international standards when making our decision, have scrutinised the nature of the cracking observed in Reactor 3 and are satisfied that it will not prevent the reactor from operating safely or impede its ability to be shutdown if required during this period of operation.

“As the independent nuclear regulator, our sole priority is the safety of site workers, local residents and the wider public who rely on ONR to regulate such safety matters. We will only allow nuclear facilities to operate if we are satisfied that they are safe to do so.”

More information about our technical assessments can be found in the Project Assessment Reports (below) and in the Assessment Reports: