The Office for Nuclear Regulation (ONR) has given permission for Reactor 4 at the Hunterston B power station to return to service for the next period of operation following an assessment of cracks in its graphite core.

Located in North Ayrshire, Scotland, the Hunterston B power station began operation in 1976. The station’s two reactors were shut down in 2018 following the discovery of hairline cracks in both graphite cores.

ONR’s team of inspectors completed an assessment of the safety case submitted by the station’s licensee EDF Energy, focusing on whether the cracking observed in the graphite cores would compromise the safety requirements of the reactor.

Following the assessment, ONR was satisfied that Reactor 4 is safe to operate and could be safely shut down. The assessment also determined that the reactor is able to fulfil its fundamental safety requirements; unimpeded insertion of control rods, unimpeded movement of fuel, adequate gas flow and appropriate moderation and thermal inertia.

ONR deputy chief inspector Donald Urquhart said: “My team of expert inspectors, along with an independent panel of graphite experts, have worked for a number of years on the issue of cracking and shrinkage within graphite cores.

“The licensee, EDF Energy, has completed an extensive programme of work to analyse the condition of the graphite cores at Hunterston B and other reactors in the UK to increase their knowledge and provide sufficient evidence to ensure safe operation.

“I have met with many of our interested stakeholders during the course of our assessment and I recognise that this is an emotive issue, but let me reassure you, it is ONR’s statutory role to examine the evidence objectively and make our decisions based on that.  Nuclear safety remains our utmost priority and we would only allow a reactor to re-start with clear evidence that it remains safe for workers and the public.”

ONR has given permission for the reactor to operate for up to 16.025 terawatt days, equivalent to roughly four months of operation. The longer-term condition of the reactor remains uncertain, and ONR will require further justification of safe operations from EDF Energy beyond this initial period.

ONR will assess the safety case to justify a further period of operation for Reactor 3, following its submission by EDF Energy on 17 June 2019.