The Office for Nuclear Regulation (ONR) has today provided EDF Energy Nuclear Generation Ltd with permission for Reactor 4 at Hunterston B to return to service for the next period of operation. Permission is for up to 16.025 terawatt days, which is approximately four months operation.

Our team of specialist inspectors has completed an extensive assessment of the detailed safety case submitted by the licensee EDF Energy, focusing specifically on whether the cracking observed in the reactor graphite core will compromise its fundamental nuclear safety requirements. Our assessment included extensive scrutiny of the underpinning evidence provided by the licensee and concluded that an adequate safety case has been provided to allow a further period of operation.

ONR is satisfied that Reactor 4 is safe to operate for the next period and can be safely shut down in all foreseeable circumstances, including that of a significant seismic event. The core would be able to fulfil its fundamental safety requirements, which are:

unimpeded insertion of control rods and unimpeded movement of fuel;

ensuring that gas flow will remain adequate 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.

In support of their assessments, our inspectors have engaged extensively with EDF Energy in technical discussions to ensure that key issues have been adequately addressed. Whilst the assessment has identified some matters that will need to be taken forward by the licensee, ONR is satisfied that none of these prevent agreement to the restart of Hunterston B Reactor 4. The issues will be tracked and monitored by ONR’s inspectors throughout the period of operation.

Donald Urquhart, ONR Deputy Chief Inspector said:

“We have made the decision to allow Reactor 4 at Hunterston B to return to service, following a long and detailed assessment of the safety case submitted by EDF Energy. “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.”

Whilst we are content to allow operation for a limited period, the longer term condition of the reactor remains uncertain and the licensee will need to justify safe operation beyond this period. ONR will continue to engage with EDF Energy to ensure all operations are only carried out with a detailed and underpinned safety case.

Our assessment of the safety case to justify a further period of operation for Hunterston B Reactor 3 will commence shortly following submission by EDF Energy on 17 June 2019.

More information on our decision is available in the Project Assessment Report (PAR), which has been published today. The detailed Assessment Reports providing further detail of our assessments will be published in the coming weeks.