Restart at Hunterston B

Consent has been requested from the HSE by the licensee, EDF Energy Nuclear Generation Ltd (EDF), under Licence Condition 30 (3) to restart Reactor 4 at Hunterston B Nuclear Power Station following its periodic shutdown.


There are two advanced gas-cooled reactors (AGRs) at Hunterston B Nuclear Power Station. During operation the reactors are cooled by high-pressure gas, which is circulated through the reactors and then fed to boilers where the heat boils water, turning it into steam, to drive turbines which then generate electricity.

To continue to operate safely and reliably, reactors require examination, inspection, maintenance and testing. Continuous improvement also requires plant upgrades to be implemented. Whilst some of these activities can take place when a reactor is generating electricity, many of them require a reactor to be shut down and at relatively low temperatures. To enable such activities each reactor at Hunterston B under goes a periodic shut down every three years. Following a periodic shutdown the reactor cannot start-up without the permission of the Office for Nuclear Regulation (ONR).

The current shut down for Reactor 4 at Hunterston B commenced on 8 July 2011.

Assessment and inspection work carried out by Office for Nuclear Regulation (ONR) in consideration of this request

During this periodic shutdown, the Licensee has conducted maintenance activities in accordance with the Station Maintenance Schedule, together with inspections to support the station safety case and comply with statutory requirements, and has carried out remedial work to rectify defects and implemented modifications requiring a reactor outage.  Where the inspection work has revealed deterioration in plant condition, the Licensee has assessed the results in accordance with its arrangements and remedial actions, where necessary, have been taken, again in accordance with the Licensee’s arrangements.

All modifications intended to be carried out, have been carried out during the outage. The station has also implemented numerous additional modifications which were not included in the initial outage scope. These have been necessary to cover emergent work or where the Licensee has taken advantage of additional work windows opening up during the outage.

Audits of selected outage activities were carried out by insurance company inspectors; responsible station engineers; the station’s QA section and also by EDF’s Nuclear Oversight (NOS), which provide independent inspection based on WANO (World Association of Nuclear Operators) procedures. No issues of significance, challenging safe reactor start-up and operation until the next periodic shutdown, were identified from any of this work.

The Licensee considers that it has successfully completed all outage related work and that the reactor can be safely returned to service and operated until the next periodic shutdown. This view is independently supported by the company’s internal Safety and Regulation Department (SRD), who will continue to monitor completion and whose agreement to restart is required by EDF’s own procedures.

The ONR Site Inspector and a number of ONR specialist inspectors have undertaken sample assessments and inspections of the Licensee’s arrangements for controlling and completing periodic shutdown work, its execution and its results, paying attention to matters of the highest safety significance.  These assessments and inspections have raised no significant issues in relation to the Licensee’s safety case for the restart of Reactor 4 and its operation until the next periodic shutdown in respect of matters related to the current periodic shutdown.

Matters arising from ONR’s work

There are no significant findings from the periodic shutdown that would prevent the reactor restarting for a further period on 3 years’ operation.


Our programme of inspections and assessments gives us confidence to issue a Consent to start-up Reactor 4 at Hunterston B Power Station following its periodic shutdown.


Consent to start up Reactor 4 should be given by issuing Licence Instrument No 531 to the Licensee.

You can read the full report here.

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