Office for Nuclear Regulation

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Statement of civil incidents meeting the Ministerial Reportable Criteria (MRC) reported to ONR - Q2 2018

1 April 2018 to 30 June 2018

A statement on incidents at nuclear installations in Great Britain and during the transport of radioactive material, which meet Ministerial Reporting Criteria (MRC), is provided to the Secretary of State for Business, Energy and Industrial Strategy and the Secretaries of State for Scotland and Wales. This statement is produced quarterly by the Office for Nuclear Regulation.

For the period, 01 April 2018 to 30 June 2018, there was one incident which met the MRC.

Hunterston B Power Station  - Reactor 3 Graphite Core - 22 March 2018

Ministerial Reporting Criterion - Examination, inspection, maintenance, test or operation of any part of the plant revealing that the safe operation or condition of the plant may be significantly affected.

INES Rating 1

On 9th March 2018, Reactor 3 at Hunterston B was safely shut down for a planned inspection to determine the extent of the progression of cracking in the graphite bricks of the core of the reactor.  The safety functions of the graphite core are to maintain the correct geometry for coolant gas flow over the reactor fuel and to maintain clear channels for control rod insertion to allow the reactor to be safely shut down. 

Cracking of the graphite bricks in Advanced Gas-cooled Reactors is expected as the reactors age.  However, the number of cracks found during the inspection of Reactor 3 led, on 2 May 2018, to EDF Nuclear Generation Ltd confirming that Reactor 3 will remain shut down whilst further inspections of the core are carried out, and to allow preparation of a robust safety case that justifies a further period of safe operation.

ONR has communicated  to EDF NGL that Hunterston B Reactor 3 cannot be returned to service until an adequate safety case has been provided, demonstrating to our satisfaction that the reactor can continue to be operated safely. Once satisfied, ONR will provide formal consent to the restart of the reactor through the provision of a Licence Instrument.

ONR is satisfied that Reactor 3 at Hunterston B remains safely shut down with adequate cooling being provided.  ONR is monitoring EDF NGL’s on-going activities to prepare the requisite safety case and we will undertake a detailed assessment  of the submission once it has been received, which will inform our decision on any future return to service. 

ONR is content that there are no immediate safety concerns for the remaining reactors in the AGR fleet, but the on-going inspections of Reactor 3 will have a bearing on the planned outage and inspections for Hunterston B Reactor 4.

An INES rating of 1 (Anomaly) is assigned to this event.