Office for Nuclear Regulation

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Planned Intervention - Hunterston B

Executive summary

Purpose of intervention

The purpose of this intervention was to undertake compliance inspections and information exchange meetings at EDF Energy Nuclear Generation Limited's [NGL's] Hunterston B [HNB] power station in line with the planned inspection programme contained in the HNB Integrated Intervention Strategy [IIS].

Interventions Carried Out by ONR

As part of this intervention, we attended the Reactor 4 start-up meeting. NGL presented the status of outstanding Engineering Change papers and Independent Nuclear Safety Assessment [INSA] approval by NGL's internal regulator, Independent Nuclear Assurance [INA] of which restart of Reactor 4 is contingent. NGL described its performance during the outage in respect of Fire and Conventional Safety; Radiological Safety; LC28 maintenance activities; status of graphite core inspections and boiler internal / external weld inspections and repairs. During the inspection period, the NGL presented an updated schedule for delivery of the Nitrogen hold-down system. The superintending inspector undertook a physical inspection of the Reactor 4 gas circulator hall, turbine generator 8, the pump-house and a familiarisation visit to the Western Link convertor station.

I undertook preliminary enquiries into the failure of two gas circulators, an event that occurred in early September 2014. This preliminary enquiry sought to acquire sufficient understanding of the incident and sufficient assurance that there are no significant shortfalls in engineering and administrative controls that might affect ONR's issue of consent to restart Reactor 4. I undertook these enquiries in parallel with but independent of NGL's own investigation.

I undertook a joint inspection with INA to sample Station's start-up sequencing and start-up tests occurring during the inspection period.

Explanation of Judgement if Safety System Not Judged to be Adequate

No safety system inspection was undertaken during this intervention.

Key Findings, Inspector's Opinions and Reasons for Judgements Made

Hunterston B is now entering an important phase of commissioning for the nitrogen hold-down system and the underpinning engineering change is progressing well. We advised station that recent efforts to enhance regulatory communications and currency of information are welcome. Station recognises the challenges to ONR assessment resource but has presented a more detailed schedule of INSA and ONR activities ahead of a Licence Instrument being sought in January 2015.

Station coordinated a comprehensive start-up meeting agenda, supported by an informative start-up report. At the time of the meeting 85% of statutory tests and 70% of LC28 maintenance activities had been undertaken. Station reported good overall safety performance and contamination control. Station reported significant investment in engineering modifications, notably an overhaul of the main boiler feed pump and its exhaust; replacement of three generator transformer phases and gas circulator exchanges as part of an accelerated programme.

Station referred to a largely successful programme of in-vessel and external weld inspections with only minimal repairs required in this outage. ONR sought and received further clarification on the reduction in scope associated with the weld inspections previously planned and were satisfied with the approach.

ONR reflected on a number of incidents that occurred both during and prior to the outage, which from initial discussions, we consider may be attributable to human error. We advised station that ONR would be considering further regulatory interventions at Hunterston B and wider across the NGL fleet to secure confidence that any shortfalls identified across fleet are addressed in a prompt manner.

I observed reactor start-up tests with an INA evaluator as well as a mode change meeting. I judged that Station's arrangements for start-up test sequencing and mode change control to be mature and well-established. During the sampling of start-up tests I observed areas of improvement relating to procedural design and reduction of human error traps in recording safety significant limits. I will be following up these findings as part of a wider human factors intervention following return to service.

My preliminary enquiries into the failure of two gas circulators have given me sufficient assurance that Station has addressed the associated direct causes to the incident. I am satisfied with the measures undertaken in the short term.

The intervention was performed in line with ONR's guidance requirements (as described in our technical inspection guides) in the areas inspected.

Conclusion of Intervention

The start-up meeting provided confidence that station has and continues to manage its outage well. Overall safety performance has been good in the context of a greater than usual complexity of outage activities. Station's management of start-up sequencing appears to be robust although some areas for future improvement have been identified.