Office for Nuclear Regulation

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Heysham 2 - 250v DC Unit Battery 7 Follow-up Inspection

Executive summary

Purpose of intervention

This intervention was to follow up enquires after an incident at EDF Energy Nuclear Generation Limited (NGL) Heysham 2 Nuclear Power Station. Elevated levels of hydrogen had been discovered adjacent to a battery room in the -4.5m level of the Turbine Hall. Due to the potential for an explosion or fire, because of the build-up of hydrogen, a conservative decision was made to declare a Site Incident and to muster staff to protect and account for all personnel on site. Actions were then taken to ensure the source of hydrogen (battery boost charging) had been stopped, and to clear the hydrogen from affected levels by increasing local ventilation. The success of these actions allowed the Site Incident to be terminated.

ONR internal hazards specialist inspectors and the nominated site inspector reviewed the circumstances leading to the incident and the licensee’s response. Additionally, the status, suitability and progress of the Licensee’s own investigation were also considered. Information was obtained from these follow up enquires to allow a decision to be made as to whether this incident required formal investigation by ONR.

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

Explanation of Judgement if Safety System Not Judged to be Adequate

Not applicable - no system based inspections were performed during this intervention.

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

The elevated levels of Hydrogen adjacent to a battery room in the -4.5m level of the Turbine Hall at Heysham 2 power station were found to have no nuclear safety consequences and the intervention did not identify any significant breach of site licence conditions.

Elevated levels of Hydrogen may have reached a concentration within the battery room in excess of that identified as good practice in HSE guidance for “using electrical storage batteries safely”, but the concentration is not directly measured, so this is uncertain until the licensee’s investigation has established a clear timeline for events.

The station has implemented its event recovery which will further strengthen its arrangements for battery charging to ensure adequate ventilation whenever boost charging is carried out. The revised plant item maintenance instruction will also require that the maintenance technician has a gas analyser set to alarm at 10% of the lower explosive limit (LEL). The station is to procure a hand held anemometer to allow the technician to confirm adequate ventilation flow. These arrangements will significantly improve the compliance against good practice whilst NGL carries out its own investigation and identifies the learning from this event.

NGL’s  formal investigation is still ongoing notwithstanding this I am content that the NGL’s investigation is being set up to address the important aspects of the event, and will lead to valuable learning.

The station’s response to the incident showed conservative decision making. Appropriate steps have been taken to ensure that safe conditions will exist in the battery room and adjacent areas whilst NGL’s investigation is progressing.

Noting the above, I am satisfied that the incident does not meet ONR’s criteria for investigation.

Conclusion of Intervention

The licensee is judged to be dealing with this incident appropriately and is applying its corrective actions programme to implement lessons learned and prevent a reoccurrence. This incident does not meet ONR’s investigation criteria. The site inspector has requested NGL’s internal investigation report on its completion and intends to review its conclusions

There are no findings from this inspection that could significantly undermine nuclear safety nor require a change to the planned interventions and inspections of Heysham 2 power station.