Office for Nuclear Regulation

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Site inspection - Wylfa

Executive summary

Purpose of intervention

This inspection has been undertaken as part of ONR's enforcement response to the pile cap coffin event that arose during maintenance of Fuelling Machine 2.

Interventions Carried Out by ONR

Interventions included inspection of the maintenance activities currently being undertaken on the fuel route and the licensee approach to identifying critical spares.

A review of site approach to the effectiveness reviews of actions arising from root cause investigations was undertaken.

A follow-up to previous enquiries relating to events associated with nuclear fire barriers was undertaken.

In addition to the inspections a number of liaison meetings were attended. These included the following topics:

Explanation of Judgement if Safety System Not Judged to be Adequate

No safety system based inspections were undertaken during this intervention.

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

Fuel Route Maintenance

The arrangements for the fuel route maintenance strategy, identification of critical spares and the delivery of repairs to DSC2 were sampled. Based on the evidence provided Magnox Ltd demonstrated that there are no significant shortfalls and for the samples inspected relevant good practice was being met; an inspection rating of Green is assigned. A number of actions were agreed to provide additional information to support further confidence in response to unplanned maintenance if/when it occurs.

Nuclear Fire Barriers

Following further consideration of information provided by Magnox Ltd in relation to the possibility of defects and penetrations in nuclear fire safety barriers, it remains apparent that Magnox Ltd is unable to robustly demonstrate compliance with the nuclear fire safety case. Assumptions made in the safety case imply no tolerance for through wall defects, and that any defects that do arise should be dealt with in a timely manner. The collection of events raised cast some doubt over whether the current safety case is robustly implemented at all times with regard to this. In mitigation Magnox Ltd have advised that the Nuclear Fire Safety case has not been reviewed and revised to reflect the defuelling operations now being undertaken and Magnox Ltd considered reasonably that requirements were likely to be less stringent given the ongoing hazard/risk reduction. Actions have been agreed to provide further reviews of events and their impact on the safety case.

Effectiveness Reviews

Magnox Ltd provided an overview of the effectiveness review process. It did not appear to be particularly challenging and a number of opportunities for a enhancing the review but focussing on specific key actions/improvements should be considered. The Wylfa OEF manager agreed to explore developing a revised approach that could be piloted on an RCI relating to a leak from the RACW system during 2015.

Conclusion of Intervention

From the information obtained and evidence gathered during this intervention, I believe there were no issues identified that were considered to adversely affect nuclear safety.