Office for Nuclear Regulation

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Sellafield Inspection of MSSS Silo Liquor Controls

Executive summary

Purpose of intervention

This intervention was to inspect the liquor control system in place on Sellafield Ltd Magnox Swarf Storage Silo (MSSS), to verify that the system and its operation remains adequate to manage the hazards that are present. This is one of seven MSSS systems identified on the Sellafield systems inspection register.

This inspection was carried out in line with the planned inspection programme for the Decommissioning Division. It was conducted by the ONR site inspector, supported by Chemical Engineering, Fault Studies and Environment Agency Specialist Inspectors.

Interventions Carried Out by ONR

I carried out a safety case based system inspection against the MSSS liquor control systems. Beginning with the facility safety case, I examined the safety case, inspected the key safety measures and features claimed, to verify if these were effective on the plant, and to confirm if adequate maintenance and training are carried out to support its safe operation. Through examination of this system I inspected compliance against Licence Conditions 10, 23, 24, 27, 28 and 34.

Explanation of Judgement if Safety System Not Judged to be Adequate

N/A - the silo liquor control system was judged adequate based on this inspection

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

Overall Sellafield Limited was able to demonstrate that it had an adequate understanding of the state and behaviour of liquors in the cells and sumps on the plant; it could recognise anomalies from normal plant behaviour and has arrangements for responding if a deviation occurred. The operations team demonstrated an adequate understanding of the plant readings and alarms, normal plant behaviour, and what the response should be under a number of normal and emergency scenarios, but there didn't appear to be a clear recognition of the defined operating rule on the plant.

During the inspection a good questioning attitude was demonstrated by the licensee, i.e. an investigation was launched when small changes in two silo levels occurred at the same time. These were eventually confirmed as normal plant response to current operations, but this indicated a positive nuclear safety culture on plant.

I inspected the flow down of the safety case requirements to the lower level procedures, and although these contained the requirements, I considered there was scope to improve the structure of some of these documents. I also identified areas where the references in the documents and the links on the management system could be improved.

I inspected the Safety Mechanisms (SM) associated with the liquor control system. These appeared to be adequately defined within the safety documentation and references connected to the operational documentation. All Safety Mechanisms appeared to fully operational during the inspection. I identified a minor issue with labelling of Safety Mechanisms, which is identified in the report, but this was of a minor nature.

During the plant inspection I identified a defect label attached to a Safety Mechanism, which had been open since August 2013. Although the item was functioning at the time of the inspection the presence of the label and its age indicated that not all Safety Mechanisms are in good working order, and Safety Mechanisms are not always corrected in a timely manner.

I inspected the Examination, Inspection, Maintenance and Test (EIM&T) arrangements related to the liquor control system. It was encouraging to note that the Systems health report had been completed for the containment systems and I considered the report to be a reasonable assessment of the system. In addition to this, there is an annual containment report, which adds specific support and additional information to containment and liquor balance aspects. The combination of these two documents demonstrated reasonable control over the condition and performance of the containment and liquor control systems.

I identified that the Plant Maintenance Schedule still identified the Vacuum Operated Slug Lift (VOSL), which is no longer operational and is in the process of being physically removed from the plant. The item was still identified on the PMS, with a defined maintenance frequency even though it is no longer maintained. I challenged how this complied with the sites arrangements for Licence Condition 28, which requires all EIM&T to be carried out within the specified period or deviations reported to ONR. In this instance the item no longer has any safety function, so I considered this to be of limited safety significance, but this issue has the potential to be more significant if this is a wider issue across the site. Prompted by this anomaly, Sellafield Limited undertook to write to me outlining the outcome of a review of their LC28 arrangements and their implementation at the MSSS.

I considered the Liquor Activity Reduction programme of work to be a positive programme of work to improve the condition of silo liquors. Plans are being developed to roll this out further to the original building and first extension in support of preparation for retrievals.

Sellafield Limited demonstrated a good programme of work had been carried out to improve the accuracy of the silo liquor balance model and maturity of trends for the silo contents. I consider this a good improvement not only in gaining an improved picture of liquor behaviour and means to identify leaks, but this will also support retrieval operations.

Conclusion of Intervention

Overall, I was satisfied that Sellafield Limited was able to demonstrate that it had an adequate safety case, appropriate understanding of the state and behaviour of liquors in the cells and sumps on the plant; it could recognise anomalies from normal plant behaviour and has arrangements for responding if a deviation occurred. A number of minor issues and actions were identified during the inspection, which will be added to the ONR issues database and followed up by the ONR site inspector as part of normal regulatory interaction.