Office for Nuclear Regulation

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System Based Inspection (SBI) of the Liquor Level Monitoring System on the Magnox Swarf Storage Silo (MSSS) on Sellafield Ltd.'s nuclear licensed sites at Sellafield, Cumbria

Executive summary

Purpose of intervention

The Office for Nuclear Regulation (ONR) undertakes all regulatory interaction with the Sellafield site licensee (Sellafield Limited, SL) against a strategy defined by ONR's Sellafield Programme. In accordance with that strategy, a system based inspection (SBI) of the MSSS Liquor Level Monitoring System was planned and undertaken in May 2016. The purpose of this inspection is for ONR to examine whether the licensee's safety case claims in respect of this system important to safety have been adequately implemented.

This inspection activity targeted the Liquor Level Monitoring System as the focus of this inspection because it provides the primary means for detecting a loss of liquid level in the MSSS silos which may indicate a loss of containment and ultimately could expose radioactive waste to air leading to a fire within the compartments. As such, the Liquor Level Monitoring System is considered to be a system important to nuclear safety.

ONR's SBI process examines evidence to determine compliance against six key licence conditions (LC's). These LC's (listed below) have been selected in view of their relevance to ensuring nuclear safety and in providing a structured approach to determining whether the safety case has been implemented adequately through the system being inspected.

Interventions Carried Out by ONR

I carried out an SBI of the Liquor Level Monitoring System over the course of two days. I was assisted by another ONR site inspector, an ONR fault studies specialist inspector and a technical support expert in the field of mechanical engineering. I was also supported on this inspection by SL staff from its internal regulatory organisation. The inspection comprised discussions with SL staff, physical viewing and inspections of targeted structures, systems and components (SSC's), and reviews of SL's records and other safety documentation.

Explanation of Judgement if Safety System Not Judged to be Adequate

The safety case supporting this system was judged to be implemented adequately.

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

From the areas I targeted and the evidence I examined during this inspection, I consider that SL has implemented adequately those safety case claims that relate to the MSSS Liquor Level Monitoring system. In particular, it is my opinion that SL is developing and implementing some good arrangements for the provision of training for its staff and for the examination, inspection, testing and maintenance of the safety system. SL has also made improvements to their arrangements for leak detection. Consequently, it is my opinion that an inspection rating of GREEN (no formal action) is appropriate for compliance against LCs 10, 27, 28 and 34.

Two areas for improvement were, however, also identified. The first relates to operating rules, where, although SL described some good practice, ONR identified a specific shortfall regarding the non-inclusion of the Severe Accident Analysis in the safety case which may therefore not be reflected in the operating rules for the liquor level monitoring system. The second relates to the adequacy of the Operating Instructions (OIs). I consider these should include instructions to confirm that alarms are physically verified on plant as part of the Alarm Response Instruction (ARI). I also consider these could be further improved by including overt control and supervision and plant configuration control elements. Consequently, I have awarded an inspection rating of AMBER (seek improvement) for LCs 23 and 24.

Conclusion of Intervention

I consider that the licensee has a good knowledge of the physical condition of the structures, systems and components reviewed during this inspection and has in place appropriate management controls to ensure on-going safety.

I have, however, identified two specific areas for improvement, which require appropriate attention from the licensee. I have therefore raised two ONR Regulatory Issues. These will ensure that progress against these concerns can be monitored and will be addressed as part of the more comprehensive SL action already underway to review the MSSS safety case.

Overall, on balance, therefore, I award an inspection rating of GREEN (no formal action) for the MSSS Liquor Level Monitoring system.