Office for Nuclear Regulation

MSSS Electrical System – System Based Inspection (SBI)

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 the ONR Sellafield Programme.

In accordance with that Strategy, a system-based inspection of the Magnox Swarf Storage Silo (MSSS) Electrical Systems was planned for May 2015.  The purpose of this inspection was for the ONR to confirm the adequacy of implementation of the licensee’s safety case claims in respect of this system.

ONR’s system-based inspections examine evidence of the adequate implementation of six licence conditions.  These licence conditions (listed below) have been selected in view of their importance to nuclear safety and are defined within ONR’s formal process for system-based inspection. .

The Electrical Systems were selected for inspection in view of their importance to safety.  Specifically:

SL is preparing MSSS for Silo Emptying Plant to be installed; this includes modification to the electrical system.

The electrical supplies to the facility’s 3rd Extension and the level detection in both the original building and 1st Extension are of particular importance to the ongoing safety of the facility.

Interventions Carried Out by ONR

I carried out a two-day, on-site, system-based inspection of the MSSS Electrical Systems, with technical support from Electrical and Control and Instrumentation (C&I) specialists from ONR and a technical support contractor.  The inspection comprised discussions with SL staff, a plant walk down and reviews of operating instructions, plant records and other documentation.  

Explanation of Judgement if Safety System Not Judged to be Adequate

The electrical supply system to the 3rd extension and the level detection in the original building and 1st extension for the silo and sump was judged to be adequate.

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

From the evidence made available to me during this inspection, I consider that SL has implemented adequately those claims within the facility safety case that relate to the 3rd Extension and the level detection in the original building and 1st Extension for the silo and sump.  Additionally, SL is continuing to refine and develop the operating rules and limits as part of the implementation of the passive ventilation modification Safety Case Implementation Plan (SCIP).

In performing this system-based inspection, I have considered if the supporting safety case for the system requires a priority assessment of its adequacy.  Based on the information gathered during this inspection, I have no reason to recommend an early assessment of this safety case.

Based on discussions with facility staff and an examination of the training records during this inspection, I consider that SL has adequate training in place to deliver an effective response to system alarms, from both a public and plant operator safety perspective.  There was evidence of a good awareness of plant parameters, the means of ascertaining them, and a good level of wider understanding of which plant indications could be drawn upon with confidence in an emergency including the changes that the SCIP will implement in the future. In general, the safety mechanisms appeared to be adequately maintained.  However, I did find evidence where the written procedures had not been followed correctly, and there was one part of the electrical supply system where maintenance had been delayed. These shortfalls draw into question the adequacy of SL’s planning and supervision of this system’s maintenance. 

As the loss of the electrical system cannot result directly in a leak or escape of radiological material or radioactive waste, I did not perform a specific inspection against LC34.  That said, the electrical and level detection systems I inspected do provide indirect prevention and protection against leakages and escapes and I was satisfied with the adequacy of these systems in this regard.

Conclusion of Intervention

From the evidence sampled during this inspection, I judge that the licensee has implemented adequately the relevant claims within its safety case, and that, with one exception, there was evidence that the formal arrangements for all the licence conditions against which I inspected are also being implemented adequately.  As such I have awarded IIS ratings of 3 (Adequate) against LCs 10, 23, 24 and 27.

However, I identified one area (LC28) where there was evidence of shortfalls against both ONR guidance and the licensee’s own arrangements, against which SL has agreed to make improvements.  As such I have awarded an IIS rating of 4 (Below Standard) against LC28 (Examination, inspection, maintenance and testing).  ONR Regulatory Issues will be raised to tracks SL’s progress in addressing these matters.

The shortfalls in regard to LC28 do not alter my conclusion in regard to the adequacy of this system to deliver its safety requirements.

My inspection also noted a number of minor observations.  These were shared with, and accepted by, the licensee during my inspection feedback.