Office for Nuclear Regulation

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Magnox Swarf Storage Silos Air System - System Based Inspection

Executive summary

Purpose of intervention

The Office for Nuclear Regulation (ONR) undertakes all regulatory interaction with Sellafield Limited (SL, the Sellafield site licensee) against a strategy defined by the ONR Sellafield Programme. The inspection plan PP3 for Decommissioning sets out a programme of regulatory inspections which includes a system-based inspection (SBI) of the Magnox Swarf Storage Silo (MSSS) Air System in July 2015.

The purpose of this inspection was for ONR to consider the adequacy of implementation of the licensee’s safety case claims in respect of this system.

 SL is preparing for MSSS Silo Emptying plant to be installed. The air system is important for nuclear safety as it is required for the stack sampling system, silo liquor and sump level monitoring and for the operation of ventilation dampers.  Of these, I selected liquor level monitoring in the original building and the operation of the ventilation dampers for my inspection as these functions are particularly important to the MSSS safety case for maintaining plant containment.

Interventions Carried Out by ONR

I carried out a two day on-site SBI of the MSSS Air System with technical support from Mechanical and Fault studies specialists sourced from a technical support contractor. My inspection comprised discussions with SL staff, a plant walk down and reviews of plant records and other documentation.

As part of my inspection we examined evidence of the adequate implementation of five 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.

LC10 requires SL to make and implement adequate arrangements for suitable training of those who have responsibility for any operations that may affect safety. I used NS-TAST-GD-027 rev 4 as the reference against which to assess SL’s compliance.

LC23 requires SL to produce an adequate safety case to demonstrate the safety of its operations, and to identify operating conditions and limits necessary in the interests of safety. I used NS-INSP-GD-023 rev 3 as the reference against which to assess SL’s compliance.

LC24 requires SL to make and implement adequate arrangements for the provision of suitable written instructions for undertaking any operation that may affect safety. I used NS- INSP-GD-024 rev 2 as the reference against which to assess SL’s compliance.

LC27 requires SL to ensure that, before a system is operated, inspected, maintained or tested, all safety mechanisms, devices and circuits are properly connected and in good working order. I used NS- INSP -GD-027 rev 2 as the reference against which to assess SL’s compliance.

LC28 requires SL to make and implement adequate arrangements for the regular and systematic examination, inspection, maintenance and testing of all plant which may affect safety. I used NS- INSP-GD-028 rev 3 as the reference against which to assess SL’s compliance.

ONR’s process for SBI also includes LC34.  LC34 requires SL to ensure that its radioactive material and radioactive waste is at all times adequately controlled and contained, and that no leak or escape of that material or waste can take place without being detected.  As the Air System does not provide any containment function, and aspects where it assists with MSSS containment are adequately addressed through other elements of my inspection, I did not perform any explicit inspection against LC34.  

Explanation of Judgement if Safety System Not Judged to be Adequate

I judged this safety system 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 level detection in the original building and the control of the ventilation dampers. 

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 in the case of a sustained loss of the air system.

The safety mechanisms appeared to be adequately maintained. I did find evidence of corrosion to the air system pipework external to MSSS.  SL has already recognised the poor state of parts of the system and is implementing a programme of work to ensure it remains fit for purpose. The corroded pipework is isolated and no longer part of the air system. I am confident that the MSSS Asset Management Plan will maintain it in that state for the period that MSSS requires it.  

From the samples inspected I consider that the air system would adequately ensure that no leak or escape of radioactive material could occur without being detected and that control of the ventilation system dampers would be maintained in the event of a sustained loss of the air system.   

Conclusion of Intervention

From the samples inspected, I judge that the licensee has implemented adequately the relevant claims within its safety case and I was satisfied that the formal arrangements for all the licence conditions against which I inspected are also being implemented adequately.

Though I raised no Regulatory Issues, I made a number of minor observations during my inspection.  These were shared with, and accepted by the licensee during my inspection feedback.  None of these adversely affect safety.