Office for Nuclear Regulation

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System Based Inspection (SBI) of the Magnox Swarf Storage Silo (MSSS) Cooling System

Executive summary

Purpose of Intervention

This intervention, conducted at the Licensee’s Sellafield Limited (SL) Cumbria licensed site, was undertaken as part of the 2018/19 intervention plan and the Sellafield sub-division strategy.

Interventions Carried Out by ONR

ONR conducted a System Based Inspection (SBI) of the Magnox Swarf Storage Silo (MSSS) cooling system. The scope of this inspection considered the operation, maintenance and structural integrity of the cooling system to control temperatures and silo chemistry to ensure nuclear safety. The delivery of this function is important to safety for MSSS and for high hazard and risk reduction operations more generally.

ONR’s SBI process examines evidence to determine compliance against six key licence conditions (LCs). These licence conditions (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.

LC10 requires SL to make and implement adequate arrangements for suitable training of those who have responsibility for any operations that may affect safety.

LC23 requires SL to produce an adequate safety cases to demonstrate the safety of its operations, and to identify and implement operating conditions and limits necessary in the interests of safety.

LC24 requires SL to make and implement adequate arrangements for the provision of suitable written instructions for undertaking any operation that may affect safety.

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.

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.

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.

Explanation of Judgement if Safety System Not Judged to be Adequate

The Licensee has not been able to demonstrate that the Structures, Systems and Components (SSCs) which have been inspected as part of the MSSS Cooling system are able to fulfil their safety duties (safety functional requirements) adequately, in line with the safety case. This judgement has been reached on the basis of the facility not being able to provide sufficient evidence to demonstrate some of the Structures, Systems and Components (specifically the MK 3 & MK 4 coolers) are fit for purpose and will fulfil their safety functional requirements on demand. However, there is no immediate risk as historically temperatures within the silos supported by these coolers have not risen sufficiently to place a demand for this cooling and this is not expected to change. It should also be noted that the Licensee has confidence in its ability to introduce cooling in a stated period of time to react to abnormal temperature rises and thus maintain the safe operating envelope of the facility. I will progress this shortfall through the Regulatory Issues raised as a result of this inspection.

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

No matters were identified as requiring immediate regulatory attention during the conduct of the compliance inspection.

From the areas ONR targeted and the evidence ONR examined during this inspection, I consider that the Licensee has not adequately implemented those safety case claims that relate to the MSSS cooling system. In particular, two of the three types of installed cooling systems have not been in an operational state nor have they been periodically and systematically maintained for a significant period of time. On balance, I applied an inspection rating of Amber (Seek improvement) for LCs 27 and 28 to reflect this. These shortfalls have been raised as Regulatory Issues which will be managed as part of normal regulatory business.

In addition, I have applied an inspection rating of Green (No formal action) for LCs 10, 23, 24 and 34. However, I have identified an area for improvement during the inspection which has been raised as minor Regulatory Issue, and which will be managed as part of normal regulatory business. This relates to changes to the clearance certificate via addendum, which should be appropriately managed such the changes to the safety case are clear and visible to the user.

Conclusion of Intervention

No matters were identified as requiring immediate regulatory attention during the conduct of the system based inspection.

I have applied an inspection rating of Amber (Seek improvement) for LCs 27 and 28.

I have applied an inspection rating of Green (No formal action) for LCs 10, 23, 24 and 34.

I consider that SL has not adequately implemented those safety case claims that relate to the MSSS cooling system.