Office for Nuclear Regulation

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ONR readiness inspection for Sellafield Ltd to implement pressurised inerting on the Magnox Swarf Storage Silo

Executive summary

Purpose of Intervention

On the 18 May 2017 the Office for Nuclear Regulation (ONR) undertook an intervention on the Sellafield site to inspect Sellafield Limited’s (SL) readiness to implement pressurised inerting on the Magnox Swarf Storage Silo (MSSS).

The purpose of the intervention was to inspect SL’s implementation of its Licence Condition (LC) 22 arrangements for this project and gain assurance that for the proposed modification, SL is in a state of readiness to safely commence project implementation. The findings of the intervention will be used to inform the regulatory decision on SL’s request for ONR’s agreement to commence the modification. 

MSSS is a legacy facility on the Sellafield site that was built in the 1960’s for the underwater storage of fuel cladding arising from decanning of Magnox fuel elements.  The facility comprises 22 vertical concrete silos, partially below ground level, that were in active use between 1964 and 1991. The ageing silo compartments contain a large inventory of radioactive waste (a mixture of intermediate level solid and liquid waste and miscellaneous beta gamma waste).

The underwater storage of radioactive waste within MSSS gives rise to the continuous chronic generation of hydrogen, primarily due to Magnox metal corrosion and requires continuous management and control to avoid build-ups that could lead to an explosion. This is normally managed by active ventilation, with quiescent passive ventilation available in the event of failure of the active systems.  However, excursions of significantly elevated hydrogen generation have occurred historically and given rise to high peak compartment hydrogen generation rates. The last such excursion was in 1980 and although SL predicts that an occurrence is unlikely, it cannot be entirely ruled out.

Following an optioneering study, SL has chosen to implement pressurised inerting as its primary hazard management strategy in the event of a future excursion.

Interventions Carried Out by ONR

The scope of this intervention was to inspect MSSS from a plant, processes and people perspective and assess the licensee’s readiness to safely commence the proposed modification. The readiness inspection was undertaken against LC 22 ‘Modification or experiment on existing plant’, which requires the licensee to make and implement adequate arrangements to control any modification or experiment carried out on any part of the existing plant or processes which may affect safety.   

The inspection comprised desktop-based discussions and facility inspection. It was undertaken against LC 22 in accordance with ONR guidance Technical Inspection Guide (TIG) NS-INSP-GD-022, LC22: Modification or experiment on existing plant, Revision 3, December 2014.

Explanation of Judgement if Safety System Not Judged to be Adequate

N/A – This was not a safety system inspection.

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

Based on the evidence sampled during this intervention, we have judged SL’s implementation of its LC 22 arrangements as Green.

With respect to ONR’s TIG NS-INSP-GD-022, SL demonstrated:

The modification is under the control of plant management and being managed in accordance with the established arrangements. This was evidenced by reviewing the proposed modification with the emergency operating instructions and discussion of training outcomes with duly authorised persons and the pressurised inerting subject matter expert.  We also gained evidence that the procedural controls for plant modifications had been recorded on the plant modification proposal.

Training has been developed and implemented for duly authorised persons (DAP); only 2 people had undertaken the training at the time of the inspection and there were some issues with the roll out and training assessment processes, however, these are considered minor and SL has resolved the issues adequately.  We judged the DAP training to be adequate.  SL is planning operator training, however, ONR was not provided with sufficient evidence that this was in a mature state at the time of the inspection. ONR noted that full implementation was not planned for approximately 2 months, following modifications to plant, providing time to complete the training. Though we were not able to sample the operator training, as the DAP training is adequate, we judged that it is sufficient to request that during internal readiness reviews, SL’s internal regulator confirms to ONR the development of a programme and materials for delivery of training and competency assurance arrangements for all key staff associated with MSSS PI (including operators). This is included a part of a recommendation in the assessment report for the permissioning decision.

Procedures and instructions have been assessed as part of the implementation of the modification. This was evidenced by subjecting parts emergency operating instructions to plant walk-throughs.  From the sample undertaken during the inspection, these were judged to be adequate. Further review will be undertaken as part of the assessments for the permissioning decision. We noted that SL has undertaken a full implementation walk-down but the report was not provided to ONR by the time of the inspection . This will be considered by the human factors assessment for assessments the permissioning decision

Ability to implement control of personnel entering MSSS during an excursion and control of personnel sent from the control room to perform tasks on plant. We judged SL’s control arrangements were on the whole adequate, however a number of questions regarding the need for operators to undertake operations near or on the operations floor, clarity of the delineation of areas (e.g. the operations floor boundary) and expectation regarding the risk assessment for personnel protection were raised.  These require further evidence to be followed up as part of the permissioning decision, noting that the risks relate to protection of personnel from radiological and conventional safety hazards (potential asphyxiation risk and hazardous flammable areas).

Ability to demonstrate flow measurements from the quiescent passive vents as required and demonstrate preparatory steps in preparation for implementation of pressurised inerting.

In relation to SL’s overall state of readiness to commence the proposed modification, it needs to complete its specified programme of work, which SL acknowledged.  We noted that SL’s plan includes plant modifications that will result in full implementation in approximately two months allowing time to complete preparations. Sufficient evidence was unavailable at the time of this intervention on certain aspects that we judge relevant to the permissioning decision. These were requested and will be considered as part of the overall permissioning decision.

In addition, during the inspection, observations were made of faults on the compartment oxygen monitor recorders (safety mechanisms) in the IGVEP control room.  Records were not available at the time of the inspection due to being stored in an archive. ONR has raised a regulatory issue to follow up these maintenance records following the inspection.

Conclusion of Intervention

Based on our findings, we judge that SL’s implementation of its LC22 arrangements related to the Plant Modification Process (PMP) is adequate. Based on the evidence sampled, no shortfalls were identified that would prevent permission being granted for SL to commence the modification.

In relation to SL’s state of readiness for the proposed modification, SL needs to complete its specified programme of work, which it acknowledged. As a result, we were unable to gather sufficient evidence on certain aspects that are judged relevant to the permissioning decision at the time of the intervention. We therefore identified a number of actions that SL’s needs to complete to inform the ONR’s permissioning decision.