Office for Nuclear Regulation

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Updated Safety Case for Plug Unit Weld Integrity

Executive summary

Purpose of intervention

ONR attended EDF Energy Nuclear Generation Limited's (NGL's) Barnwood office to participate in a level 4 technical and programme exchange meeting. This was part of ongoing ONR interventions into assurance of plug unit weld integrity at Dungeness B power station (DNB) and improvements of the management of the Neutron Scatter Plugs (NSPs). The meeting focussed on Fuel Plug Units (FPUs) since this type of plug unit is of most safety significance. The meeting was attended by the ONR site inspector for DNB, as well as structural integrity and fault studies specialist inspectors. Attendees for the Licensee included representatives from various project disciplines.

The two ONR interventions relating to this meeting are NGL-072 "Dungeness B - Plug Units" and NGL-175 "Dungeness B Neutron Scatter Plug (NSP) Management". ONR Level 2 compliance issue 660 "Plant Condition - Structural Integrity - Fuel Plug Unit" also refers.

The main safety and radioactive waste management activities dealt with by these interventions are the unloading of affected components from the reactor cores, the handling of affected components in the fuel route and the storage of components that cannot be reused.

Purpose of Intervention

This meeting was arranged as part of ongoing interventions to enable the Licensee to provide an update on the progress of the safety case for plug unit weld integrity and the NSP management programme including longer term options. No regulatory judgement or decision was required.

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

Although no regulatory judgement has been made for this intervention, it is my opinion that NGL is taking relevant actions to understand and manage the weld integrity concerns associated with the plug units and how these will influence safety. The ongoing examination and test work will provide further evidence for a follow on long term safety case. ONR suggested that in order to support the follow on safety case, the Licensee should develop a strategy for selecting FPUs to give the optimum results for testing and analysis. This includes developing test criteria for push out testing and a technical specification for materials analysis.

Conclusion of Intervention

The Licensee is progressing with the development of a strategy and safety case for managing the replacement of the plug units with identified weld integrity issues. The outcome of the proposed test work will determine the final content and structure of the long term safety case. ONR will assess the current safety case and provide feedback for the Licensee to incorporate into the long term safety case.

To meet the long-term objective of removing all affected components from the reactor cores, storage of the NSPs is reliant in the short term on emplacement of NSPs in the Main Active Waste Vault. A project is also underway to design a NSP Secondary Vault (NSPSV). These aspects are being dealt with via ONR intervention project NGL-175.

Generally, the Licensee is proceeding in line with the ONR expectations. Maintaining communication with ONR and providing visibility of this programme of work is seen as key for future assessment and permissioning as required for the future stages of this project.

Recommendations

ONR should assess the current "Updated Safety Case for Plug Unit Weld Integrity", Engineering Change 335466, to inform the Licensee's follow on long term safety case.

ONR should continue to engage with the Licensee on the NSP management programme.