This was a planned intervention at EDF Energy Nuclear Generation Ltd’s (NGL’s) Dungeness B (DNB) power station, undertaken as part of the planned Integrated Intervention Strategy (IIS) for the Civil Nuclear Reactor Programme (CNRP) of the Office for Nuclear Regulation (ONR).
The intervention included a system based inspection (SBI) of post trip interlocks to demonstrate compliance with the safety cases for this system. This inspection was performed by the ONR nominated site inspector and two control and instrumentation specialist inspectors.
The key inspections during the post trip interlocks SBI were for licence compliance covering the following licence conditions (LC):
A separate licence condition inspection was undertaken for the following LC:
During this intervention, the following key activities were also undertaken:
For LC 23 and 27, the technical specifications, which specify the availability requirements for the post trip interlocks were reviewed. The technical specifications were found to reflect the safety case and compliance with the specific technical specifications would ensure sufficient safety circuits were available. Some concerns were identified over the interpretation of the technical specifications, but the site staff were confident that they would be interpreted consistently with the safety case and, given the safety significance of the system, it was judged that this would be the case. Nonetheless, an action was placed on the site to make the interpretation clearer in the technical specifications. The implementation of the limits and conditions as required by LC 23 was judged to be adequate. The technical specifications for the post trip interlocks ensured that there would be sufficient safety circuits available as required by LC 27 and therefore IIS ratings of 3 were assigned for both LC 23 and LC 27.
For LC 24, a sample of operating instructions was reviewed, a walkdown to demonstrate the instructions on the plant was witnessed and records from reactor trips were inspected. The records were found to be adequate, the operators demonstrated knowledge and understanding of the plant and the records demonstrated that the instructions had been followed. Operating instructions for the Main Guard Line post trip interlocks and Diverse Guard Line post trip interlocks were judged to be adequate and therefore an IIS rating of 3 was assigned.
For LC 28, a number of the maintenance procedures were sampled and judged to provide adequate maintenance instructions for the Main Guard Line post trip interlocks. In addition completed maintenance instructions were sampled and found to have been completed satisfactorily, within specified timescales and appropriately signed off. However there was no evidence of maintenance instructions for the Diverse Guard Line post trip interlocks, although they are routinely tested. An action to address this shortfall was agreed with NGL. Compliance with LC 28 was judged to be adequate and allocated an IIS rating of 3.
The post trip interlocks have a major role in ensuring that the post-trip cooling systems perform satisfactorily in the event of a reactor trip and hence preventing a major radioactive release. However, they do not directly provide a containment or detection function for radioactive material or radioactive waste. Therefore, it was judged that LC34 was not applicable to this system.
For LC 10, a sample of training records and training programmes for staff involved in maintenance and operation of the post trip interlocks were inspected. All staff were found to be up to date with post trip interlock related training and the training was judged to be appropriate. Training was judged to be adequate and hence an IIS rating of 3 was assigned.
The plant performance and condition were inspected by reviewing the licensee’s assessments of the system and inspecting the plant. The inspection confirmed that station had arrangements in place to ensure that the Post Trip Interlocks system was meeting the requirements of the safety case and that overall examination, inspection, maintenance and testing was being conducted in a satisfactory manner to ensure the Main Guard Line post trip interlock control and instrumentation equipment was maintained to a good standard and remained fit for its intended purpose. It was judged that the plant performance and condition was satisfactory.
For LC 14, the arrangements for the production and assessment of safety cases were inspected and judged to be adequate. A sample of the licensee’s safety cases that had been produced and assessed using the arrangements for LC 14 were inspected and the arrangements found to be adequately implemented. Compliance with LC 14 was judged to be adequate and an IIS rating of 3 assigned.
In relation to the other activities undertaken in this intervention, no key findings arose.
The intervention was performed in line with ONR’s guidance requirements (as described in our technical inspection guides) in the areas inspected.
From the evidence obtained during this intervention, no matters have been identified that are likely to have a significant impact on nuclear safety or protection of the public in the event of an emergency on the Station at this time. Therefore, no additional regulatory action arising from this visit is considered necessary at this time.