Sizewell B - Reactive unplanned LC7 inspection
- Site: Sizewell B
- IR number: 19-024
- Date: May 2019
- LC numbers: 7
Purpose of Intervention
The purpose of this intervention was to undertake a reactive unplanned inspection of compliance with Licence Condition (LC) 7 (incidents on the site) at EDF Energy Nuclear Generation Limited’s (NGL) Sizewell B power station, in response to events that had been reported to ONR.
Interventions Carried Out by ONR
The licensee’s processes for identifying and investigation faults, and for notifying ONR, were inspected using three examples:
- The failure of plant control system instrumentation (NOT the nuclear safety-related plant protection system);
- The failure to comply with the tech specs in isolating one of two redundant containment isolation valves providing double isolation;
- The failure of a weld in a spent fuel rack containing a burnable poison array in the spent fuel pond.
Explanation of Judgement if Safety System Not Judged to be Adequate
Key Findings, Inspector's Opinions and Reasons for Judgements Made
Overall I am satisfied that my sample of the above incidents was investigated and reported appropriately by the licensee, based on the following observations:
- As the reactor was stable when plant control system indication was lost, and the plant protection system remained working at all times, the loss of indication did not pose an immediate risk to nuclear safety. The fault finding was carried out quickly and efficiently, and the control system was reinstated in a timely fashion. The existing plant control system will be largely replaced during Refuelling Outage 16 (RO16).
- The failure to comply with tech specs appears to have been caused by a high workload in the MCR (the event occurred during the plant control system event reported above), and the licensee has committed to providing supplementary training as a result. Nuclear safety was not compromised, as one of the two containment isolation valves remained shut throughout (further, the primary circuit was intact, so the release from containment even if both valves had been open is unlikely to have been significant).
- The immediate response to the fuel rack weld failure was thorough (a camera inspection of all fuel racks in both regions of the pond). The licensee has sought international operating experience to inform its decision making. Safety analysis concludes that there is no threat to the seismic safety case and the fact that no fuel is stored in that location in the fuel pond means that there is no other threat to nuclear safety.
Conclusion of Intervention
There are no findings from this intervention that undermine nuclear safety at this time. Therefore, no additional regulatory action is needed over and above the planned interventions at for Sizewell B.