The temperature of the Hot Box Dome (HBD) on Heysham 1 (HYA) and Hartlepool (HRA) reactors has been gradually increasing over time with reactor operation. This has the potential to limit reactor life as the structural integrity safety case HBD temperature limit is approached. Given the potentially significant implications, EDF Energy Nuclear Generation Ltd (NGL) has been managing this issue for a number of years through the HBD project within NGL's fleet critical programme.
The intervention enabled NGL to provide ONR with an update on the current status of the HBD on HYA and HRA reactors, proposals for future mitigation strategies and to demonstrate that the issue is being managed appropriately by NGL.
The intervention included:
The intervention related primarily to Licence Condition 23 which concerns operating rules and includes a requirement for the licensee to produce an adequate safety case, and to Licence Condition 22 which concerns plant modifications.
No safety system inspection was undertaken, hence this is not applicable.
HBD temperatures continue to rise slowly on HYA and HRA reactors. The smallest margin to the HBD temperature limit was 3.7 oC on HYA Reactor 2. However, recent improvements to the methods used to demonstrate compliance with the HBD temperature limit has resulted in this margin increasing to 9.5 oC. Further improvements to the compliance method are being developed.
A "hot spot" safety case that will aim to justify small areas of the HBD exceeding the current limit is being developed by NGL. It is intended that the safety case will be presented as a Category 1 modification in early 2015. The safety case is likely to require assessment by structural integrity and fault studies specialist inspectors.
NGL has put a significant amount of effort into developing a cowl that it is planning to fit to the central control rod channel of HRA Reactor 1 during the 2014 statutory outage. The cowl is designed to divert bleed flow onto the upper surface of the HBD to provide additional cooling. Noting that the modification is designed to be reversible the most significant aspect from a nuclear safety perspective is the potential consequences of the cowl becoming detached. NGL reported that this has been considered and the potential consequences have been determined to be minor. In order to help inform ONR's intervention strategy with respect to the installation of the cowl NGL agreed to provide ONR with a report on the consequences of cowl failure in service.
ONR inspectors observed a training exercise for operators who will be fitting the cowl to HRA R1 during the 2014 outage. The training is being performed on the full scale single quadrant rig at AMEC in Warrington. Whilst the procedure for fitting the cowl is challenging it was completed successfully during the training exercise. Overall the trial provided evidence that appropriate training is being performed and that a cowl could be successfully deployed at HRA R1.
The aims of the intervention were well met. Whilst HBD temperatures continue to rise, improvements to the compliance calculation route and bleed hole modifications have provided valuable mitigation and helped to manage the issue. Further measures to provide additional cooling to the HBD by fitting a cowl and the development of a "hot spot" safety case may also provide significant benefit. Overall the HBD project continues to be managed well by NGL.
Once the NGL report addressing the consequences of a cowl becoming detached has been received and considered by specialist inspectors and the HRA site inspector ONR's intervention strategy for the cowl modification will be determined.
A number of actions relating to the provision of documentation were agreed at the meeting and will be tracked through the ONR issues database.