The Office for Nuclear Regulation (ONR) has an expectation that the Control Rod Drive Mechanism (CRDM) penetration welds in the Reactor Pressure Vessel (RPV) heads are as defect free as practicable. Operating experience indicated that difficulties had been encountered in the manufacture of similar welds for the Flamanville 3 EPR™. Therefore, ONR deemed it important to gather evidence to confirm that the required quality would be achieved and that the licensee, NNB Generation Company (HPC) Ltd (NNB GenCo), had suitable arrangements in place to specify, control and oversee CRDM welder training.
The purpose of the intervention was to gather evidence on:
the licensee’s ability to specify, control and oversee the welder training;
the licensee's processes for the specification, generation and approval of welder training documentation; how operating experience from previous EPR™ projects had been used to develop sound processes to manufacture the RPV CRDM penetration welds for Hinkley Point C (HPC); and that the technical requirements of the design would be met.
ONR carried out a one day inspection at the Framatome St Marcel facility through observation of CRDM welder training activities and discussions with relevant NNB GenCo and Framatome personnel.
The intervention is relevant to Licence Condition (LC)12: Duly authorised and other suitably qualified and experienced persons, which requires the licensee to make and implement adequate arrangements to ensure that only suitably qualified and experienced persons perform any duties which may affect the safety of operations on the site.
Not applicable; this was not a system-based inspection.
The licensee and Framatome have assessed operating experience and carried out a development programme to mitigate the risk of weld defects occurring during welding of the HPC CRDM penetrations. Within this programme, the joint design has been modified to reduce constraint and the occurrence of cracking. This design change had been presented to and accepted by NNB GenCo’s Design & Safety Case Committee (DSCC).
Welder training documents (welder training qualification procedures and internal training instructions) are not subjected to the licensee's review and acceptance process. Consequently, ONR has placed an action on the licensee to confirm the adequacy of CRDM welder training documentation, and to justify how they determine the adequacy of training documentation for other welder training activities. A regulatory issue has been raised to capture these actions.
The licensee confirmed that the ultrasonic technique and acceptance criteria applied during the manufacturing of the two additional mock-ups as part of the welder qualification were the same as those to be applied during production.
I am content that appropriate welding requirements have been specified for CRDM welder training. However, actions have been placed on the licensee to confirm that these requirements have been implemented into applicable welder training documents. A level 4 regulatory issue has been raised to capture the actions.
The licensee has indicated that the findings can be justified and resolved in appropriate timescales. In addition, the licensee has appropriate arrangements in place to monitor and oversee welding during the CRDM welder qualification activities. Therefore, an inspection rating of GREEN (no formal action at this stage) is appropriate.