Executive summary
Date(s) of inspection:
March 2024
Aim of inspection
The aim of the inspection was to assess the adequacy of NNB Generation Company (HPC) Ltd's (NNB GenCo) arrangements, specifically those relating to Licence Condition (LC) 19, for the oversight of the optimised local post weld heat treatment (LPWHT) applied to the unit 2 steam generator (SG) Partie Inférieur Fini (PIF) weld at Framatome St Marcel. This optimised LPWHT is based on experience from the unit 1 SGs.
Framatome St Marcel is manufacturing all the Hinkley Point C (HPC) SGs, who is the primary contractor for the manufacture and installation of the primary circuit. It is important that NNB GenCo maintains adequate oversight and control over all the manufacturing activities of the SGs, that is commensurate with the high integrity classification of these components.
The inspection objectives were to examine:
- implementation of NNB GenCo's surveillance arrangements;
- the development, and ensure adequate underpinning, of the optimised LPWHT arrangements to ensure that desired results could be achieved;
- the maintenance and calibration of the equipment used for the purpose; and
- the project plan and training undertaken, and review of the risks.
Subject(s) of inspection
- LC19 - Construction or installation of new plant - Rating: Green
Key findings, inspector's opinions and reasons for judgement made
During the inspection I reviewed the theoretical underpinning of the LPWHT, considering the impact on both the primary and the secondary sides of the SG. There was good consistency in predicting the thermal distributions at the tube sheet following the treatment by two independent streams of work for the primary and secondary sides of the SG, considering those distributions to be the main driver for the displacement controlled residual stresses at the PIF weld location.
I noted from the studies that the impact on the secondary sides (e.g. tube bundle, anti-vibration bars etc.) was predicted to be minimal and real data would be used from conformant locations to compare with the unit 1 data.
There was adequate surveillance and oversight provided by Direction Industrial (DI) and the Independent Third Party Inspection Agency (ITPIA) whilst the LPWHT arrangements were being prepared.
From discussion with operators and sampling their activities I gained confidence that the work was being undertaken correctly and the operators were aware of the relevant procedures and contingency measures, if needed.
Based on examination of the data recording arrangements, Framatome St Marcel had sufficient control and supervision during the operation and there were adequate arrangements for post processing the measured data to validate the theoretical predictions.
Framatome kept the licensee involved and engaged at all the important steps along the process using the formal “General Communications” process.
Conclusion
Overall, I was content based on the information presented by Framatome St Marcel, relevant documents sampled and speaking to various personnel (e.g. DI, ITPIA and Framatome St Marcel operators) associated with the activity, that the proposed LPWHT arrangements seemed to be robust and being managed adequately. Whilst I did not see any evidence of exclusive defect tolerance calculations, considering the confidence gained with the LPWHT arrangements, albeit everything working well, I am of the opinion that the LPWHT improves compliance with the high integrity component requirements for the unit 2 SG PIF welds.
I consider that, in accordance with ONR guidance on the application of inspection ratings, a GREEN rating (no formal action) against LC 19 is appropriate for this inspection.