Date(s) of inspection:
- June 2023
Aim of inspection
The local post-weld heat treatment (PWHT) applied to the Hinkley Point C (HPC) steam generator tubesheet to primary head weld (the PIF weld) has generated high residual stress such that an adequate defect size margin (DSM) cannot be demonstrated using the initial qualification examination defect size of 10mm x 60mm. Improvement of the DSM requires a demonstrable capability for the non-destructive testing (NDT) to detect defects half the size of the original (5mm x 30mm). Since the qualification of the NDT for the initial qualified examination defect size (QEDS) has been completed, and the end of manufacture inspection of the PIF welds has been performed, much of the work to support demonstration is being performed retrospectively.
The aim of this Licence Condition (LC) 19 compliance inspection was to establish that the activities to support the revised NDT requirements were robust and to judge whether the revised inspection system would be capable of reliably detecting defects with a size of 5mm x 30mm.
The evidence gathered during the inspection will inform ONR’s decision to permission the release of the first steam generator (under enhanced implementation, monitoring and control).
Subject(s) of inspection
- LC19 – Construction or installation of new plant – Rating: Green
Key findings, inspector’s opinions and reasons for judgement made
The inspection was conducted at Framatome’s premises at St Marcel, France. Framatome (the manufacturer of the steam generators) presented evidence, in the form of a technical justification to support the detection and rejection of defects having a size of 5mm x 30mm and larger. This evidence included experimental data and modelling predictions and was supported by practical trials to demonstrate the efficacy of the new scans developed to improve the inspection performance.
I had been provided with the technical justification prior to the inspection, and from my preliminary review, I concluded that adequate margins existed for detecting the smaller defects. I also noted that the new scans for applying the time of flight diffraction technique provided significant improvements for characterising defects. While the evidence was, in general, comprehensive, I noted that it was based upon smooth defects with qualitative extrapolations to rough defect cases. I will consider this further in the ONR structural integrity assessment report used to inform the permissioning decision for release of the first steam generators from Framatome.
In addition to Framatome presenting its evidence, I held discussions with the Inspection Validation Centre (IVC) which was the assigned independent qualification body. The IVC had assessed the revised inspection procedures and the technical justification and concluded that the evidence supported high reliability for detecting defects with a size of 5mm x 30mm. It had just completed applying the new scans to a test piece containing representative defects and was analysing the data to assess whether the practical trials also supported the inspection performance. From discussions with IVC staff, and my review of the facilities, I concluded that adequate arrangements were in place for conducting the qualification.
While the full demonstration of the capability to detect 5mm x 30mm defects was yet to be completed, I was satisfied that the process being adopted was thorough and in line with that for qualifying the end of manufacture inspections for other high integrity component welds. From my limited review of the evidence provided to support the inspection capability, I expected that the qualification, when completed, would confirm that the inspection system was capable of detecting 5mm x 30mm defects.
I judged that the facilities and conditions provided by Framatome to conduct the qualification were sufficient and these, along with the IVC’s own arrangements, protected the confidential nature of the test pieces. I had assurances from the IVC that it had not experienced pressure to make technical compromises nor had it been subject to schedule pressures.
On this basis I gave the inspection a green rating.