Office for Nuclear Regulation

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Hunterston B Power Station LC28 Inspection

Executive summary

Purpose of Intervention

EDF Energy Nuclear Generation Limited (NGL) has shutdown Hunterston B (HNB) Reactor 4 (R4) to perform a schedule of examinations, maintenance, inspections, and testing as required by Licence Conditions (LC) 28 and 30 of its Nuclear Site Licence. As part of these activities NGL are committed to performing inspections of the graphite core.

Purpose of Intervention

This intervention report records the findings of my intervention on 28th September 2017 of NGL's graphite core inspection activities. The Purpose of the intervention was to sample the adequacy of the NGL's arrangements for the graphite core under LC28. The minimum inspection requirements for the graphite core are:

Interventions Carried Out by ONR

NGL's full programme of graphite core inspection consisted of dimensional and visual inspection of 31 fuel channels, 26 from the central region and 5 from the outer ring of the fuelled region. 6 fuel channels were selected for sample trepanning and a target of 36 samples from 6 channels. Eddy current inspection will be performed on the 6 trepanning channels, an additional reserve channel and two channels containing keyway root cracked (KRC) bricks (channels 12:80 and 28:84). Finally, control rod channel 33:71 has been selected for TV inspection. I conducted the following intervention scope as part of my inspection of HNB LC28 arrangements:

Key Findings, Inspector's Opinions and Reasons for Judgements Made

The observation of one main population KRC after 22 channels of inspection is thus far consistent with the first observation of KRC in HNB R3. No new bore cracks were found in previously inspected channels. In newly inspected channels 7 circumferential type bore cracks were found in newly inspected channels. No full height axial bore cracks were found in the newly inspected channels.

The inspection quality was adequate and consistent with many other inspections and certainly capable of detecting new KRC. However, it had revealed that closure of pre-existing bore cracks where potentially difficult to image.

Based on the sample of records I consider that the inspections are being conducted by Suitably Qualified and Experienced Personnel (SQEP) and that the process is being properly managed.

I support HNB's continued development and validation of the Fuel Grab Load Trace (FGLT) technique because of the potential safety benefits of this technique's ability to provide some measure of changes in the core change during operation.

I consider that the Monitoring Assessment Panel (MAP) meeting provided a suitable forum for discussion and sentencing of trending analysis of core monitoring parameters. I suggested that because the onset of KRC has now increased concern over core distortion that greater consideration should be given to all of the available data, including visual inspection and channel measurements, when concluding if trends in the analysis are associated with core distortion.

Conclusion of Intervention

Overall, I consider that HNB R4 graphite core inspection activities were compliant with the requirements of LC28.

I will consider NGL's findings from the graphite core inspections once they have been completed and record my conclusions in an assessment report. This will form part of ONR's consent to allow NGL to return HNB R4 back to power.