Office for Nuclear Regulation

LC28 Site intervention during Reactor 8 2020 Periodic Shutdown: Peripheral Shielding Brick Inspections

Executive summary

The intervention took place at Heysham B (HYB) on 27th February 2020 during its periodic reactor 8 shutdown as part of the licensee’s, EDF Energy Nuclear Generation Limited’s (NGL), compliance with Licence Condition (LC) 30.  The purpose of the intervention was to inspect the adequacy of the licensee’s examinations and inspections of the peripheral shielding bricks and determine compliance with LC 28(1). The findings from the graphite core inspections were also discussed as these had just been completed at the time of the intervention.

At HYB and Torness (TOR), the reactor core is surrounding by peripheral shielding bricks to help guide the gas flow around the reactor. Peripheral shielding is composed of 16 faces surrounding the core, each face being made of either 10 or 11 aligned graphite blocks.

Cracking in a number of peripheral shielding bricks has first been observed at TOR R2 in 2015, during routine inspection of the reactor internals. Similar types of cracks have also been observed in HYB R8 (2016), TOR R1 (2017) and HYB R7 (2018). It is therefore the second inspection of the peripheral bricks for HYB R8 since peripheral brick cracking was discovered. To justify the tolerability of the extent of cracking observed in these bricks, NGL has produced and maintains a safety case which is reviewed and updated after each inspection campaign of the peripheral brick walls at HYB and TOR.

This intervention was carried out to determine whether NGL’s inspection arrangements of the peripheral shielding bricks were appropriate and would allow for a suitable sentencing of the cracks observed.

Purpose of Intervention

The purpose of this intervention was to establish the suitability of the peripheral shielding brick inspections in relation with LC 28:  Examination, inspection, maintenance and testing.

Interventions Carried Out by ONR

In relation to the graphite core and the peripheral brick inspections, I carried out the following inspections in order to determine compliance with LC 28:  Examination, inspection, maintenance and testing:

Explanation of Judgement if Safety System Not Judged to be Adequate

Not applicable.

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

At the time of my intervention, 3 out of the 12 targeted peripheral brick face inspections had been completed. No significant defect had been found in the faces inspected. There was no apparent issue which might prevent the scope of these inspections from being completed.

In my opinion, the station staff I spoke to during the intervention was knowledgeable and suitably experienced. The peripheral shielding brick inspections I observed appeared to have been carried out with appropriate rigour. The training records I sampled were up-to-date and relevant to the tasks carried out by the operators. The Quality Assurance records I sampled during the intervention were appropriately completed and up-to-date. The video footage of the peripheral shield wall bricks was of reasonable quality. In my view, the licensee’s arrangements for the peripheral shielding brick inspections were appropriate. I commented that the viewing facilities for the second line assessors could be improved. However, I did not observe any shortfall in NGL’s inspection arrangements.

The graphite core inspection team had completed all 16 fuel channel inspections which were planned for inspection. A doubly cracked brick was found in the layer 6 brick in channel D77. All the other fuel channels inspected were free of defects. This is the first time that a full-height axial crack is observed at HYB/TOR although similar cracks have been found in other AGR graphite cores. I observed a GAP meeting, composed of NGL’s graphite specialists. This meeting was convened to review and sentence the findings from the latest graphite core inspections. The group was quorate in accordance with their terms of reference, complied with their crack sentencing procedures and was composed of suitably qualified and experienced personnel. Concerning the cracks observed in layer 6 of channel D77, the GAP reviewed the bore measurements from the channel and the visual information produced during the inspection. The conclusions of the GAP and the safety justification following this observation will be summarised and submitted to ONR as part of the return to service documentation.

Conclusion of Intervention

Overall, from the activities I sampled during my intervention I found that the licensee, EDF Energy Nuclear Generation Limited, were complying with LC 28 with respect to the peripheral shielding brick inspections. I considered that an inspection rating of ‘Green’ is warranted.

A forthcoming assessment report will consider whether the results from the examinations and inspections are consistent with the safety case in order to support ONR’s return to service consent activities.