Office for Nuclear Regulation

Dungeness B - Planned inspections

Executive summary

Purpose of Intervention

The purpose of this intervention was to undertake a Licence Condition (LC) 28 compliance inspection at EDF Energy Nuclear Generation Ltd’s (NGL’s) Dungeness B (DNB) power station, in line with the planned inspection programme. This intervention is one of a number of LC28 compliance inspections performed by ONR during the periodic shutdown of DNB Reactor 22 (R22) to inform the ONR decision whether to issue a license instrument granting consent for the return to service of DNB R22 following its 2018 statutory outage.  This intervention specifically focused on arrangements for graphite core examination, trepanning and gas chemistry.

Interventions Carried Out by ONR

I carried out an intervention at Dungeness B DNB on 12th to 13th September to examine EDF Energy’s graphite core inspection arrangements with respect to LC 28 compliance. This intervention was carried out by two ONR graphite specialists and one ONR chemistry specialist.

During our intervention, we focussed on the following items:

During the intervention, we also carried out a walkdown of the pile cap and of the gas chemistry control panels relevant to the primary gas coolant.

Explanation of Judgement if Safety System Not Judged to be Adequate


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

At the time of my visit, the licensee had completed the first two inspection channels, one fuel channel and one control rod channel. There was nothing to suggest that the licensee could not meet the target requirements within the planned schedule. An assessment report will be produced subsequently to consider whether the work performed was sufficient and whether the findings are consistent with the current safety case. I have listed the key graphite findings from this inspection below:

I consider that reactor gas chemistry data presented by NGL for Reactor 22 was in line with my expectations. However, the Reactor 21 chemistry data presented to us were below expectations; specifically methane compliance on R21 has been poor over the previous 10 months with sustained operation with methane concentration in outside of normal operating parameters. I am content, however, that NGL have followed their internal processes.  I am unconvinced that proposed repairs to the affected plant will be successful, and have requested that NGL provide an update of the outcome of modifications towards maintaining coolant chemistry within the targets. Based on the above, I judge that an IIS rating of Amber – Seek Improvement is appropriate.  ONR Issues 5971 and 1737 will be updated to reflect the findings of this intervention.

Conclusion of Intervention

Concerning the graphite core inspections, based on the sampling undertaken as part of this intervention, I did not identify any significant shortfall in NGL’s arrangements. I am therefore satisfied that the LC 28 arrangements in place are adequate and have been adequately implemented. I judge that the licensee is in a good position to complete its full scheduled programme of graphite inspections. The discussion with NGL staff was open and informative.

Concerning the recent gas chemistry performance, I judged that chemistry compliance of Reactor 22 had been adequate in the six months I sampled as part of my intervention. However, gas chemistry compliance for Reactor 21 has been below expectations.

At the end of our intervention, we expressed our views regarding the graphite core inspections and gas chemistry in a meeting with Dungeness B Technical Station Safety Manager (TSSM) and with the Station Director. We expressed concerns about the lower than expected recent gas chemistry performance for Reactor 21. NGL was aware of the issues and planned repairs to fix the issues with the gas bypass plant during the double reactor outage. ONR will follow this issue as part of regulatory engagement with NGL.

Given the recent gas chemistry performance with Reactor 21, I have therefore given this inspection an ONR rating of Amber – Seek Improvement.

A forthcoming assessment report will consider whether the results are consistent with the graphite safety case, once the inspection schedule has been completed and make a recommendation with regard to the return to service of DNB R22.