Office for Nuclear Regulation

This website uses non-intrusive cookies to improve your user experience. You can visit our cookie privacy page for more information.

Corrosion under Insulation / Concealed Systems Intervention

Executive summary

Purpose of Intervention

This report presents a summary of the discussions with NGL staff and observations made during a corrosion focussed inspection at Dungeness B power station (DNB). Two ONR Structural Integrity Inspectors attended site on 25 and 26th July 2018. The DNB Corrosion Co-ordinator facilitated the discussions that took place throughout the inspection. NGL also sent the Fleet Corrosion Co-ordinator and Materials Performance Specialist from Barnwood to assist with the inspection.

I concentrated on reviewing the progress that the licensee had made on site in response to the required actions raised in previous ONR enforcement communications. I also focussed my attention on the actions that had previously been identified in existing ONR Regulatory Issues.

Interventions Carried Out by ONR

The primary aim of the inspection was to perform a follow up inspection at DNB to assess the adequacy of arrangements related to their corrosion management programme. This is in response to actions raised from previous ONR inspections which had identified shortfalls with respect to the management and prioritisation of corrosion related plant inspections and subsequent defect remediation activities.

Additionally, I took the opportunity to conduct follow up discussions relating to maintenance schedule activities for the CO2 plant and discovery of corrosion on the Buffer Store Back up Cooling Water system, along with its subsequent remediation.

The inspection reviewed DNB’s procedures and sampled compliance arrangements for selected systems against licence condition (LC)28 (examination, maintenance, inspection and testing) to judge the adequacy of the arrangements in place.

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

Defect Tracking Spreadsheet

The defect tracker spreadsheet used at DNB now contains columns for the System Engineer and corrosion team to add their judgements. However, from my sample of the tracker, a number of entries did not contain judgements. Several of the defects that were sampled during our inspection contained comments in the “judgements” columns of the spreadsheet that were no more than observations and did not represent a summary of a justifiable decision based upon evidence. A large number of high priority defects had no apparent planned dates for remediation.

From the information presented in the defect tracker spreadsheet and during discussion throughout the inspection, I do not consider that DNB demonstrated robust and embedded arrangements for prioritising defect remediation based upon risk and evidence from any previous inspections.

For Category 1 and Category 2 defects on it Corrosion Vulnerable 1 and Corrosion Vulnerable 2 systems (now classed by DNB as High-High to Medium-Medium likelihood vs consequence defects), DNB were unable to provide dates for their remediation. A plan was being prepared for 2019 activities; however it is unclear how DNB will account for the known risk from plant failure within the plan, along with any uncertainties associated with incomplete or unrepresentative past inspection results.

I do not consider that DNB has provided evidence to close actions identified in previous enforcement communications, or addressed actions identified in existing Regulatory Issues. In my opinion, DNB have not been able to demonstrate a robust process that shows how it is prioritising the large volume of potentially significant corrosion related defects that remain; I have particular concerns about the potential discoveries to be made when high risk trenches are inspected.

Prioritisation of Inspection and Remediation

During this inspection I witnessed and discussed examples of plant degradation relating to the Buffer Store Back up Cooling Water (BS BUCW) pipework and supports, Back up Feedwater Tanks (BUFW) and Essential Cooling Water Reactor (ECWR) pipework. In these examples, I do not consider that DNB has demonstrated it used this evidence of degradation to revise or re-prioritise inspection and remediation of similar components in service at the station.

Sections of the ECWR pipework in trench 124B are planned for renewal during the 2018 R22 outage, however, the pipework has been in a degraded state for an extended period of time and DNB could not justify why it had not been remediated sooner. In my opinion, this provides evidence to challenge the judgement process behind DNB’s prioritisation of trench inspection and remediation work.

I consider that DNB responded appropriately once it had discovered significant corrosion associated with the BS BUCW trenched pipework and supports in January 2018. However, there had been evidence of past water ingress and corrosion within the trenched pipework; external areas of the system were found to be degraded in February 2017. DNB made the decision not to bring forward any trench inspections, or carry out any sample inspections, to ensure that the pipework it contained remain safe to operate. I consider this is an example of an ineffective decision for the prioritisation of the 42 high priority trenches that remain to be inspected.


Out of the 138 trenches at DNB, 42 have been identified as high priority for inspection, based upon the significance of systems contained, materials of construction, presence of insulation, potential for trench flooding and known issues from past inspections.

I was informed by DNB that it is anticipated to take until the end of 2020 for the 42 high priority trenches to complete its inspection. DNB was unable to provide evidence that adequately demonstrates how it has prioritised the inspection of these trenches or how it has justified the systems contained within the trenches remain safe to operate until inspection and potential remediation takes place. I do not consider that DNB has provided suitable evidence to demonstrate a robust process for prioritising the inspection of its 42 high priority trenches. Evidence from the ECWR and BS BUCW trenched pipework degradation gives concerns regarding the potential discoveries to be made when these trenches are inspected.

CO2 System

I am content that DNB was able to demonstrate that maintenance routines exist for pipework inspection, in addition to those required for PSSR compliance. However I consider that improvements could be made in relation to the unexplained differences in routines for seismically qualified and non-qualified pipework. It was also challenging for DNB to identify which areas of pipework had been inspected, since the detail and quality seen within reports varied.

During the inspection, I was informed that DNB has identified a shortfall in its maintenance routines for the CO2 system. No additional routines exist that require inspection of the tanks to aid compliance with LC28; they currently appear to address PSSR requirements alone. Taking into consideration the past focus on the CO2 plant across the fleet following the Heysham 1 failure in March 2015, I consider the lack of robust and embedded maintenance routines for this system to be a significant shortfall. I do not consider that DNB has provided adequate evidence to close the actions identified in existing Regulatory Issues related to this system.

Corrosion Remediation Team

From my observations during this and previous inspections at DNB, the team involved in the remediation of corrosion appear to be engaged in their work and enthusiastic for its success. They demonstrated that they were able to complete remediation in the order specified in the corrosion management process. What is not clear is whether or not the team are carrying out remediation in an appropriate order based upon the risk that the degradation poses to nuclear safety.

Conclusion of Intervention

DNB falls short of where I would have expected it to be at this stage following previous ONR inspections and in comparison with the progress made by the rest of the fleet. DNB was unable to demonstrate that it fully understands the challenges posed to the station from corrosion. I remain concerned that DNB is not yet able to demonstrate that the threat posed by the corrosion of concealed systems is being adequately managed.

I am also not satisfied that DNB has provided sufficient evidence to close actions raised in my previous inspections and monitored via existing Regulatory Issues.

I have identified recommendations for DNB to provide evidence to demonstrate how they may address these concerns.

Based on the evidence sampled, I do not consider that adequate progress has been made to address the shortfalls identified; I judge that DNB has failed to adequately deliver improvements previously identified in enforcement communication. Therefore, I consider that an ONR inspection rating of RED is appropriate.  In light of this rating I will apply the Enforcement Management Model (EMM) to determine the most appropriate enforcement action.  This decision will be captured in a separate report and will take forward the recommendations made within this intervention report.