- Site: Torness
- IR number: 21-133
- Date: October/November 2021
- LC numbers: 24
Executive summary
Purpose of Intervention
During the assessment of EDF Energy Nuclear Generation Limited’s, further known as the licensee, safety case justifying the risks from seal ring groove wall debris (NP/SC 7808) evidence related to the Fuel Grab Load Trace (FGLT) was presented. Specifically the licensee stated “FGLT assessment provides an opportunity to detect SRGW debris between inspection outages”. The FGLT is a record of the variation in the load on the Fuelling Machine hoist when a fuel assembly is charged or discharged from the reactor. The licensee claims that variations in FGLT can highlight anomalies, such as abnormal graphite shrinkage, a keyway root crack (KWRC) or even seal ring groove wall (SRGW) debris, within a specific channel.
Currently there is limited means of preventing fuel movement issues caused by SRGW debris which would trigger the hoist protection systems. Hence, the potential for the FGLT to reveal the presence of SRGW debris could provide a means of protection against operation in a potentially undesirable(?)state.
This intervention was intended to gain further knowledge of the formal safety case claims on the FGLT data, recording of the FGLT data and sentencing of the FGLT data. A discussion was also had around the potential use of the FGLT data for the identification of SRGW debris.
Purpose of Intervention
The purpose of this intervention was to provide evidence to inform the ONR judgement on NGL’s ability to monitor the progression of graphite core degradation (e.g. keyway root cracking and seal ring groove wall debris) through the FGLT.
Interventions Carried Out by ONR
This intervention took place in three parts:
- A discussion with safety case and analysis personnel at the licensee’s offices in Barnwood.
- A site visit to Torness (TOR) power station to discuss the arrangements to collect the FGLT data and view the FGLT data collection process.
- The observation of the Monitoring Assessment Panel (MAP) where FGLT data is sentenced and, if appropriate, further action is recommended.
Key Findings, Inspector's Opinions and Reasons for Judgements Made
The licensee stated that the primary formal claim on FGLT data is through the need to confirm safe sit down of fuel during refuelling operations. To prevent unwanted underload trips, which could unnecessarily stress the fuel and lifting mechanism, the underload protection is vetoed when the fuel is ~40-60 cm from the bottom of the channel. The licensee claimed that from this height there is negligeable risk to the fuel from a potential drop event. However there is a need to confirm the absence of potential false bottoming or fuel sleeve gapping. The licensee stated that an acetate overlay is used to detect anomalies in the sit-down portion of the FGLT.
From my activities at TOR I was content that the arrangements in place to collect the FGLT data are adequate. Whilst I have only sampled those arrangements at TOR, the licensee stated that the arrangements are similar for Heysham 2 (HYB) and the personnel are subject to similar training. At this time I am content that the activities performed at TOR are sufficient to provide confidence for both HYB and TOR.
The FGLT captures all of the variations in load as the fuel is raised and lowered during refuelling and graphite inspections. This is used by the licensee to identify significant changes in the channel / graphite brick shape. Specifically, the licensee claimed that FGLT analysis can provide helpful evidence for the state of the core and can provide an indication of defects or anomalies in the channel. The licensee also explained the work that was ongoing to generate advice for analysts to identify potential SRGW debris. From the information I sampled and discussions with the personnel present, I am content the licensee has adequate arrangements for the review and analysis of FGLT. I am also satisfied that the personnel present were adequately trained, and the arrangements were being complied with. I have confidence that anomalies will likely be identified and communicated to the MAP where they will be sentenced, and where appropriate, further action recommended.
During my observation of the MAP meeting I was content that the review of the FGLT data, which included the FGLT data from the recent November 2021 HYB R8 refuelling campaign, was adequate and in line with the arrangements. From this observation I am content that the MAP functioned as described and I have confidence that it will sentence the FGLT appropriately and feed up recommendations for future inspections
Conclusion of Intervention
Through my activities I am content that the licensee has adequate arrangements to satisfy the current safety claims on FGLT data (e.g. confirmation of fuel sit down) and that these arrangements are complied with at station. I am also content that the licensee has adequate arrangements for the review of FGLT data to identify issues with the graphite core condition. This includes new work to provide advice on the identification of SRGW debris. I noted that the licensee has appropriate arrangements for sentencing observations and a route for escalation to ensure that appropriate actions are taken.
Based on the findings of this intervention, I have allocated an overall ONR rating for this inspection of ‘GREEN’ (no action required) against LC 24, Operating Instructions.