Office for Nuclear Regulation

Hunterston B - Working Practice Inspection - Graphite Safety Case

Executive summary

Purpose of Intervention

The Inspection at Hunterston B Power Station (HNB) was undertaken in line with the operating reactors sub-division inspection plan.  The Inspection was a planned Working Practice Inspection of the Nuclear Safety Group (NSG).  Working Practice Inspections focus on a particular site group’s fulfilment of safety case requirements - in this case NSG - rather than the site’s fulfilment of the safety case requirements of a specific system as in a System Based Inspection (SBI).

The opportunity was taken to strategically align this inspection with the most significant issue currently facing HNB. Therefore, the focus the inspection on the fulfilment of the requirements of the graphite safety case.

The site inspector also held a number of routine interactions with the station, which examined preparations being made for transition to defuelling and decommissioning.

Interventions Carried Out by ONR

The inspection focussed on the following License Conditions:

The focus of my inspection was the role of the NSG with regards to the requirements specified in the graphite safety case. In particular the procedures and operation of the monitoring assessment panel (MAP) and the arrangements for coolant activity monitoring.

At the time of the Inspection graphite core inspections were taking place at site, and so the opportunity was taken to observe the Graphite Assessment Panel (GAP), which is the body which assesses and sentences the results of the graphite inspections.

The Inspection was structured into the following topics:

Explanation of Judgement if Safety System Not Judged to be Adequate

No System Based Inspection was undertaken.  The inspection found the implementation of the graphite safety case was adequate.

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

In each topic area my inspection looked for adequate arrangements under LC23, LC24, LC28, and that the ‘golden thread’ flowed from the safety case, through the operating rules, procedures, and into the surveillance and maintenance schedules.  I then examined a random sample of records of completed surveillance and maintenance tasks to assess compliance with the procedures in place at Hunterston B.  I also checked on the training records of the people completing the tasks to check for adequate LC10 arrangements.

I found that each topic area had adequate arrangements under each of the relevant License Conditions inspected, and that the requirements of the graphite safety case were adequately implemented by the station’s Nuclear Safety Group.

This inspection identified that there was no criteria for rating an amber rating of any anomaly in the Fuel Grab Load Trace analysis under the Monitoring Assessment Panel processes.  The consequence of this is that an anomaly which could be indicative of core degradation would not be tracked, and thus the capability of the Monitoring Assessment Panel to monitor the core condition and potentially highlight a degrading condition is reduced.  I did not view this as a significant shortfall in the implementation of the requirements of the safety case, and therefore I don’t consider this to be significant enough to affect the inspection rating.  However, I raised a Level 4 regulatory issue on this topic.

I rate the inspection as Green (No Formal Action) for LC10, LC23, LC24, & LC28.  The results of this inspection do not require any changes to the sub-division inspection plan.

The nominated site inspector held several discussions with site personnel on the topics of preparations for defueling, preparations for the return to power, the ongoing graphite core inspections, and two incident reports.

Conclusion of Intervention

This inspection found that the station Nuclear Safety Group’s discharging of it responsibilities under the graphite safety case was adequate.