Office for Nuclear Regulation

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Licence Condition 23 safety case inspection at Hunterston B

Executive summary

Purpose of intervention

The intervention was held at EDF NGL's Hunterston B AGR power station on 31st Aug and 1st Sep 2016. It addressed issues covered in the Consolidated Hazards Safety Case (CHSC) relating to Extreme Ambient Temperature (EAT), seismic and wind hazard protection measures as part of an overall fleet wide strategy for external hazards.

Interventions Carried Out by ONR

This was a Licence Condition 23 safety case inspection and verification of as is plant. On a sampling basis plant items relating to EAT, seismic and wind hazards were selected.

Explanation of Judgement if Safety System Not Judged to be Adequate

The systems inspected were found to be adequate.

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

A PSR2 Identified Corrective Action (PICA 1280) was discussed concerning temperatures in the gas circulator hall and heating and ventilation efficiency. Many of the electrical and electro-mechanical systems are rated at 40°C and high EATs might raise temperatures inside the circulator hall above 40°C. The site is addressing this, but we sought clarification of the current status of this action. The HNB Nuclear Safety Group Head reported that for PICA1280 an EAN was produced (DAO/EAN/JICC/145/AGR/14) which made a number of recommendations to be completed under normal business arrangements. The only remaining assignment for HNB is one to update a number of our System Based Views within the suite of Living Safety case Documents. It was noted that this is due to be progressed next year.

We observed that some of the backup cooling system (BUCS) pipework has trace heating for low EATs, although we could not confirm that all of it has and we are seeking clarification of this as part of normal business.

When we viewed it, the pipework on BUCS and RFTs was being inspected/lagged/ refurbished. We sought clarification of what was being done. The HNB Nuclear Safety Group Head reported that the RFT pipework was being examined as part of the corrosion management programme. This is currently identifying systems at risk of corrosion under insulation (CUI) which could lead to degradation of the system integrity. Where appropriate lagging is being replaced with a new type designed to minimise this risk.

The Consolidated Seismic Safety Case discusses several isolation valves on the feed to the bypass plant and on the return leg. Operation of these valves has been seismically qualified for local operation for the bottom line event. In some instances manual closure of these valves would be required. We confirmed that these valves in the circulator hall can be easily accessed for manual closure in an emergency when operators may be required to wear breathing apparatus to allow for CO2 leaks.

Although the wind hazard safety case was found to be adequate in terms of nuclear safety, it was noted that there could be significant damage to non-essential plant following design basis extreme wind conditions. There is the potential for partial collapse of the Deaerator Tower, and permanent reactor shutdown, although the safety factors leading to partial collapse appear to be borderline in the CHSC. If partial collapse of the Deaerator Tower should occur due to failure of the bracing system, this could result in loss of the normal boiler feed system but post-trip cooling would be provided from the BUCS. To confirm the CHSC judgement stated in this paragraph we visited the areas listed there and were satisfied that this claim is reasonable.

Conclusion of Intervention

The Intervention Record includes some specific queries/observations/suggestions, but none of these are considered to be significant nuclear safety issues.

On a sampling basis the points for verification relating to EAT, seismic and wind hazards following assessment of the CHSC were found to be satisfactory.

No regulatory issues were raised and no further regulatory action is required.