- Site: Heysham 1
- IR number: 21-071
- Date: August 2021
- LC numbers: N/A
Executive summary
Purpose of Intervention
This was an unplanned inspection of EDF Energy Nuclear Generation Ltd.’s (NGL’s) Heysham 1 Power Station following the total loss of 400kV supplies on the 22 July 2021 caused by the failure of an offsite National Grid current transformer. The loss of supplies resulted in both reactors automatically and safely shutting down and post trip cooling being established.
The purpose of the inspection was to gain assurance that NGL had adequately investigated the challenges introduced by the incident and where appropriate implemented reasonably practicable improvements to systems and arrangements in preparation to restart both reactors.
Interventions Carried Out by ONR
I, accompanied by Electrical, Control and Instrumentation and Fault Study Inspectors, carried out interventions on the following systems and arrangements:
- Demineralised water stock management used for post trip cooling purposes
- Emergency boiler feed post trip logic systems
Explanation of Judgement if Safety System Not Judged to be Adequate
No system based inspection was undertaken, therefore this is not applicable for this intervention.
Key Findings, Inspector's Opinions and Reasons for Judgements Made
Demineralised water is pumped through boilers during routine reactor at power and trip operations to remove heat from the coolant gas which has circulated past the nuclear fuel. Following a reactor trip, although the nuclear fuel is no longer critical, it remains hot and continued cooling of the fuel remains an essential safety function. Without off site electrical supplies, onsite back-up systems provide the necessary power supplies for, amongst many other things, emergency boiler feed pumps and coolant gas circulators.
I can confirm that Heysham 1 adequately managed the stocks of demineralised water following the total loss of 400kV supplies on the 22 July 2021. However NGL recognise that improvements to water stock management are necessary and have introduced and trained shift staff on the use of improved decision making instructions to reduce losses within the system. In addition, a permanent demineralised water trailer is now on site which will add resilience to the capability of the existing make up water treatment plant. I was content with measures taken by NGL to improve the management of demineralised water on site.
NGL also carried out modification to the post trip logic associated with the starting of emergency boiler feed pump systems to improve the resilience of the automatic nature of this feature. ONR inspected this modification, it’s justification, substantiation and testing, and was satisfied with NGL’s due process to adequately implement the modification. I was content with measures taken by NGL to improve the post trip logic associated with the emergency boiler feed pumps.
Conclusion of Intervention
The intervention did not reveal any regulatory issues which would prevent NGL from restarting both reactors at Heysham 1.
However three regulatory issues were identified which will require NGL at the corporate centre and locally at Heysham 1 to examine the wider learning from the 22 July 2021 loss of grid event. These regulatory issues will be subject to routine regulatory follow up.