During the reporting period from 1 July 2021 - 30 September 2021 there was one incident at a nuclear licensed site within Great Britain that met the Ministerial Reporting Criteria (MRC) as defined within the Nuclear Installations (Dangerous Occurrences) Regulations 1965 and ONR guidance in relation to notifying and reporting incidents and events.
On the 22 July 2021 at 14:57, following failure of a National Grid transformer located offsite, Heysham 1 experienced a complete loss of 400kV power supplies. Both reactors were operating at nominal full power prior to the incident, and both tripped automatically.
Post trip cooling was successfully established by the automatic start of one of the four station’s Emergency Boiler Feed Pumps (EBFP). There are four EBFPs, any one of which can maintain effective post trip cooling. One EBFP was out of service for planned maintenance and the two remaining EBFPs failed to initiate on demand because of an automatic control system issue. The two additional EBPFs were started manually after 45 minutes.
Post trip cooling was effective, and the reactors were safely shut down.
No one was injured in the incident and there were no radiological consequences.
The event has been rated Level 2 (Incident) on the International Nuclear Event Scale (INES).
Note – If no EBFPs are available, post trip cooling would be adequately delivered through initiation of the High Pressure Back Up Cooling (HPBUC) system. HPBUC is an independent system of three diesel engine powered pumps any of which are adequate at delivering post trip cooling and which remained available.
EDF initiated their emergency arrangements and declared a site incident. Post trip cooling was established, and teams were despatched to start additional emergency boiler feed pumps and to ensure adequate stocks of boiler feed demineralised water were maintained.
The site incident was stood down after approximately 16 hours. EDF’s internal investigation was completed, the learning from which resulted in:
ONR completed a formal investigation into the circumstances surrounding the incident which concluded that although a complete loss of grid fault sequence is reasonably foreseeable, the incident revealed a shortfall in the operation of the post trip logic equipment which was not reasonably foreseeable.
ONR’s investigation did not reveal any significant shortfalls in compliance and confirmed that the dutyholder has taken appropriate measures to learn from the incident.
ONR carried out a return to service readiness inspection which, following the adequate implementation of modifications, did not reveal any regulatory issues which would prevent EDF from restarting both reactors at Heysham 1.
Fleet wide, Hartlepool is the only other station that is of similar design to Heysham 1 in terms of post-trip cooling. However, the post trip logic equipment at Hartlepool was confirmed to be resilient to this type of fault sequence. Therefore, ONR does not consider this a fleet issue.