Automatic trip of both reactors following loss of off-site electric supplies
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.
The back-up power supply required by the Limits and Conditions of Operation (LCO) was provided by automatic start of two gas turbines. There are four gas turbines in total. One gas turbine is sufficient to provide adequate electrical supplies to both tripped reactors.
Adequate post trip cooling was established by the automatic start of one Emergency Boiler Feed Pump (EBFP). The LCO require a minimum two EBFPs to be available when the reactors are operating at power.
There are four EBFP on site, any one of which can maintain effective post trip cooling for both reactors. One was out of service for planned maintenance and the two other EBFPs shutdown during their start sequence because of an automatic control system issue. These were started manually after 45 minutes.
Post trip cooling was effective, and the reactors were safely shut down.
There were no radiological consequences as a result of this incident and no harm to workers or the public.
Note: If no EBFPs were available, post trip cooling would be adequately delivered through initiation of any one of three available diesel-powered High Pressure Back Up Cooling (HPBUC) system pumps.
Event date: Thu, 22-07-2021