The company identified an unexpected result during routine ultrasonic inspection of a boiler spine during the Heysham 1 Reactor 1 planned statutory outage last year.
A conservative decision has been taken by the company to close Heysham 1 Reactor 2 and Hartlepool Reactors 1 and 2 which feature similar design to conduct further inspections over the next few days and to equip equipment to enable monitoring of the condition of the boiler spines.
The inspections will satisfy the UK nuclear regulator before the planned return to service of the four reactors.
Each reactor at Heysham 1 and Hartlepool has eight boiler units and are arranged around their associated reactor in four quadrants with each quadrant containing two boiler units.
The Heysham 1 Reactor 1, located on the north west coast of England near Lancaster, will remain shut down and work will continue to characterise the nature of the defect.
The investigations are expected to take around eight weeks.
