The expected restart dates of Heysham 1-1, Heysham 1-2 and the Hartlepool 1 and 2 reactors have been pushed back by about a week to March, the firm's outage website page showed. The Heysham 1 reactors have been offline since December and the Hartlepool units have been out of action since January.
The Heysham 1 power plant, which has two reactors with combined capacity of around 1.2 gigawatts (GW), was taken offline late last year for inspections on steam valves at the boilers.
As a protective measure, the Hartlepool plant - also with two reactors with combined capacity of about 1.2 GW - was taken offline in January because it has the same reactor design.
"Since the valve issue at Heysham 1 in late December 2023 we have been carrying out modifications, replacements and safety case work for all four units with the same valves," an EDF Energy spokesperson said.
"This work is nearing completion but has needed some extra time to conclude. We have changed the restart dates to accommodate this and expect the units to start coming back into service over the coming weeks."
EDF said in January that it aims to keep four nuclear plants in Britain - Torness, Heysham 1, Heysham 2 and Hartlepool - running longer than originally planned, subject to regulatory approval, and will make a decision by the end of this year.
Last year it extended the life of its Hartlepool and Heysham 1 nuclear plants by two years to 2026.
