Four nuclear reactors operated by EDF Energy, shut down last month after a defect had been discovered in one of them, will be put back online, EDF said.
Although investigations are still underway, EDF Energy is positive it will be able to restart the reactors between end of October and December.
"Depending on the progress of the programme and any necessary modifications, the company expects there to be a phased return to service between the end of October and the end of December, 2014,” EDF said in a statement.
"Dates for returning the stations to service depend on the findings and completion of the inspections. EDF Energy will give further updates on the progress of the programme when it is able."
Two reactors at Heysham 1 and two in Hartlepool were switched off after a crack was discovered in one of the Heysham 1 reactors during a regular inspection. The crack in the boiler spine of the reactor, a metal tube supporting the weight of the reactor, prompted closure of the other three reactors with identical design.
EDF said the fault discovered is specific for this particular design and doesn’t affect its other nuclear reactors.
The Heysham 1 and Hartlepool reactors provide about 10 per cent of the UK's nuclear capacity, enough to supply three million homes. The closure thus put additional pressure on National Grid, as it is facing uncertainty over electricity supply over the upcoming winter.
Earlier this week, National Grid said it would pay mothballed generators to be online over the period.
EDF Energy’s engineers are now investigating the cause of the Heysham 1 defect and designing mitigation measures to fix the crack in the boiler spine. As part of the measures, they will have to develop a safety case for reopening the boilers, which will have to be presented to the Office for Nuclear Regulation before the reactors can be given the final go-ahead for the restart.
The French-owned EDF Energy also operates nuclear plants at Torness in Dunbar, Hunterston B in West Kilbride, Scotland, Sizewell B in Suffolk, Hinkley Point B in Somerset, and Dungeness in Kent.
The firm is leading a consortium which plans to build the new £16bn Hinkley Point C nuclear plant in Somerset, due to be completed in 2024.