The Sizewell B nuclear power plant is generating electricity again after a three-month switch off for maintenance work.
The Suffolk power station was turned off in February for the £80m project, which should keep the plant ticking over for the next 18 months.
Sizewell B originally said the shut down would last 66 days but the station has been closed for a month longer than initially estimated while the work was carried out.
One of the plant's two turbine rotors was replaced, together with one of the four coolant pumps in the reactor while valves and motors were serviced.
Sizewell B said more work costing £300m would be carried out over the next three years to improve the station's reliability and performance, possibly extending Sizewell B's life to 2055.
Turning off the reaction allows engineers to check the pipes in the plant for any signs of stress or corrosion.
Sizewell B said the circuit had been given a clean bill of health.
Robert Gunn, Sizewell B station director, said the team were glad to get the power station up and running again.
“This was a particularly important outage for us as we deliver the 10-year plan for the station. We know the importance of energy security and the role nuclear power plays in limiting carbon emissions.
"Since we switched the station off in February we have systematically worked through a checklist of thousands of tasks to ensure the station has a clean bill of health and is set up for a successful future.”
Preparation work for the outage began at least two years in advance of refuelling the reactor as part of a 10-year strategic plan at the power station.
Sizewell B was switched off on 17 February and brought back online on 22 May.
Protesters have brought a High Court challenge against the building of Sizewell C, claiming insufficient attention has been paid to the environmental impact of the project.
But EDF, which is behind Sizewell B and Sizewell C, planned to be built next door, say the projects will train a new generation with skills to help the UK move away from fossil fuels.
The government granted permission for Sizewell C last year but campaigners are worried about the impact on Suffolk.
Sizewell B said it will increase its workforce by a tenth to support the long term operation of the existing power station.
It was connected to the grid on 14 February 1995 and has so far saved 85m tonnes of Co2 emissions, says EDF.
The site employs 800 people and is worth £40m to the local economy each year.
By 2028 Sizewell B is due to be the only generating nuclear power station from the existing fleet.
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