EDF Energy, Britain's largest nuclear power producer, restarted its 640-megawatt (MW) Torness 2 nuclear reactor on Friday, a spokesman said.
We can confirm that R2 at Torness power station was re-synchronised to the grid on 18 November, he said.
The reactor shut down unexpectedly on Monday after a fault on a fuelling machine, which has now been resolved, he added.
Eight other nuclear reactors remain out of service in Britain on Friday.
(Reporting by Karolin Schaps)