Reactors at a nuclear power station in Somerset will not be back in action until the New Year.
The plant normally provides 3% of the UK's electricity
The two gas-cooled reactors at Hinkley Point B nuclear power station were shut down. Cracks have been found in boiler pipes and in a reactor's graphite core.
British Energy says the reactors will only generate power at 70% of their normal capacity from January onwards.
This is to reduce wear on the plant, which normally produces around 3% of the UK's electricity.
In a separate development, Roy Anderson stepped down as chief nuclear officer and member of the board of British Energy on Friday.
In a statement, the company said chief executive Bill Coley will take direct control of nuclear operations .
"We are implementing certain further organisational changes to improve station performance," it read.