Three-yearly maintenance checks resulted in the first reactor station being out of action for several weeks.
Problems with the steel supports were noticed and when the second reactor was checked, similar safety concerns were raised and that had to be shut down as well.
Work has also been carried out on one reactor to strengthen fuel plugs.
A spokeswoman for British Energy, which runs the power station, said “There are no safety implications – it is a question of dealing with minor issues before they become serious.
“One of the reactors was shut down as standard procedure, but repairs to the plugs took longer than originally thought.
“The other was then shut down to enable work to be carried out on the supports.
“It is unusual for both reactors to be shut down at the same time and no electricity to be generated at all.”
The company admitted it had taken a “commercial hit” because of the works.
“We have no electricity being produced and we also have to pay for the repair work,” said the spokeswoman.
“Also we have contracts with customers to supply power and so we have to buy supplies in – and we are charged a premium rate.
“We cannot say when the reactors will be back online, but it’s fair to say shortly. I think we are on the home strait.”