The electricity company EDF has temporarily shut down two of its nuclear power stations, Heysham 1 and Hartlepool.
It said it was a precautionary measure after finding a defect, in June, in one of the boilers at Heysham 1.
The problem was first detected during a routine boiler inspection, which led to one of the station's two nuclear reactors being shut down.
All four reactors at the stations will be shut as they are of similar design.
The company said all the boilers associated with the two nuclear reactors at Heysham 1, and with the two nuclear reactors at Hartlepool, would be inspected to make sure they were safe.