EDF Energy switched on another nuclear reactor at the Hartlepool power plant in the early hours of the morning.
Hartlepool was closed for safety inspections in August after a crack was found in the boiler spine of its sister plant, Heysham 1.
Both the plants have eight boilers, which channel heat away from the reactor core and creates the steam which drives the electricity-generated turbines.
The boilers each have a number of boiler tubes with a central cylindrical boiler ‘spine’ which supports the tubes.
The power company said it was back online after the “successful completion” of the unit’s boiler inspection. This was to check there were no other “defects” to the boiler spine which could develop into cracks.