One of two reactors at Hunterston nuclear power station in Ayrshire had to be shut down on Sunday after engineers found potential problems.
Hunterston was shut down on Sunday night
An investigation is now under way into what caused the fault with the temperature controls.
Operator British Energy was only given permission to restart the plant last month after repairs to cracked pipes.
A spokesman said the outage in reactor three occurred at 2300 BST and there were no safety concerns.