Part of Dungeness nuclear power station has shut down because of an equipment fault.
EDF said the Dungeness B reactor 21 shut down because of an issue on a turbine condenser.
The energy firm said staff on site at the plant in Kent were working on a repair to return the unit power.
National Grid data showed the 550-megawatt reactor stopped sending power to the system and it shut down at 03:50 GMT, EDF said.
Dungeness B has two reactors and reactor 22 remains operational.
Dungeness A has been decommissioned.
Both Dungeness B reactors automatically shut down during the St Jude's Day storm on 28 October, when electricity to the site was cut off.
After the storm, the reactors were restarted using the power station's own generators and the plant was later reconnected to the National Grid on 6 November.