Authorities in Fukui Prefecture, central Japan, have confirmed smoke and sparks at a vent of the Tsuruga nuclear power plant's No.1 reactor.
Firefighters say workers at the plant reported at around 6:30 a.m. Thursday that smoke was coming out of the reactor's turbine building.
The plant's operator, the Japan Atomic Power Company, says a fire alarm for an air conditioning system inside the turbine building went off.
Workers later found smoke and sparks coming out of an exhaust fan at the turbine building.
There have been no reports of injury so far.
The reactor has been undergoing decommissioning work since 2017.
Fukui prefectural officials say they have detected no change in data from radiation monitoring posts around the nuclear plant.