Authorities in Fukui Prefecture, central Japan, have confirmed that a small fire at the Tsuruga nuclear power plant's No.1 reactor was put out in about one hour.
Firefighters say workers at the plant reported that smoke was coming out of the reactor's turbine building at around 6:30 a.m. on Thursday.
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.
The smoke and sparks subsided when workers unplugged the exhaust fan.
Firefighters confirmed the fire extinguished shortly after 7:30 a.m.
The plant operator says impact of radiation on the environment has not been confirmed.
Fukui prefectural officials say they have detected no change in data from radiation monitoring posts around the nuclear plant.
The reactor has been undergoing decommissioning work since 2017.