Technical failure has forced the closure of a nuclear reactor at Japan's Ohi power station, 220 miles west of Tokyo.
The decision will place further pressure on the country's already strained power supplies.
Operators at Japan's Kansai Electric Power Company said on Saturday that while no radiation leak had occurred, they would manually close the No. 1 reactor for checks at 9pm local time.
It is unknown when the reactor will be restarted.
A spokesman said pressure in a tank containing boric acid water had fallen on Friday night for unknown reasons, according to Reuters. The pressure has reportedly since returned to normal.
The closure will compound power supply problems that followed the earthquake and tsunami that his the east coast of Japan in March, triggering a radiation crisis at Tokyo Electric Power Co's Fukushima plant.
Fukushima is continuing to leak radioactive material, while ongoing power problems have hurt Japan's manufacturing industry, along with the economy.
The BBC said that the Ohi closure will leave Japan with just 18 operational reactors out of 54.