Japan's Ohi nuclear power station is being shut down, after a technical fault.
Pressure in a safety tank fell for no apparent reason, and although it is now back to normal, the plant's operators said they would "give the top priority to safety and find out the cause".
There was no release of radioactive material.
The closure will compound power difficulties in the wake of March's earthquake and tsunami.
There has been growing public disquiet over the continuing crisis at the Fukushima Daiichi nuclear plant, which was damaged by the disaster.
Fukushima continues to leak radioactive material.
Pressure in a tank containing boric acid, which is used to slow down nuclear fission in emergencies, dropped on Friday at Ohi's No 1 reactor.
The pressure levels have since returned to normal, but the reactor will be completely shut down by 2100 (1200 GMT).
The reactor, 350km (220 miles) west of Tokyo, has a capacity of 1.18m kW.
With the closure at Ohi, only 18 of Japan's 54 reactors remain operational.
The Kansai Electric Power Company, which operates Ohi, could not say when the reactor would be restarted.