The Kansai Electric Power Company says it plans to manually shut down the No.1 reactor at its Ohi nuclear power plant in western Japan due to technical problems but says no radiation leakage has occurred.
The reactor, located 350 km west of Tokyo, will be closed for checks from Saturdy evening after pressure in a tank containing boric acid water fell for a time on Friday night for unknown reasons.
A Kansai Electric spokesman says the pressure has since come back up to the normal range.
The shutdown will further compound a tight power supply linked to long-running attempts to resolve the radiation crisis at Tokyo Electric Power Co's Fukushima Daiichi nuclear power plant, which has been out of action since the 11 March earthquake and tsunami.
Currently 35 out of the 54 reactors in Japan remain shut. Some of the closures are due to the disaster, but about half are due to regular maintenance and inspections.
The reduction in generating capacity has been stifling manufacturing activity and hurting Japan's weak economy.
Meanwhile, the Nuclear & Industrial Safety Agency (NISA) in Japan says it cannot estimate how long will it need to review safety tests of nuclear plants.
Plans for stress tests of nuclear reactors were announced last week by the government.
NISA submitted plans to the Nuclear Safety Commission on Friday on how to conduct the tests.
''We cannot assume how comprehensive the reports from the utilities will be," NISA director Tomoho Yamada said afterwards.
''It is difficult to say at this point how long it will take for us to make the assessments.''
Nuclear power accounted for nearly one-third of Japan's power supply before 11 March.