Kansai Electric Power Co. shut down the No. 2 reactor at its Takahama nuclear power plant in Fukui Prefecture early Thursday after workers noticed a steam leak during a test run.
The Fukui Prefectural Government said the 826,000-kw pressurized water reactor was shut down at 1:31 a.m.
Kansai Electric officials said there was no danger of radiation leakage.
The utility said the workers found steam leaking from two places in pipes near a low-pressure turbine at around 9:30 p.m. Wednesday. The steam did not contain radiation, the officials said.
They said the company decided to halt the reactor manually after steam continued to leak despite efforts to halt the leak.
The Osaka-based power company activated the reactor Monday and started the test run Wednesday afternoon in the final phase of a regular checkup.