Vapor leaked Saturday from a moisture separator-heater in the turbine house adjacent to the No. 1 reactor at the nuclear power plant in Mihama, Fukui Prefecture, the prefectural government said.

The vapor contained no radioactive substances, it said.

According to the prefecture's nuclear safety department, an employee of Kansai Electric Power Co. found a small volume of vapor hovering around a pipe connected to one of the turbine house's two moisture separator-heaters at around 10 a.m.

The vapor leaked out from the welded part of the thermometer installed on the pipe, it said.

Kepco said it has been operating a test run of the pressurized water reactor since late August after completing a periodical check that started in late April.

The utility will investigate the leak to identify and fix its cause, leaving the reactor to continue to operate at 50 percent of capacity, the department said.

When the periodical checkup was conducted on the No. 1 reactor, engineers inspected the device from which the vapor leaked, it said.

On Aug. 9, 2004, super-heated nonradioactive steam leaked from the No. 3 reactor at the Mihama plant, leaving five workers dead and six seriously injured. The accident was blamed on pipes that had not been inspected for 28 years.