Kyushu Electric Power Co. said Sunday it found a 1-cm hole in a pipe believed to have caused a small steam leak at one of the reactors at the recently restarted Genkai nuclear power plant in Saga Prefecture.

The leak at Genkai's No. 3 reactor occurred in part of the steam production system and did not involve any radiation leaks. The reactor remains online and the utility is inspecting it.

The hole was spotted in an air vent pipe in a device used to remove oxygen and other gases that flow into the steam generator, Kyushu Electric said. The generator is in the reactor's primary containment vessel.

Unit 3 resumed operation on March 23 after a seven-year hiatus caused by new safety requirements prompted by the March 2011 Fukushima disaster.

The reactor was expected to enter commercial operation on April 24 but will now be delayed. The utility's plan to restart unit 4 reactor in May is also likely to see a delay.