Kyushu Electric Power Co. stopped generating electricity at the Genkai nuclear power plant in Saga Prefecture on Saturday after a steam leak was detected in a reactor the previous day.
The utility said that no radiation leaked and that it will inspect the reactor, which resumed operation just a week ago.
According to Kyushu Electric, at around 7 p.m. Friday staff discovered steam leaking from the pipe of a device used to remove oxygen and other dissolved gases from feedwater used in steam generators.
Reactor 3 resumed operation on March 23 after being offline for over seven years amid concerns about how to evacuate residents near the plant in a serious accident. It resumed power generation two days later.
Kyushu Electric said its plan to restart reactor 4 in May could be delayed in light of the problem.
Reactor 3 unit was suspended for a regular inspection in December 2010, three months before the March 2011 quake and tsunami sparked the man-made crisis at the Fukushima No. 1 nuclear plant.
It cleared the Nuclear Regulation Authority's safety screening in January 2017 under stricter, post-Fukushima regulations and was later approved for reactivation by both the Genkai Municipal Government and the Saga Prefectural Government.