One of the systems that pumps seawater into the condenser of the Fugen advanced thermal reactor in Tsuruga, Fukui Prefecture, stopped working around 10:50 a.m. Wednesday, prefectural officials said.
The officials said the reactor, operated by Japan Nuclear Cycle Development Institute, is still operating, although at reduced output, because the other pumping system is running smoothly.
No radioactive leaks have been detected, the officials said.
The prefecture officials and the reactor's state-run operator said an electrical problem with the pump's motor may have caused the problem.
The water circulation system, which supplies seawater to cool steam used to propel the power generator's turbine, was checked in a regular inspection completed in June, they said.
The 165,000-kw reactor was shut down April 21 after an increase was detected in the radioactive density of the coolant water. It was also shut down April 9, one day after resuming operations following an 11-month hiatus, because a small leak of steam containing radiation was detected in a pressure gauge.
The reactor resumed trial operations June 3 and full operations June 28.
The reactor, in a nuclear complex on the Sea of Japan coast, is scheduled to be shut down permanently at the end of March.