A reactor at the Takahama nuclear plant in Fukui Prefecture on the Sea of Japan coast was automatically halted on Monday following an emergency signal.

Japan's Nuclear Regulation Authority says the No. 4 reactor was stopped at around 3:20 p.m. after an alarm went off, signaling that the level of neutrons inside the reactor had plunged rapidly.

The nuclear watchdog says no problems have been reported with the nuclear fuel cooling systems. It says there is no change in the values of the monitoring posts that measure radiation levels in the surrounding area.

Both the NRA and the plant's operator, Kansai Electric Power Company, are looking into what happened.

The reactor in question was reactivated in November and resumed commercial operation the following month.

The NRA says this was the first time since 1988 that the signal went off, resulting in the automatic halting of a reactor. Last time, it occurred at the No. 3 reactor at the same nuclear power plant.