UPDATE2: Takahama nuclear reactor in Japan's Fukui halted after alert goes off

A reactor at Kansai Electric Power Co.'s Takahama nuclear power station in Japan's Fukui Prefecture was automatically halted Monday after an alert went off warning of a rapid decrease in the number of neutrons within the unit, the complex's regulator and operator said.

The No. 4 reactor was halted at 3:21 p.m., the Osaka-based utility said, adding that there has been no indication of the incident causing environmental contamination. The reactor's cooling function is normal, according to the Nuclear Regulation Authority.

Abnormalities were picked up by at least two of the four neutron detectors installed outside the reactor vessel, according to the operator.

It was the first automatic shutdown of a reactor since the No. 3 unit at the same plant was halted in 1988 due to a similar warning, Kansai Electric said.

The Fukui prefectural government said the incident could have been caused by a problem with the neutron detectors or a falling control rod. Control rods inside the reactor are used to control nuclear chain reactions by absorbing neutrons.

No abnormality was detected when Kansai Electric conducted maintenance work on the detectors on Thursday, the utility said.

The No. 4 reactor was reactivated on Nov. 4 following regular inspections and resumed commercial operations on Dec. 1. It had been halted automatically in 2016 due to a different problem.

The seaside plant has four reactors and faces the Sea of Japan.

==Kyodo