Neutron detectors have triggered an automatic shutdown at unit 4 of Kansai Electric Power Co's Takahama NPP in Japan's Fukui Prefecture. The reactor was automatically stopped after an alert warned of a rapid decrease in the number of neutrons within the unit, according to Kansai and the Nuclear Regulation Authority (NRA).

Kansai Electric said there was no indication of the incident had resulted in any environmental contamination. Abnormalities were picked up by at least two of the four neutron detectors installed outside the reactor vessel. NRA said the reactor's cooling function was normal.

Kansai Electric said it was the first automatic shutdown of a reactor since the Takahama 3 was halted in 1988 due to a similar warning.

The Fukui prefectural government said the incident could have been caused by a problem with the neutron detectors or a falling control rod. However, no abnormality was detected when Kansai Electric conducted maintenance work on the detectors subsequently.

Takahama 4 resumed commercial operations in December following routine maintenance. 

Image: The No. 4 reactor at Takahama nuclear power plant (courtesy of KYODO)