Photo/Illutration The No. 4, right, and the No. 3 reactors of Kansai Electric Power Co.'s Takahama nuclear power plant on Nov. 14, 2020, in Takahama, Fukui Prefecture (Tatsuo Kanai)

An alarm signaling a marked drop in neutron levels forced a reactor at the Takahama nuclear power plant in Takahama, Fukui Prefecture, to shut down on Jan. 30 while the cause is investigated.

The drop was detected at 3:21 p.m. and triggered the automatic shutdown of the No. 4 reactor, according to plant operator Kansai Electric Power Co.

Radiation levels remain normal at the plant, and the incident caused no damage to the core and other key components of the reactor, KEPCO said.

KEPCO officials are not sure if something went wrong with the reactor or if it was a false alarm. They will start investigating the cause after the reactor is in a cold shutdown.

The No. 4 reactor went back online in December 2022, two weeks behind schedule, following abnormalities detected during the inspection that began six months earlier.

This was the first automatic shutdown of the No. 4 reactor since February 2016 and the third in its history.

The reactor will reach its 40th year of operation in 2025, the legal lifespan of a reactor.

In 2022, KEPCO inspected the plant’s No. 3 and No. 4 reactors, conducting a special assessment required for an extension of the operational period beyond 40 years.

Following the assessment, KEPCO concluded the reactors “have no problem with operating up to 60 years” and announced its plan to make a request for an extension of 20 years to the Nuclear Regulation Authority in November 2022.