TOKYO, Feb. 9 (Xinhua) -- Japan's Tokyo Electric Power Company (TEPCO) on Monday reactivated a reactor once again at its Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture, after determining the cause of an alarm that halted the facility last month.

The logo of Tokyo Electric Power Company is seen at an entrance to the company's headquarters in Tokyo on February 9, 2026. (Photo: AFP)

The company restarted the No. 6 reactor at the plant on Jan. 21 for the first time in about 14 years but shut it off the following day after an alarm from the monitoring system sounded during the process of removing the control rods.

TEPCO explained that the issue was caused by an error with the alarm's settings. An investigation found no equipment abnormalities but identified rare instances of delayed electric current flow in one of the three cables, which were detected and triggered the alarm. The company changed the alarm settings and confirmed normal current flow in all control rods.

The Kashiwazaki-Kariwa plant, about 220 km northwest of Tokyo, is the world's biggest nuclear power plant by potential capacity. The facility had been offline since Japan pulled the plug on nuclear power following the March 2011 core meltdowns at TEPCO's tsunami-stricken Fukushima Daiichi plant.

The restart of the No. 6 reactor at the seven-unit complex marked the first TEPCO-run unit to go back online since the Fukushima nuclear disaster, which has met local opposition amid criticism that the plant sits on an active seismic fault zone.