Japan: Startup of Reactor 6 at the World’s Largest Nuclear Power Plant Kashiwazaki-Kariwa Halted After Alarm

Tokyo (Germany) - Following the Fukushima nuclear accident in 2011, all 54 nuclear power plants in Japan with a combined capacity of around 49 GW were shut down. Under the new, strict safety standards of the Nuclear Regulation Authority (NRA), some reactors are now gradually resuming operation.

The world’s largest nuclear power plant, Kashiwazaki-Kariwa, comprises a total of seven reactors, all of which had been shut down. Reactor 6, one of the seven units, was now restarted for the first time. However, a malfunction occurred during the startup process.

Tepco suspends startup of Unit 6 at the Kashiwazaki-Kariwa nuclear power plant

Tokyo Electric Power Company Holdings (Tepco) suspended startup activities for Reactor 6, with a gross capacity of 1,356 MW, at the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture on January 22, 2026. Just a few hours after the restart of this individual unit, an alarm was triggered during control rod inspections, prompting an immediate halt to the work. Tepco emphasized that no radiation was released and that there is no risk to the public. The cause of the alarm is currently being thoroughly investigated, while the company is working closely with Japan’s Nuclear Regulation Authority to ensure the safety of the facility.

Restart of Reactor No. 6 following inspections

The restart of Reactor 6 took place after multi-stage inspections and approvals. Tepco submitted an initial pre-startup confirmation to the Nuclear Regulation Authority as early as November 2024, loaded the reactor with nuclear fuel in June 2025, and conducted initial integrity tests. Subsequently, on December 24, 2025, Tepco submitted a modified pre-startup notification authorizing final system tests and trial operation. In January 2026, the NRA granted approval for the startup, which began on January 21, 2026.

About the world’s largest nuclear power plant, Kashiwazaki-Kariwa

With a total installed capacity of approximately 7,965 MW, Kashiwazaki-Kariwa is the largest nuclear power plant in the world and consists of seven reactors. The older units 1 through 5, each with a capacity of around 1,100 MW, have remained shut down since the Fukushima accident in 2011 and are expected not to be restarted. The modernized Advanced Boiling Water Reactors, Units 6 and 7, each have a capacity of 1,356 MW. With the recent startup, only Reactor 6 was activated to test system functionality after 14 years of shutdown and to stabilize the regional power supply.

Tepco is now carefully examining the cause of the alarm and will implement all necessary safety measures before any renewed startup of Reactor 6. A restart of Reactor 7 may be possible in the long term but depends on further safety reviews. The company emphasizes that safety is its top priority and that all steps will be transparently documented and communicated to the public.

Source: IWR Online, 23 Jan 2026