A technical malfunction has postponed the anticipated restart of the world’s biggest nuclear power plant at Kashiwazaki-Kariwa, its operator Tokyo Electric Power Company Holdings (TEPCO) announced on Monday.

TEPCO said the scheduled restart of Unit No. 6, part of the Kashiwazaki-Kariwa facility in Niigata Prefecture, originally slated to resume operations on Tuesday, January 20, 2026, has been deferred by a day or two as engineers conduct additional safety checks following a glitch in the plant’s alarm system.

The alarm, designed to trigger warnings in key safety situations, malfunctioned during routine testing over the weekend, the utility said, prompting it to halt the restart preparations to ensure all systems meet regulatory standards. While the issue was reportedly fixed by Sunday, the company has yet to set a new definitive date to bring the reactor back online.

TEPCO spokesman Isao Ito said the utility will brief Japan’s nuclear safety authorities on the incident and complete final examinations before moving forward. Officials have indicated that the verification process may take one to two days, subject to regulatory consultation.

The Kashiwazaki-Kariwa complex, with a total capacity of about 8.2 gigawatts, is the world’s largest nuclear power station. The restart of Unit No. 6 would mark TEPCO’s first reactor revival since the 2011 Fukushima Daiichi nuclear disaster, when a massive earthquake and tsunami triggered a triple meltdown, forcing the shutdown of nuclear units nationwide.

Japan has progressively moved to restore nuclear capacity after years of stringent post-Fukushima safety reforms, driven by the need to reduce reliance on imported fossil fuels, curb carbon emissions and meet rising energy demands. A successful restart at Kashiwazaki-Kariwa is viewed as pivotal in reviving Tokyo’s nuclear energy portfolio.

However, nuclear power remains divisive among the Japanese public, with critics voicing concerns over safety and emergency preparedness. On Monday, protesters gathered outside TEPCO’s headquarters in Tokyo, chanting “No to the restart of Kashiwazaki-Kariwa!” and demanding a comprehensive investigation before any reactor is restarted.

Takeshi Sakagami, president of the Citizens’ Nuclear Regulatory Watchdog Group, said the delay underlines the need for a thorough safety review and that if a significant flaw is discovered, the affected reactor should be permanently shut down.

While Japan’s Prime Minister Sanae Takaichi has publicly supported the nuclear energy revival, balancing energy security and climate goals with public trust and reactor safety continues to be a key challenge for policymakers and regulators.