Japan's nuclear regulator on Wednesday effectively banned Tokyo Electric Power Company Holdings Inc. from restarting its largest nuclear plant due to serious safety flaws, dealing a blow to the utility's efforts to turn its business around following the 2011 Fukushima nuclear disaster.

The Nuclear Regulation Authority formally issued the order not to transport nuclear fuel stored at the Kashiwazaki-Kariwa plant in Niigata Prefecture or loading it into reactors until corrective actions are taken to address the complex's security breaches.

The order was the first of its kind issued to a commercially operated nuclear facility in Japan.

TEPCO President Tomoaki Kobayakawa pledged to reshape the company's corporate mindset toward the security of nuclear materials as he attended a prefectural assembly session in Niigata to explain the problems, saying, "We have grave concerns about whether we can continue to operate the nuclear power generation business."

Photo taken from a Kyodo News plane on April 13, 2021, shows the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture. (Kyodo)

The company has seen restarting the seven-reactor Kashiwazaki-Kariwa complex, once one of the world's largest nuclear plants by output, as a main pillar of its business restructuring plan.

The plant was found to have been vulnerable to unauthorized entry at 15 locations since March last year because of defective intruder detection systems and backups, according to the NRA. It is the first time the regulator has issued a corrective order for a commercial nuclear reactor.

The punitive measure is expected to remain for more than a year until the authority concludes additional inspections. Although the plant's Nos. 6-7 reactors cleared safety screenings by the regulator in 2017, all seven reactors remain idle.

The delay in resuming the plant's operation would be a setback for the central government's push to restart more nuclear plants to cut greenhouse gas emissions while a shift to weather-dependent renewable power generation is promoted.

The Kashiwazaki-Kariwa plant has been plagued by security issues, including an incident in which an employee used a colleague's ID pass last September to enter the central control room without authorization.

The NRA also gave a warning Wednesday to the top three officials of its secretariat for failing to report the ID mishandling to the NRA even though they had been informed about it the day after the incident.

TEPCO has been facing huge compensation payments and other costs stemming from the Fukushima disaster. It is aiming to restart the Kashiwazaki-Kariwa plant to reduce its dependence on costly coal-fired power generation.

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