Tokyo Electric Power Company Holdings (Tepco) has restarted unit 6 at its Kashiwazaki-Kariwa NPP in Niigata Prefecture, after technical problems in January delayed the process. Tepco initially tried to restart unit 6, a 1356 MWe Advanced Boiling Water Reactor (ABWR), on 21 January but closed it down again the following day after a monitoring system alarm sounded.
The alarm had picked up slight changes to the electrical current in one cable even though these were still within a range considered safe, Tepco officials told a news conference. The alarm setting has since been changed. The unit’s power output will now be gradually increased with the aim of starting commercial operation on or after 18 March following another comprehensive inspection. Commercial operation had earlier been targeted for 26 February.
The start-up procedure involves removing 205 rods in batches of 26. Tepco said after the failed restart in January that it had suspected a malfunction in the control rod operating system after the alarm sounded but, after a component was replaced, the problem had persisted. That first restart had already been delayed by a day to investigate why an alarm had failed to sound during a test withdrawal of control rods.
“I still believe it was my responsibility to decide to shut the plant” to conduct deeper investigation into the issue, said Takeyuki Inagaki, the superintendent of the Kashiwazaki-Kariwa facility during a news conference before the latest restart. Tepco will proceed with each step of the restart carefully, he said.
According to a September 2025 survey, public opinion in Niigata is deeply divided with around 60% of residents opposed to the restart, of the NPP while 37% support it. Although Kashiwazaki-Kariwa, unlike Fukushima Daiichi, was unaffected by the March 2011 earthquake and tsunami, all seven of the plant’s reactors had already been offline for two to three years following the earlier 2007 Niigata-Chuetsu earthquake, which caused damage to the site. Work has since been carried out to improve the plant’s earthquake resistance.
Tepco submitted information on safety upgrades across the site and at the newest units (6&7), which began commercial operation in 1996 and 1997 and were the first Japanese boiling water reactors (BWRs) to be considered for restart. The National Regulation Authority (NRA) cleared safety screenings for the two units in 2017 but security breaches and delays in completing safety upgrades caused delays. The Kashiwazaki-Kariwa complex has been fitted with a 15-metre-high (50-foot) tsunami wall, elevated emergency power systems and other safety upgrades.
Fuel assemblies had been loaded into unit 7 in April 2024 in preparation for its restart. However, as the unit cannot be restarted before completion of the antiterrorism facility, which is scheduled for August 2029, the fuel was removed the following September. Tepco then prioritised restarting unit 6, where fuel loading was completed in June 2025. The company has until September 2029 to implement similar anti-terrorism safety measures at unit 6, but was authorised to operate until then, pending local approval.
Kashiwazaki-Kariwa is the first of Tepco’s nuclear units to restart since 2011. The company also operates the Fukushima Daiichi plant, which is now being decommissioned, along with the neighbouring Fukushima Daiini NPP.
Kashiwazaki-Kariwa 6 is the 15th restart since all 54 of Japan’s nuclear power reactors were closed in the wake of the Fukushima accident. Following the permanent closure of some units, Japan now has 33 operable reactors. Most of the restarted units are pressurised water reactors (PWRs) as boiling water reactors (BWRs), like those at Fukushima and Kashiwazaki-Kariwa require more complex upgrades to satisfy NRA requirements. The PWR units which have restarted operation include Kansai Electric Power Co’s Takahama 1-4, Ohi 3&4 and Mihama 3; Kyushu Electric’s Sendai 1&2 and Genkai 3&4; and Shikoku Electric’s Ikata 3. Two BWRs restarted in 2024: Tohoku Electric’s Onagawa 2 and Chugoku Electric’s Shimane 2.
Before 2011, nuclear power accounted for nearly 30% of Japan’s electricity with plans to increase that to 50% by 2030. Its 2025 energy plan targets a 20% nuclear share by 2040. Despite its decreasing population, Japan predicts growing electricity demand from AI data centres.
