Tepco restarts reactor at world's largest nuclear plant for first time since Fukushima – only to suspend it hours later due to an alarm malfunction.
Tokyo Electric Power Company (Tepco) restarted Reactor No. 6 at the Kashiwazaki-Kariwa Nuclear Power Plant in Niigata Prefecture in early 2025, marking the first activation of a reactor at the world's largest nuclear facility since the 2011 Fukushima disaster. The restart was suspended hours later due to an alarm malfunction, with no radioactive impact reported.
Reactor Restart and Suspension
The No. 6 reactor at Kashiwazaki-Kariwa, located approximately 220 kilometers (135 miles) northwest of Tokyo, was restarted on Wednesday. The operation had been delayed by one day due to a separate technical issue involving an alarm malfunction.
During the startup process, plant workers began removing neutron-absorbing control rods from the reactor core. An alarm sounded after 52 of the 205 control rods had been removed, indicating a malfunction. Tepco spokesperson Takashi Kobayashi stated the reactor remained stable and there was no radioactive impact outside the facility.
Tepco suspended operations “during reactor-start-up procedures” as a safety measure. Plant chief Takeyuki Inagaki stated that the reactor would not be restarted until the cause of the malfunction is identified and corrective measures are implemented, noting that the resolution would not be swift. The reactor had been scheduled to begin commercial operation in the month following the restart.
Approval Process
The restart followed a decision by Niigata Governor Hideyo Hanazumi to grant the prefecture's consent. The plan then proceeded to the prefectural government assembly for discussion and approval, followed by authorization from Japan's nuclear regulator. The approval initially targeted the No. 6 reactor, with the No. 7 reactor planned for a later restart.
Background: Fukushima Disaster and Nuclear Shutdown
Japan shut down all 54 of its nuclear reactors after the March 11, 2011, 9.0-magnitude earthquake and tsunami caused a meltdown at the Fukushima Daiichi nuclear plant. The disaster resulted in reactor flooding, radiation leakage, and the evacuation of over 150,000 residents. An independent government report labeled the incident a “man-made disaster.” A court later cleared three Tepco executives of negligence related to the event.
Since 2015, 15 of Japan's 33 operable reactors have been restarted. The Kashiwazaki-Kariwa plant is the first Tepco-operated facility to attempt a resumption. The No. 7 reactor at the plant is projected to restart by 2030, while the remaining five reactors may be decommissioned.
Public Opinion and Safety Concerns
A survey conducted by Niigata Prefecture found that 50% of residents supported the plant's restart, while 47% opposed it. Approximately 70% of respondents expressed concern about Tepco's management of the facility. Protests have occurred outside Tepco's headquarters and the Niigata prefectural assembly.
Following the Fukushima disaster, 70% of the Japanese population supported a reduction in nuclear power use in 2012 surveys. A 2022 poll by Nikkei indicated that over 50% of people would support nuclear power if safety could be ensured.
The Kashiwazaki-Kariwa plant has been involved in security-related incidents. In 2023, an employee lost confidential documents and another employee mishandled documents. Tepco reported these incidents to the Nuclear Regulation Authority (NRA) and stated intentions to improve security management.
Earlier in 2025, the NRA suspended its review for restarting Chubu Electric's Hamaoka plant after the company was found to have manipulated seismic data. The company apologized, and authorities indicated a commitment to penalize data falsification.
Spent Fuel Storage and Waste Management
Japan has limited capacity for storing spent nuclear fuel and no permanent disposal plan. Cooling pools at Kashiwazaki-Kariwa and two other plants are projected to reach full capacity within five years, per the Federation of Electric Power Companies of Japan. As of December 2025, 17 Japanese nuclear plants held over 17,000 tons of spent fuel, using nearly 80% of storage capacity, according to the Ministry of Economy, Trade and Industry.
The government is considering Minamitorishima, a remote Pacific island administered by Tokyo, as a potential site for high-level radioactive waste disposal. Industry Minister Ryosei Akazawa requested a feasibility study from Ogasawara village. The government has offered subsidies of up to 2 billion yen for the first stage of site selection and up to 7 billion yen for the second stage.
Japan's policy has been to recycle spent fuel, but a key reactor designed for plutonium reuse has not succeeded, and reprocessing capacity is insufficient. Tepco is transferring spent fuel within the plant and considering dry cask storage.
Energy Policy and Economic Factors
Before 2011, nuclear power generated nearly 30% of Japan's electricity, with plans to increase this to 50% by 2030. The current energy plan targets 20% by 2040. Nuclear power accounted for 8.5% of Japan's electricity in 2023.
The No. 6 reactor, if operational, can generate 1.35 million kilowatts of electricity, sufficient to power over a million households. The plant's total output capacity is 8 million kilowatts, though Tepco currently plans to resume only two of its seven reactors.
Operating costs for nuclear reactors have increased due to new safety checks requiring significant investments. This poses a financial dilemma regarding subsidies or passing expenses to consumers. Tepco is managing the cleanup at the Fukushima site, estimated at 22 trillion yen ($139 billion).
Globally, the International Atomic Energy Agency projects a potential doubling of worldwide nuclear power capacity by 2050. Japan aims to achieve net-zero carbon emissions by 2050. Prime Minister Sanae Takaichi has emphasized nuclear power's role in energy self-sufficiency, citing anticipated demand increases from data centers and semiconductor manufacturing.