Japan’s Nuclear Regulation Authority (NRA) has suspended its safety review of the Hamaoka NPP in Shizuoka Prefecture after concluding that plant owner and operator, Chubu Electric Power Company, had deliberately understated the dangers of an earthquake at the site.
This came two days after Chubu Electric disclosed that it had used inappropriate data to estimate the earthquake standard ground motion during a safety review of units 3&4.
Standard ground motion represents the maximum potential shaking from an earthquake, which a NPP must be designed to withstand.
Hamaoka NPP comprises five units – four boiling water reactors (BWRs) and one advanced boiling water reactor (ABWR). Units 1 and 2, the two oldest (540 MWe and 840 MWe) were permanently shut down in 2009 and are currently being decommissioned. Hamaoka units 3-5 were shut down after the Fukushima accident in 2011 and underwent significant safety upgrades. In 2014, Chubu Electric applied to the NRA to restart them by initiating a safety review. Chubu Electric had aimed to finalise both the plant facility screenings – covering reactor buildings and equipment – and the investigations of on-site geological faults by the spring of 2026.
In 2023 the NRA had viewed Chubu Electric’s standard ground motion at the Hamaoka plant as “generally appropriate”. However, in February 2025, NRA received information from an external source that Chubu Electric was engaged in fraudulent activity, prompting it to demand that the company submit materials and provide an explanation. Chubu conducted an internal investigation and told the NRA in December that it had “confirmed fraudulent activity”.
NRA subsequently stated that the quake data was arbitrarily manipulated using a method different from what was indicated in the review.
According to Chubu Electric, at a regulatory review meeting in 2019, the company had told the NRA that it would calculate a set of 20 seismic ground motion estimates based on different conditions to serve as data for calculating a representative or average figure for the safety standards at the Hamaoka NPP.
However, prior to 2018, instead of preparing a single set consisting of 20 simulations of seismic motion, the company had been creating multiple sets and selected just one of them to submit to the NRA. At some point after 2018, the company had selected one simulation that was not closest to the average, and then cherry picked 19 others for the set, so that it would appear to be the average.
Tetsuya Toyoda, head of the company’s nuclear power department, said he believed there had likely been intent “to make the seismic motion appear smaller”. The Ministry of Economy, Trade and Industry has requested Chubu Electric to report by 6 April in detail on what caused the inappropriate practices and to propose measures to prevent future recurrence.
NRA Chairman Shinsuke Yamanaka told a news conference: “This is an outrage against safety regulations. The review itself needs to be redone.” He added: “This is a case of fabricating safety-related review data. It is extremely serious and grave”. He also ordered the NRA to respond independently without waiting for the results of Chubu Electric’s internal investigation.
“They manipulated diagrams to make their data appear plausible,” Koshun Yamaoka, NRA Commissioner in charge of reviewing earthquake and tsunami data, said. “Compared with research misconduct, this is equivalent to fabrication or falsification. This is a serious matter that fundamentally overturns the premise of the review.”
NRA Commissioner Reiko Kanda said Chubu Electric had wasted national funds. “We have invested human resources and conducted our review in good faith,” he said. NRA Commissioner Tomoyuki Sugiyama said the safety review should remain halted until the reliability of Chubu Electric’s data can be assured. “I was utterly disappointed. Although we are temporarily suspending the review, there is no way we can resume it,” he noted.
Local leaders and officials from Omaezaki, the municipality where the plant is located plant were highly critical of Chubu Electric. “The premise of the review under the new regulatory standards has collapsed. It is a deeply regrettable event that causes a loss of trust,” said Shizuoka Governor Yasutomo Suzuki. He cited a series of scandals at the utility, noting: “I will be closely watching its future initiatives, including corporate culture reform.” He said he will demand an explanation from Chubu Electric after receiving a report from a third-party committee investigating the false data and will petition the central government for guidance.
Omaezaki Mayor Masaru Shimomura said it will be difficult to restart the plant without the trust of the region. “If the reliability of the submitted data itself is shaken, then everything is shaken,” he said.
At a press conference earlier at Chubu Electric’s headquarters in Nagoya, Company President Kingo Hayashi revealed that quake data provided to the NRA was not calculated as it should have been and may have been handled inappropriately. The company will establish an independent panel to investigate the incident, he said. “There is a risk that this could have had a serious impact on the Nuclear Regulation Authority’s review,” Hayashi added, apologising for the potential mishandling. “It undermines trust in our operations and could strike at the very foundation of our business.” He added: “We are in no position to comment on the resumption of the screening or the timeline,” said Hayashi. “Our only option is to proceed while fundamentally restructuring our nuclear division.”
In the wake of these events 65-year old Hayashi said that he will resign, making the announcement at a news conference. He reportedly said: “I’ve decided to resign in order to focus on investigating the facts and finding the cause,” also apologising again for the lapse.
However, Yamanaka ordered the NRA to respond independently without waiting for the results of Chubu Electric’s internal investigation and the NRA has now decided that its secretariat will conduct an on-site inspection of Chubu Electric Power. The inspection is expected to target Chubu Electric’s headquarters in Nagoya but could also be extended to the NPP itself.
NRA will also now consider demanding a legally mandated report and conducting a nuclear regulatory inspection that includes on-site access to Chubu Electric’s headquarters and the Hamaoka plant. Such inspections are compulsory and carry penalties for refusal to cooperate.
In the wake of the data sandal, NRA has cancelled the screening of the plant for a possible restart. In addition, NRA will issue an order for Chubu Electric to report back on the details of the data fraud under the nuclear reactor regulation law, with the deadline set for the end of March. The company will face punishment if it refuses the order or makes false statements. The authority plans to urge other power companies to prepare appropriate documents for the NRA’s reactor screenings.
