Japan’s ‘most dangerous’ nuclear power plant admits to manipulating earthquake safety data

Regulator halts review to restart Hamaoka power plant after 14 years

A Japanese power plant operator has admitted to cherry-picking critical safety data to pass the screening process of the nuclear safety regulator to restart two of its offline reactors.

Chubu Electric said on Monday that it had set up an independent panel of experts to investigate possible misconduct in compiling data as part of a process to restart two reactors at the Hamaoka nuclear power plant.

The plant originally had five reactors but two were permanently shut down in 2009. The remaining three reactors were taken offline in the wake of the 2011 Fukushima disaster.

Concerns about data manipulation mean the power plant is unlikely to restart anytime soon. It’s also a likely setback for Japan’s efforts to shift back to nuclear power to boost energy security and cut greenhouse gas emissions.

Chubu told regulators it had selected an earthquake wave model closest to the average of 20 possible patterns to calculate the Hamaoka plant’s “standard seismic motion”, the maximum shaking the reactors could withstand. However, the company admitted, employees in charge could have deliberately chosen that model to make the plant appear safer and speed up the screening process.

“We sincerely apologise for the incident,” Chubu Electric president Kingo Hayashi told a press conference. “The actions could potentially shake the foundations of the nuclear power business.”

The regulator learned about the misconduct last February after it was contacted by a whistleblower.

A senior agency executive called the matter “unbelievable” saying it broke trust in the operator and would make the people question its eligibility.

The industry ministry has now ordered Chubu Electric to submit a detailed report by 6 April explaining the cause of the misconduct and outlining measures to prevent it from happening again.

The Hamaoka power plant, 200km south-west of Tokyo, has been described as the “world’s most dangerous” nuclear power facility by some seismologists and anti‑nuclear campaigners. Government forecasts have predicted an 87 per cent chance of a powerful quake in the area, which sits on two major subterranean faults. A major accident would be likely to force the evacuation of Greater Tokyo, home to 28 million people.

Professor Katsuhiko Ishibashi, a seismologist and a former member of a Japanese government panel on nuclear reactor safety, said in 2003 that Hamaoka was “the most dangerous nuclear power station” in Japan because of the potential for an earthquake to trigger a nuclear disaster.

He assessed at the time that such an incident would devastate a broad area between Tokyo and Nagoya, destroying more than 200,000 buildings and resulting in a huge tsunami.

The plant was ordered to shut down reactors 4 and 5 and cancel the planned restart of reactor 3 following the Fukushima disaster, when a magnitude 9 earthquake triggered tsunami waves up to 15m high. Hamaoka was built to withstand only an 8.5-magnitude quake and an 8m tsunami.

Chubu applied for a review to restart the Hamaoka reactors between 2014 and 2015, and it was approved for standard seismic motion in September 2023.

Shares of Chubu Electric dropped 8.2 per cent, its steepest fall since April 2025, after the latest revelations.