NAGOYA - Chubu Electric Power Co. may have begun manipulating earthquake resistance data for its nuclear power plant in central Japan after a 2009 earthquake subjected one of its reactors to shaking beyond its design limits, a source familiar with the matter said Thursday.
The latest revelation came after the utility admitted earlier this year that it had, by no later than 2012, begun cherry-picking favorable data to set earthquake-resistance standards for the Nos. 3 and 4 reactors at its Hamaoka nuclear power plant, which it has been seeking to restart.
The data rigging was initially believed to have begun under tougher safety requirements imposed after the Fukushima Daiichi nuclear power plant disaster, which was triggered by a massive earthquake and tsunami in 2011. But the source said the practice may have begun even before the disaster, as part of efforts to restart the No. 5 reactor after it was shut down following the 2009 earthquake.
In the magnitude-6.5 earthquake that struck Omaezaki, Shizuoka Prefecture, in August 2009, the No. 5 reactor experienced seismic shaking several times stronger than that recorded at the Nos. 1 to 4 reactors, partially exceeding its design limits.
Chubu Electric found that unusual ground conditions had amplified the seismic waves. After the No. 5 reactor automatically shut down in the quake, the company allegedly began generating seismic data and selecting favorable results to avoid being ordered by the government to implement additional earthquake-resistance measures, according to the source.
The company switched to a different contractor around the same time, allowing it to generate seismic wave data more quickly and "creating the perfect circumstances and motive," the source said.
The nuclear regulator at the time approved the safety of the No. 5 reactor in late 2010, paving the way for its restart in February 2011.
The fresh allegation could mean Chubu Electric engaged in data manipulation involving the Hamaoka plant's three operable reactors, further undermining confidence in the country's nuclear safety screening process.
"Details of what happened will be determined through a third-party investigation. We will cooperate sincerely with the inquiry," Chubu Electric said in a statement.
Located on the Pacific coast in central Japan, the Hamaoka complex sits near the expected epicenter of a potential massive earthquake in the Nankai Trough. It was shut down in May 2011 at the request of then Prime Minister Naoto Kan over safety concerns.
Chubu Electric applied for state safety screenings between 2014 and 2015 to restart the Nos. 3 and 4 reactors, but the review process has been halted amid the scandal.
According to a report compiled by Chubu Electric in March this year on its data misconduct involving the Nos. 3 and 4 reactors, the utility told regulators it had generated 20 sets of seismic motion data under different conditions and selected the one closest to the average -- known as the "representative wave" -- to establish a benchmark for earthquake-resistant design.
In reality, however, the company had been generating numerous combinations of seismic motion data and representative waves before selecting a set from among them. In 2018, it began deliberately selecting the representative wave and ensuring the other data were consistent with it.
The Nuclear Regulation Authority also recently revealed it had learned from an external source that Chubu Electric continued manipulating data even after the investigation began in May 2025.
