TOKYO, Jan 6 (Reuters) - Chubu Electric Power (9502.T) , opens new tab shares tumbled by the most in more than 13 years on Tuesday after the Japanese utility disclosed possible problems with how it evaluated seismic waves at an idled nuclear plant during a regulatory review required for a restart.

The company's stock led declines on the benchmark Nikkei gauge, falling 9.9% and poised for the steepest one-day decline since October 2012.

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Chubu Electric said on Monday it might have used a different method than explained to regulators to select representative seismic waves during the review of two reactors at its Hamaoka nuclear power station. They have been idled since a nuclear disaster at the Fukusihima Daiichi power station in 2011 led to an eventual shutdown of all reactors in Japan.

The discrepancy could "significantly impact" the regulatory review of the Hamaoka plant in central Japan, Chubu Electric said in a statement.

There is suspicion that the representative wave used as the benchmark for expected ground shaking was "deliberately chosen to underestimate seismic motion," Chubu Electric President Kingo Hayashi told reporters on Monday.

The Nuclear Regulation Authority (NRA) promptly suspended its review of the No.3 and No.4 reactors at Hamaoka after receiving a report from Chubu Electric on the matter on December 18, an official said, adding that the committee would discuss next steps.

The disclosure could dampen momentum to rebuild trust in Japan's nuclear power industry, just after Tokyo Electric Power's (9501.T) , opens new tab Kashiwazaki-Kariwa nuclear power plant and Hokkaido Electric Power's (9509.T) , opens new tab Tomari nuclear plant had secured approval from local communities and were moving toward restarts.

https://www.reuters.com/sustainability/boards-policy-regulation/chubu-electric-falls-tokyo-trading-after-nuclear-plant-test-disclosure-2026-01-06/