Chubu Electric Power understated the seismic ground motion used as the basis for quake-resistant design at the Hamaoka nuclear plant in Shizuoka Prefecture by at least 10% to 20%, the Nuclear Regulation Authority said at a regular meeting on October 28.
It is the first time the level of the ground motion before the data was tampered with has been made public. The utility had improperly manipulated figures for the "standard seismic ground motion" — the benchmark shaking a plant must be built to withstand.
A report compiled in September by Chubu Electric's external investigation committee said the pre-manipulation values were "results that could exceed the safety standard values for the facility and the ground." That leaves open the possibility the utility will be required to carry out additional reinforcement work if it seeks to restart the reactors.
Chubu Electric withdrew its application for the safety screening of reactors No. 3 and No. 4 in September. It plans to recalculate the standard seismic ground motion by the correct method and publish the results.
Why the figures matter
The standard seismic ground motion is the design basis for a plant's earthquake resistance, so a figure set too low understates the shaking the facility is built to survive. The manipulation at Hamaoka is the kind of lapse in a safety filing that has weighed on public confidence in nuclear power since the 2011 Fukushima Daiichi accident.