Nuclear Regulation Authority (NRA) former deputy chairman Kunihiko Shimazaki asked the nuclear power watchdog on June 16 to recalculate the maximum possible earthquake estimate -- known as the standard ground motion -- for some nuclear plants in western Japan using a different formula, since the current calculation may include underestimated figures.
The NRA will hold a meeting on June 20 and discuss handling the matter to recalculate figures for Kansai Electric Power Co.'s Oi Nuclear Power Plant in Fukui Prefecture that is now under safety screening.
After retiring from the NRA position in 2014, Shimazaki, professor emeritus at the University of Tokyo, examined one of the calculation formulas used to work out the standard ground motion for the Oi nuclear plant. The formula is known as the Irikura-Miyake recipe. As a result, Shimazaki claims that he has confirmed that in cases where an active fault at the estimated hypocenter is straight up-and-down or close to vertical, the scale of the calculated motion becomes smaller compared to the figures calculated with other formulas. The figures worked out with the Irikura-Miyake recipe for the recent Kumamoto Earthquake did not match data recorded in the actual tremor.
Shimazaki has submitted a statement to an ongoing appeal trial at the Nagoya High Court Kanazawa branch over the suspension of the No. 3 and No. 4 reactors at the Oi plant pointing out the problem with the calculation formula.
Shimazaki told NRA members, including Chairman Shunichi Tanaka, at the June 16 meeting that different formulas should be used and other measures should be considered such as re-evaluating the earthquake resistance of the plant if necessary.
According to the NRA, in addition to the Oi plant where the fault at the estimated hypocenter is either vertical or close to vertical, the Irikura-Miyake recipe is being used to calculate the standard ground motion at Kansai Electric's Takahama nuclear plant in Fukui Prefecture that has passed the NRA screening and Kyushu Electric Power Co.'s Genkai nuclear power station in Saga Prefecture that is now under safety screening.
After the meeting with NRA officials, Shimazaki told reporters that first and foremost figures for the Oi plant should be recalculated. While he suggested that recalculation should be considered for the Genkai plant, there will be little effect on the Takahama plant as it is located away from an active fault.
Kojiro Irikura, professor emeritus at Kyoto University who developed the formula in question, said the method has been scientifically proven to be effective in estimating the scale of an earthquake. He admits, however, when used to predict ground motion the formula may provide smaller figures for the scale of a tremor in cases where the angle of an active fault is close to vertical. He added, "The formula should be used with caution to avoid an underestimate."