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NRA to scrap recalculated quake ground motion data for Oi nuclear plant

The chairman of Japan's nuclear watchdog admitted in a news conference on July 20 that its secretariat's renewed method of calculating standard ground motion from earthquakes at Kansai Electric Power Co.'s Oi Nuclear Power Plant in Fukui Prefecture was insufficient.

"It was hasty. We are reflecting on this," Shunichi Tanaka, chairman of the Nuclear Regulatory Authority (NRA), said. He said the secretariat's recalculated data would be taken back to the drawing board, but expressed the view that the NRA had no intention of reviewing Kansai Electric's own calculations.

Ground motion is measured in gals -- a unit of acceleration -- with 1 gal defined as 1 centimeter per second squared.

Kunihiko Shimazaki, former deputy chairman of the NRA, had earlier pointed out that the Irikura-Miyake method initially used to calculate standard ground motion at the Oi Nuclear Power Plant produced an estimate that was too low. On July 13, the NRA secretariat released new calculations based on the separate Takemura method, which predicted that shaking at the nuclear plant could reach 644 gals. However, this was smaller than the 856 gals calculated by Kansai Electric, resulting in criticism of the calculation method. Shimazaki estimated that under the Takemura method, ground motion acceleration could reach up to 1,500 gals.

In the news conference on July 20, Tanaka said that calculations based on the Irikura-Miyake method would continue, but that the secretariat's renewed calculations based on the Takemura method could not be trusted and therefore would not be adopted.

During the same news conference, however, a representative of the secretariat expressed a divergent view, saying that the results from the two different calculation methods could be compared in relative terms.

On July 13, based on the results of its recalculation, the NRA secretariat dismissed Shimazaki's views. However, after Shimazaki met with secretariat officials on July 19, it made a turnaround and acknowledged its insufficiencies, stating, "The recalculation was stretched." Tanaka said he was aware of problems with the renewed calculation on July 14, the day after the announcement.

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