The Nuclear Regulation Authority (NRA) on July 27 concluded that there is no need to review the maximum possible earthquake estimate -- known as the standard ground motion -- for Kansai Electric Power Co.'s Oi Nuclear Power Plant in Fukui Prefecture.
The NRA reached the conclusion at a regular meeting after former acting NRA chairman Kunihiko Shimazaki pointed out that Kansai Electric had "underestimated" the calculated standard ground motion for its Oi plant. The NRA said that the result of Kansai Electric's calculation was reasonable. The NRA then dismissed Shimazaki's argument by saying that calculation methods other than the current one used for the Oi plant "have not reached a degree of scientific and technological maturity."
Shimazaki had earlier suggested that the so-called "Irikura-Miyake method" used by Kansai Electric was the cause of the underestimated standard ground motion. The NRA's secretariat checked the validity of other methods such as the "Takemura method," but it concluded that ways of taking into account the "uncertainties" involved in predicting standard ground motions have not been established. Five NRA commissioners approved the secretariat's verification results.
A string of issues over the calculations of standard ground motions raised questions about the NRA's expertise.
After recalculating the estimated standard ground motion for the Oi plant using the "Irikura-Miyake method" -- the same method used by Kansai Electric -- the NRA secretariat found that the recalculated estimate was 356 gals, "gal" being a unit of acceleration. Its recalculation based on the "Takemura" method showed 644 gals. These two figures fell below Kansai Electric's estimate of 856 gals. Therefore, the NRA secretariat determined that Kansai Electric's figure was not "underestimated." The NRA approved the secretariat's findings on July 13.
On July 19, the NRA secretariat effectively withdrew its findings, saying that "They were unreasonable calculations." Thus, it came to light that the NRA had confirmed the secretariat's findings without verifying the validity of the calculations. It also came to light that the NRA had not grasped the detailed process of Kansai Electric's calculation as the secretariat's calculation result conflicted with that of Kansai Electric. The NRA approved Kansai Electric's calculation of the standard ground motion in the autumn of 2014, but questions were subsequently raised about the way in which the screening was conducted.
Among the five NRA commissioners is a geologist, but there is no expert on ground motion. At a news conference on July 27, NRA Chairman Shunichi Tanaka acknowledged that his group was lacking expertise, saying, "That's what we need to reflect on." But when he met Shimazaki on July 19, Tanaka bluntly said, "There is no room for listening to outside experts nor am I in a position to do so." As the biggest lesson learned from the Fukushima nuclear crisis ought to be that the most up-to-date expertise should be reflected in safety measures, the NRA is urged to listen to arguments and suggestions from outside experts.