28 Juillet 2016
July 28, 2016 (Mainichi Japan)
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.
July 27, 2016
Japan's nuclear watchdog says it will not change the assumed maximum quake level for a nuclear power plant in central Japan in its screening for a restart.
Two reactors at the Ohi nuclear power plant in Fukui Prefecture, on the Sea of Japan coast, are being screened by the Nuclear Regulation Authority to see if they meet requirements introduced after the Fukushima Daiichi nuclear accident in March, 2011.
The authority said on Wednesday that the current quake calculation for the 2 reactors is safe enough. It said the lengths of relevant fault lines are estimated as being longer than the projected lengths.
The decision came in response to a request from a former authority member for the size of assumed maximum jolts to be recalculated.
The former member told the authority last month that the maximum jolt calculated using the current formula may be too small for some reactors.
The authority conducted a recalculation using a different formula. But the result showed a smaller quake level.
The authority concluded on Wednesday that the recalculation method is not trustworthy and the figure obtained cannot be used for a comparison with the initial one.
Some experts are calling for a more detailed calculation method, or are questioning whether the current method is appropriate.
NRA Chairman Shunichi Tanaka has indicated that the authority will not review its calculation method unless experts present new opinions on the issue.