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No established method to evaluate potential of active faults

December 21, 2012


NRA to examine method for fault impact forecast on nuke plants





In the wake of Japan's nuclear watchdog's recognition that the faults running under the Higashidori nuclear power plant in Aomori Prefecture are active, experts voiced difficulties in evaluating the potential effect of active faults on other nuclear facilities situated nearby, suggesting the need to examine methods for such fault impact forecasts.

The team of experts under the Nuclear Regulation Authority (NRA) concluded on Dec. 20 that the 10 faults running under the premises of the Higashidori Nuclear Power Station operated by Tohoku Electric Power Co. are a sequence of active faults, raising the need to review the facility's quake resistance even though active faults are not running directly underneath such key facilities as the reactor building.

The NRA has started examining methods for assessing the impact of active faults on nuclear facilities on the assumption that such faults could be found running close by key facilities at other nuclear power stations one after another. Based on the new evaluation methods, the NRA will determine whether relevant nuclear facilities can be reactivated or not.

"There are no established methods to adequately evaluate (the effect of) active faults (running near nuclear plants)," said NRA Commissioner Kunihiko Shimazaki after a panel meeting on Dec. 20.

Unlike the active faults found right beneath the reactor building at the Tsuruga Nuclear Power Station in Fukui Prefecture, the active faults under the Higashidori plant are not running right under its sole reactor building. Although the central government has not authorized construction of such key facilities as reactor buildings above active faults, it has approved the operation of reactors away from active faults as long as the anticipated seismic intensity and its effect on the facility were predicted based on calculations and the facility's safety was confirmed.

However, the method to precisely predict the effect of a tremor triggered by active faults as close as 200 meters from a nuclear plant has yet to be established. The NRA had earlier reviewed such evaluation methods at a separate expert panel meeting.

At a meeting on Dec. 7, Hiroyuki Fujiwara, a chief researcher at the National Research Institute for Earth Science and Disaster Prevention (NIED), said, "The method to evaluate active faults only 1 to 2 kilometers away (from nuclear facilities) has not been established. It would be difficult to improve evaluation methods in a short period of time." In response, Shimazaki said, "There are no adequate methods."

Shimazaki, however, stated following the Dec. 20 meeting, "Based on experience, it may be possible to predict that (the quake intensity) would be no greater than this or that. It's true there are no existing methods, but it doesn't necessarily mean we can't do anything."

The expert panel reviewing the evaluation methods for close-range active faults is poised to present a draft outline as early as January next year. "We won't be able to judge unless we come up with criteria. We'd like to wait for the discussions (at the expert panel)," said NRA Chairman Shunichi Tanaka.

When Tohoku Electric Power Co. applied for permission to construct the No. 1 reactor at the Higashidori plant in 1996, the utility reported to the central government that there were no active faults under the plant's premises. Furthermore, the utility stated in its interim report submitted to the government in 2008 in response to the 2006 revision to the government's seismic-resistance design screening guidelines that "There are no active faults in the neighborhood within 5 kilometers (from the Higashidori plant)."

"The old sketches (of layers studied by Tohoku Electric) lack credibility," said Shimazaki.

Tohoku Electric has heretofore anticipated an earthquake whose intensity is estimated to reach 450 gals based on a magnitude-6.8 common scenario earthquake used at the country's all nuclear plants without specifying the epicenter.



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