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August 20, 2013

NRA experts agree fault found in test trench south of Oi nuclear plant is not active



Experts from the Nuclear Regulation Authority (NRA) have shared the view that the fault found in a test trench dug to the south of the Oi Nuclear Power Plant in Fukui Prefecture is not active.

The consensus was reached while the research team of NRA experts discussed on Aug. 19 whether the so-called "F-6 crush zone" that crosses an emergency water intake channel, a key facility at the nuclear complex, is an active fault.

Some of the experts expressed their cautious views that the fault is part of the F-6 fault, but it has become possible that the team of experts would conclude that the fault is not active at their meeting to be held as early as September.

Kansai Electric Power Co. applied with the NRA for safety screening of the Oi Nuclear Power Plant as a step toward reactivating the reactors there after regular inspections in September. But the NRA has put the screening on hold until after determining whether the fault is in fact active. If the NRA endorses the experts' view that the fault is not active, it will likely resume the screening process for the nuclear power station.

The emergency water intake channel is designed to send water necessary to cool down the reactors for the No. 3 and No. 4 units at the nuclear complex. It is forbidden to build a key facility right above an active fault, and therefore whether or not the F-6 fault is active has been a key factor in deciding whether to give the green light for the reactivation of the reactors.

The research team of experts conducted a survey last year of a test trench, or the "Daibahama" trench, dug near the northern fringe of the premises of the nuclear complex, as well as other facilities. But the experts were divided over whether the trench is an "active fault" or a "landslide." Another test trench called the "mountaintop trench" was dug near the emergency water intake channel, but no layers of earth that could be used to estimate active periods of the fault were left there. Therefore, the NRA instructed Kansai Electric to dig a trench on the south side of the premises of the nuclear complex as one of a few research points where old layers of earth remain.

During the meeting held on Aug. 19, the experts shared the view that the crush zone at the south side trench was not an active fault that otherwise needs to be taken into account for seismic-resistant design based on data analysis of volcanic ash and other materials in the layers.

Kansai Electric insisted that the crush zone is part of the F-6 fault. In their response, multiple NRA experts expressed their cautious view that it could not be determined whether the crush zone is part of the F-6 fault due to a lack of data.

The research team decided to discuss the issue again at their next meeting, while urging Kansai Electric to analyze existing data again. On the fact that a final conclusion was not reached, a Kansai Electric official said, "We want them to process our application quickly. We want them to understand how we feel."


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