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information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise

Final verdict on Ohi fault (2)

No active fault under Oi plant, NRA team says




A team from the Nuclear Regulation Authority said Monday that no active geological faults exist under the key facilities of the Oi nuclear power station in Fukui Prefecture.

The five-member team reached its conclusion after inspecting the Kansai Electric Power Co. facility from November to July and determining that the “F-6 crush zone” fault under one of the facilities is not active.

NRA Commissioner Kunihiko Shimazaki, who personally led the team, said an effective consensus had been formed and that further discussions will be held before it drafts its report to the NRA. The future talks will involve outside experts, he said.

The No. 3 and No. 4 units at Oi are Japan’s only active commercial nuclear reactors. No. 3 was set to be taken offline Monday for a routine checkup and will be followed by No. 4 on Sept. 15, marking only the second time Japan has without atomic power.

To ensure the reactors can be restarted as soon as possible after the checkups, Kansai Electric applied in July for an NRA inspection of the Oi complex under new nuclear safety standards drafted after the Fukushima crisis, which began in March 2011 at Tokyo Electric Power Co.’s Fukushima No. 1 power station in Fukushima Prefecture.

But the new nuclear watchdog hasn’t yet started the inspection because it was busy conducting research on faults and crush zones under the Oi plant. It is now known how long the inspection of the Oi plant will take.

NRA experts agree 'crush zone' at Oi nuclear plant is not active fault




Experts from a Nuclear Regulation Authority (NRA) investigative team have agreed that the so-called "F-6 crush zone" that crosses an emergency water intake channel, a key facility at the Oi Nuclear Power Plant in Fukui Prefecture, is not an active fault.

The consensus was reached at a meeting on Aug. 2 of NRA experts. Kansai Electric Power Co., the operator of the nuclear power station, had filed applications with the nuclear watchdog for reactivation, after routine inspections in September, of the plant's No. 3 and No. 4 reactors, the only reactors in the country online. The NRA, however, put the screening on hold until after it determined whether the fault is in fact active. In line with the consensus among the NRA experts, the nuclear regulators are likely to resume the screening.

The NRA has been conducting research at six nuclear facilities to see whether they stand above active faults, but this is the first time regulators have deemed a suspected active fault inactive.

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 Oi nuclear complex, and it cuts across the F-6 crush zone that stretches roughly north to south through the premises of the nuclear plant. Under the NRA's new safety regulations, building a key facility directly above an active fault is forbidden, 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.

At their previous meeting held on Aug. 19, the NRA experts agreed based on age-dating analysis of volcanic ash contained in the geological strata that the fault found in a test trench dug in the southern part of the Oi nuclear station is not active. At the Sept. 2 meeting, the NRA experts concluded that the crush zone in the southern trench and the F-6 fault observed in another test trench called the "mountaintop trench" -- which cuts through the emergency water intake channel -- were active at roughly the same time partly because of similarities in how the strata had slid. Thus, the NRA experts rejected the possibility that the F-6 fault that cuts across the emergency water intake channel is active.

NRA commissioner Kunihiko Shimazaki said, "We have reached a consensus. We now have a certain course of action to take." Nevertheless, because some experts have raised questions about Kansai Electric's assumptions regarding potential dangers to the plant, the NRA experts will continue to discuss the applications for the Oi reactors' reactivation. In addition, the NRA plans to consult with other experts before endorsing the investigative team's view that the F-6 crush zone is not an active fault.


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