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Suspicions at Oi remain

December 29, 2012


Suspicions of active fault under Japan's only operational nuke plant persist


A team from the Nuclear Regulation Authority conducts a survey of geological layers at the Oi nuclear power plant in Oi, Fukui Prefecture, on Dec. 28. (Mainichi)




Suspicions remain that the Oi Nuclear Power Plant in Fukui Prefecture sits on an active quake fault after a second survey at the site on Dec. 28.

A team from the Nuclear Regulation Authority (NRA) conducted the survey to determine whether a zone of crushed rock is an active fault, and will continue on Dec. 29 with results to be evaluated at a meeting in January. The Oi plant is now the only nuclear power station in Japan that is online.

The crush zone is thought to be located below an emergency channel for carrying seawater to cool the No. 3 and 4 reactors, currently in operation.

The survey team is composed of NRA acting chair Kunihiko Shimazaki and four experts from other organizations. The survey on Dec. 28 was done at the northern edge of the plant, with an excavation trench from the last survey expanded east by around one meter and west by around 40 meters.

The first survey in November found a mismatch in geological layers at the site, explained by plant operator Kansai Electric Power Co. as due to a landslide, which would leave a U-shaped bend in the layers. According to the power company, the mismatch was found at the eastern edge of the landslide, and it continued to insist on this explanation after the latest survey on grounds that a layer mismatch was also found in the western part of the expanded trench.

The survey team members all withheld their conclusions on the mismatch's cause, saying they wanted time to consider the matter. However, one of them, Toyo University professor Mitsuhisa Watanabe said he "had doubts" about the landslide hypothesis. Shimazaki, Shinshu University associate professor Daisuke Hirouchi and National Institute of Advanced Industrial Science and Technology head researcher Norio Shigematsu all say they are not satisfied with the landslide explanation. However, Ritsumeikan University professor Atsumasa Okada said, "The possibility (that it was from a landslide) is fairly high."

NRA team starts second fault survey at Oi plant



OI, Fukui Pref. — A Nuclear Regulation Authority team started a second survey Friday at the Oi nuclear power plant to determine whether there are active faults beneath it.

If the team confirms the presence of an active fault in the two-day study, Kansai Electric Power Co. will have to stop reactors 3 and 4, currently the only active reactors in Japan.

The previous on-site survey was conducted in November.

The team is investigating a crush zone, or a fractured area in bedrock, that is believed to run underneath a seawater intake channel for the emergency cooling system for reactors 3 and 4.

After the first study, the team was unable to rule out that the crush zone known as F-6 is an active fault. But the team did not reach a final conclusion because it could also not exclude the possibility that F-6 was caused by landslides.

The second survey includes inspections of pits newly dug by Kansai Electric, which is engaged in drilling work under an order from NRA deputy chief Kunihiko Shimazaki, who heads the team.

Kepco has dug deeper and wider pits in a northern corner of the plant's premises to see whether F-6 reaches the area. More drilling requested by the authority has not finished, but Shimazaki said the team may be able to reach a conclusion without waiting for that to happen.

At a conference held prior to the second survey, Shimazaki noted that Kepco people have complained that at least some of the research appears unnecessary. "But we do not see anything wasteful," Shimazaki stressed.

F-6 is believed to run north to south between units 2 and 3. If F-6 is active and moves, the emergency seawater intake channel for units 3 and 4 may be affected.

The channel is a crucial facility that would be used if the reactors lose normal cooling functions. Japan bans construction of nuclear plants on active faults.




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