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

So, is it safe or not?

November 6, 2012


Experts split over Oi plant fault / Inability to agree raises questions over whether reactors are safe

Experts remain divided over whether an active fault runs under Kansai Electric Power Co.'s Oi nuclear power plant in Fukui Prefecture, raising questions about whether it is safe to continue operating two reactors at the plant.

On Friday, outside experts participated in the Nuclear Regulation Authority's inspection of the No. 3 and No. 4 reactors, currently the only active reactors in the country. The experts were unable to agree at a meeting held Sunday to assess the results and determine whether a crush zone underneath the plant constitutes an active fault.

The NRA, which was inaugurated in September to revamp the country's nuclear regulatory framework, decided to hold another meeting Wednesday.

The inspection was the NRA's first survey of the area beneath a nuclear plant. It was held after the now-defunct Nuclear and Industrial Safety Agency, the nuclear authority's predecessor, told Kansai Electric in July to again look into a crush zone, known as F-6, running directly underneath an emergency water intake channel for reactors at the Oi plant.

The agency made the request to the power utility during the course of reviewing the assessment of faults under nuclear plants around the nation after the Great East Japan Earthquake.

The agency claimed data on geographical features was not sufficient to prove the crush zone beneath the Oi plant is not an active fault.

At the end of October, Kansai Electric reiterated in an interim report to the government there is no data suggesting the crush zone is an active fault. In the report, Kansai Electric said traces of F-6 could not be confirmed at a trench made near the beach to conduct the fault survey, and that the crush zone might be shorter than expected.

This is why the discovery in the same trench, by a team of experts sent by the NRA, of a trace of displaced layers that could have been caused by the movement of an active fault, came as an unexpected development.

In particular, Mitsuhisa Watanabe, a professor at Toyo University, asserted the displacement of layers at the location clearly suggests movement of the fault in the period up to 120,000 to 130,000 years ago.

Watanabe, who had raised questions about the crush zone, also said Kansai Electric was wrong in its projection of the location of F-6. "The active fault we confirmed is the real F-6," Watanabe said.

Daisuke Hirouchi, associate professor at Shinshu University, said Watanabe's interpretation of inspection results is consistent.

But Atsumasa Okada, a professor at Ritsumeikan University, was cautious in the interpretation of the survey results and said such displacement of layers can be caused by landslides and other factors.

Norio Shigematsu, a senior researcher at the National Institute of Advanced Industrial Science and Technology, said the direction of forces acting on bedrock in the area did not match the direction the fault is likely to take.

This difference in opinion is due to the difficulty in determining by mere inspection of displacement traces whether layers were moved by an earthquake.

The same traces could result from the sudden breakup of bedrock or more incremental moves due to events such as landslides.

The location and shape of the fault will need to be examined in detail at various spots before drawing a final conclusion.

Under guidelines for earthquake resistance of nuclear plants, faults are considered active if they have moved in the last 120,000 to 130,000 years.

However, according to the government's Headquarters for Earthquake Research Promotion, which calculates earthquake probability, faults are considered active if they have experienced movement within the past several hundred thousand years.

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