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Tsugara should be decommissioned straight away

May 15, 2012


Warnings over fault below nuke reactors in Fukui were ignored



The possibility that a fault right below the nuclear reactor buildings at the Tsuruga Power Station in Fukui Prefecture may move in conjunction with nearby active faults has been repeatedly pointed out since 2008, but the government regulator and the plant's operator failed until recently to take any measures.

The Japan Atomic Power Co. (JAPC), the operator of the Tsuruga nuclear power plant, only released its plan on May 14 this year to survey the area to examine the possibility that the fracture zones -- a type of fault -- right below the plant's nuclear reactor buildings could in fact be active faults. The planned survey -- scheduled to be completed by November -- was approved by the government's Nuclear and Industrial Safety Agency (NISA) later the same day.

If the fracture zones are recognized as active faults that had moved sometime after around 120,000 to 130,000 years ago, the Tsuruga nuclear plant is likely to be decommissioned. It has been confirmed that there are 150 to 160 fracture zones on the premises of the plant in Tsuruga. On April 24, NISA surveyed three fracture zones, including two running below the building housing the plant's No. 2 reactor, raising the possibility that they could move in tandem with an active fault called the Urasoko Fault, located some 150 meters northeast of the No. 2 reactor.

On May 14, NISA held a meeting of experts to discuss the issue and approved JAPC's plan to report the survey results by November. While JAPC is poised to underscore its claim that the fracture zones are "not active faults" by conducting boring surveys at five locations at the nuclear plant, the plant cannot be reactivated unless the operator can provide evidence supporting these assertions.

Yuichi Sugiyama, the head of a research team at the National Institute of Advanced Industrial Science and Technology (AIST), which surveyed fault lines at the Tsuruga nuclear plant, said at the NISA meeting on May 14, "The possibility that the fracture zones are active faults cannot be ruled out at the moment. We should obtain reliable survey results, even if it takes time."

The presence of the fracture zones below the Tsuruga nuclear plant was mentioned in the application for permission to construct the No. 1 reactor, which was approved in 1966. However, it was determined at the time that the fracture zones were small-scale "dormant faults" from extremely old times, and they were not taken into consideration for the plant's seismic-resistant design. The presence of the Urasoko active fault -- located some 250 meters away from the No. 1 reactor -- had not been known by that time.

It was in 1991 that the presence of the Urasoko Fault came to surface. While it had initially been thought that the fault was about 3 kilometers long, several faults were later discovered to exist along its extension. Experts pointed out the risk of the faults moving together, but JAPC only acknowledged in March 2008 that they were active faults about 25 kilometers long.

Several experts had also earlier pointed out the possibility that the fracture zones at the Tsuruga nuclear complex could move in conjunction with the Urasoko Fault and could have a critical impact on the nuclear reactors. However, JAPC submitted a report to NISA in 2008, insisting that the fracture zones' activity period dates back to earlier times and that they would not move in tandem with the Urasoko Fault.

Mitsuhisa Watanabe, professor at Toyo University and a specialist in active faults, was skeptical about the JAPC report at the time. "Old fracture zones are consolidated and become stiff, but the report does not use such expressions as 'stiff' at all." The distribution of the fracture zones at the Tsuruga plant also convinced him that they were apparently linked to the Urasoko Fault. Watanabe has thus repeatedly pointed out the possibility of the faults moving together during academic meetings and other occasions since 2008.

However, JAPC and NISA failed to take immediate action. Even after an opinion was voiced demanding an in-depth examination of the issue during a council meeting at NISA in September 2010, the agency did not conduct an on-the-spot survey at the Tsuruga power station. It was only after the nuclear disaster at the Fukushima No. 1 Nuclear Power Plant in March last year that NISA finally moved ahead to conduct a field survey at the Tsuruga plant, in April this year.

During the survey, the fracture zones at the Tsuruga plant were found to be soft when scraped with sickles and extend linearly on the land surface. All four experts who took part in the survey agreed that the fracture zones "cannot be determined to be old faults." Masaru Kobayashi, director at NISA's seismic safety office, said remorsefully, "I should've ordered a survey much earlier."

Says professor Watanabe, "Why did they fail to conduct the survey for such a long time on something that can be so easily understood by visiting the spot? It's not academic research but an argument for safety. The plant should be decommissioned right away."

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