11 Décembre 2012
It has been more than 30 years since the Urasoko fault, which lies near the Tsuruga nuclear power plant in Fukui Prefecture, was first suspected of being active.
"If we had known that there was an active fault on the Tsuruga site, we would never have built nuclear reactors there," Nuclear Regulation Authority (NRA) commissioner Kunihiko Shimazaki, head of the team of experts who examined the Tsuruga nuclear station, said at a news conference on Dec. 10.
Not only has it been more than three decades since the Urasoko fault was suspected of being active, the central government took two years to acknowledge a crush zone under the Tsuruga plant's No. 2 reactor could move in tandem with the major fault line after that fact had been pointed out.
Nagoya University professor Yasuhiro Suzuki says that suspicions have surfaced again that "unreasonable interpretations of inspection results" had been made repeatedly, resulting in lax safety assessments. A similar pattern is also seen at other nuclear plants, meaning many other reactors may face challenges to reactivation.
The Urasoko fault was unknown at the time construction of the Tsuruga station's No. 1 reactor was approved in 1966, but in the 1970s experts began pointing out the fault could be active. Tsuruga plant operator Japan Atomic Power Co. applied to build the plant's No. 2 reactor in 1979, insisting that the Urasoko fault was not active. Although the central government instructed Japan Atomic Power to conduct additional research and dig a test trench during the screening process, it accepted Japan Atomic Power's assertions and green-lit the No. 2 reactor in 1982.
Showing drawings from additional research at the NRA panel meeting on Dec. 10, Suzuki said, "We can clearly see a fault structure. It is regrettable that such information was not used."
In "Active Faults in Japan: A New Edition," published in 1991, the Urasoko fault was introduced as an active fault. When Japan Atomic Power applied to build a third and fourth reactor at Tsuruga in 2004, however, the utility declared that the Urasoko fault was not active.
In 2005, the now-defunct Nuclear and Industrial Safety Agency (NISA) at the Ministry of Economy, Trade and Industry instructed Japan Atomic Power to conduct additional research. As instructed by the agency and experts, the company dug a trench and found traces of fault movement less than 4,000 years ago. At long last in 2008, the company acknowledged that Urasoko was an active fault.
Experts estimated that the 25-kilometer-long Urasoko fault could cause quakes as large as magnitude 7.2, resulting in ground motion of up to 800 gals -- far higher than the 532 gals projected previously. Japan Atomic Power, however, has insisted that its reactor and other buildings would have no problem surviving such a temblor. In April 2010, NISA concluded that Japan Atomic Power's assertions were reasonable.
Multiple experts disagreed, and issued a warning stating, "Because the Urasoko fault lies extremely close (to the Tsuruga plant), the possibility of crush zones on the plant premises, including those beneath reactor buildings, jolting out of alignment must be taken into account."
Despite this, Japan Atomic Power did not conduct any further on-site checks before the Great East Japan Earthquake of March 2011. In November 2011, NISA instructed Japan Atomic Power to conduct inspections. During inspections made in April together with experts, suspicions emerged that the crush zone right beneath the No. 2 reactor is an active fault that could move in tandem with the Urasoko fault.
Furthermore, in March this year a team of experts from the National Institute of Advanced Industrial Science and Technology concluded that the Urasoko fault is in fact at least 35 kilometers long and could trigger 7.4-magnitude earthquakes that release twice the energy previously estimated.
NISA started to conduct inspections of faults on the premises of all nuclear power plants across the country in July this year, and the NRA took over the mission thereafter.