Japan Atomic Power Co. may have to decommission one of its reactors after seismologists concluded the plant is sitting over an active fault line, potentially the first permanent shutdown of a nuclear unit in Japan since the Fukushima disaster last year.
A panel of seismologists and geologists with Japan's Nuclear Regulation Authority (NRA) has been reviewing geological records and this month visited the Tsuruga nuclear power plant to watch the results of drilling and other tests.
"There is no way we can carry out the safety assessments (that are required) for a restart," the chairman of the NRA, Shunichi Tanaka, said on Dec. 10 at an open meeting after being presented with the panel's assessment that there is an active fault under the plant's No. 2 reactor.
The government in Japan, one of the world's most seismically active countries, does not allow nuclear plants to be situated over active fault lines.
But about 160 geological fault lines cut across the premises of the Tsuruga nuclear power plant, including some that lie directly beneath its two reactor buildings. In addition, the Urasoko fault, which is active, runs about 200 meters east of the reactor buildings.
Much attention has been given to whether the fault lines situated directly beneath the reactor buildings could slide in tandem with the Urasoko fault when the latter shifts.
A fault line called D-1, situated directly beneath the No. 2 reactor building, was assessed to have moved in the past in tandem with the Urasoko fault and thus likely active, according to Kunihiko Shimazaki, deputy chairman of the NRA and head of the seismic panel.
"You could call it an active fault," Shimazaki told reporters after the Dec. 10 meeting. "It likely represents simultaneous movement induced by the motion of the Urasoko fault."
The experts intensively studied the vicinity of the point where the D-1 fault branches out from the Urasoko fault during an on-site survey on Dec. 1-2. The survey revealed a new fault line near D-1, and the experts agreed that the fault line was formed under approximately the same stress that drives the activity of the Urasoko fault. The panel concluded that future slippage along that fault line could not be ruled out.
Though Tanaka has no authority to order a permanent shutdown, his comment implies he will not allow the reactor to be restarted, forcing a decision on Japan Atomic over whether to mothball the unit.
A Japan Atomic official who attended the meeting said the company would carry out further seismic studies.
The results of the Dec. 10 expert panel meeting will be presented to a regular meeting of the NRA on Dec. 12, when the agency may make an official announcement on the 1,160-megawatt reactor, the larger of two at the plant in western Japan. The No. 2 unit started operating in 1987, while the 357-megawatt No. 1 reactor started in 1970.
The NRA is reviewing possible fault lines under or near Tsuruga and five other nuclear plants as part of moves to beef up safety, and Tanaka has said any reactors sitting above won't be allowed to restart.
All but two of Japan's nuclear reactors are idled for safety checks after the Fukushima disaster, forcing the country to spend tens of billions of yen extra on fossil fuels to run power stations.
An earthquake and tsunami in March last year knocked out cooling and power at Tokyo Electric Power Co.'s Fukushima No. 1 plant north of the Japanese capital, causing the biggest release of radiation since Chernobyl in 1986.
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