Japan Atomic Power Co may have to decommission a reactor after seismologists found it was sitting over an active faultline.
"There is no way we can carry out safety assessments for a restart," the chairman of Japan's Nuclear Regulation Authority (NRA), Shunichi Tanaka, said this week at an open meeting after being presented with an assessment there is an active fault under the No. 2 reactor at the Tsuruga nuclear plant.
The government in Japan, one of the world's most seismically active countries, does not allow nuclear plants to be situated over active faultlines.
An NRA panel of seismologists has been reviewing geological records and this month visited Tsuruga to watch the results of boring and other tests.
A fault line extending from below the reactor was assessed to have moved in the past in tandem with another nearby fault, Kunihiko Shimazaki, an NRA commissioner who led the seismic panel, told the meeting.
While 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 agency will meet at a later date to 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 faultlines under or near Tsuruga and five other nuclear stations 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 billions of dollars 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 Daiichi station north of the Japanese capital, causing the biggest release of radiation since Chernobyl in 1986.