Researchers have found a possible active fault directly under the No. 2 reactor at the Tsuruga nuclear power plant in Fukui Prefecture, a discovery that could lead to the decommissioning of the reactor.
Central government rules prohibit the construction of a reactor right above a fault that could move during an earthquake.
The reactor is currently offline for a periodic inspection.
Japan Atomic Power Co., which operates the Tsuruga plant, has long argued that the faults lying under the plant would not move in an earthquake.
The finding emerged when researchers assigned by the Nuclear and Industrial Safety Agency studied the area around the Tsuruga plant.
According to Japan Atomic Power officials, about 160 faults have been found within the Tsuruga plant site.
The study on April 24 was conducted by four specialists from the National Institute of Advanced Industrial Science and Technology, Kyoto University and Fukui University. They inspected the incline at six locations near the No. 1 and No. 2 reactors as well as the proposed sites for the No. 3 and No. 4 reactors and around the Fugen reactor now being decommissioned by the Japan Atomic Energy Agency.
The scientists agreed that a fault running in a north-south direction directly under the No. 2 reactor would likely shift if an earthquake struck the area.
Although it does not lie directly under any reactor, the Urasoko Fault has been confirmed as an active fault within the Tsuruga plant site. The four researchers were also in agreement that if the Urasoko Fault triggered a quake, the fault found directly under the No. 2 reactor would most likely move at the same time.
Masaru Kobayashi, the director of the Seismic Safety Office at NISA, pointed to the possibility that the fault had moved in the past.
"It was likely influenced by the Urasoko Fault in a comparatively recent time," he said.
Japan Atomic Power officials said that the fault under the No. 2 reactor had never triggered a quake through a movement of its own and that calculations have shown that even if the Urasoko Fault does move, the fault under the reactor would not.
Company officials have long argued that the fault under the reactor would not have to be taken into consideration when compiling anti-quake design measures for the reactor.
However, faults within Fukushima Prefecture that had been thought unlikely to move did in fact move in last year's Great East Japan Earthquake, leading to debate about revising views on faults.
NISA officials instructed Japan Atomic Power officials to more thoroughly check the active faults under the Tsuruga plant.
"As long as new study results do not say otherwise, we will make a judgment based on our latest findings,” Kobayashi said. “There is a high possibility that the fault we checked could move in line with movement of the Urasoko Fault."
The No. 2 reactor at the Tsuruga plant began operations in February 1987 and has been offline since August 2011 for periodic inspection.
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