25 Avril 2012
April 26, 2012
A panel of the Nuclear and Industrial Safety Agency (NISA) has suggested that faults beneath the Tsuruga nuclear power plant in Tsuruga, Fukui Prefecture, may be active--potentially putting the plant in violation of the government's criterion that prohibits building nuclear reactors above active faults.
NISA, which is examining earthquake-resistance capabilities of nuclear power plants, released Tuesday the results of the hearing panel's inspection of the plant of the Japan Atomic Power Co. (JAPC)
The burden is now on JAPC to prove the faults are not active. The hurdle for reactivating the reactors at the Tsuruga plant has thus become extremely high.
What the NISA panel sees as problematic are faults in zones of crushed rock left fragile by past earthquakes.
There are about 160 such crushed-rock zones on the plant's premises, including spots just below its Nos. 1 and 2 reactors.
Though JAPC knew of the crushed-rock zones when it applied for permission to construct the plant in 1965, the company's geological research apparently led it to believe the zones showed no significant signs of seismic activity.
However, the Great East Japan Earthquake changed the stress patterns applied to the layers of rock beneath Japan, and it may now be easier for earthquakes to be triggered by different mechanisms from past ones. Therefore, reexamination of past research on faults has become necessary.
The latest research found a possibility that the crushed-rock zones beneath the nuclear plant may move together with a nearby active fault, known as the Urasoko fault.
JAPC has conducted research on the possibility since February.
On Tuesday, four experts inspected four locations, including places where parts of the crushed-rock zones are visible on the surface of the ground, and confirmed clear signs of the existence of faults.
Shinji Toda, associate professor of Kyoto University's Disaster Prevention Research Institute, who conducted the inspection, said, "It's highly likely that they have moved, being pulled by the Urasoko fault, within the past several hundred thousand years."
The other three experts concurred.
Based on the new research, NISA asked JAPC to conduct additional drilling research and more detailed analysis of components of the geological layers near the Urasoko fault.
JAPC has already submitted first-stage results of stress tests on the Tsuruga plant's No. 2 reactor as a precondition to reactivating the reactor.
But before evaluating the stress test results, a prerequisite to restarting the reactor, JAPC now must prove with detailed geological research that faults under the reactor buildings are not active and will not move together with the Urasoko fault.
Thus it has become extremely uncertain whether the plant's reactors will be able to be reactivated.
Experts have also voiced doubts about interlocking active faults beneath Monju, a fast breeder reactor of the Japan Atomic Energy Agency in Tsuruga; Hokkaido Electric Power Co.'s Tomari nuclear power plant; Tokyo Electric Power Co.'s Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture; and Chugoku Electric Power Co.'s Shimane nuclear power plant in Matsue.
Detailed research on those nuclear plants is also under way.
The Tsuruga plant is the nation's oldest commercial nuclear power plant. Its No. 1 reactor, with an output capacity of 357,000 kilowatts, began operation in 1970. The No. 2 reactor, with an output capacity of 1.16 million kilowatts, started operation in 1987.
Both reactors have been idled since their most recent regular inspections. There are plans to build two more reactors at the plant, but the government's pre-construction safety checks have been suspended because of the crisis at the Fukushima No. 1 nuclear power plant.
Active faults are those with a record of repeated activity that also exhibit the potential to move in the future.
The Cabinet Office's Nuclear Safety Commission revised in 2006 its safety guideline on nuclear power plants' abilities to withstand earthquakes.
The revised guideline stipulates that faults believed to have moved since the Late Pleistocene age--120,000 to 130,000 years ago--are called active faults.