The Nuclear and Industrial Safety Agency (NISA) will reopen its investigation into whether Tohoku Electric Power Co.'s Higashidori plant in Aomori Prefecture is sitting on active seismological faults.
Studies of faults underneath Higashidori, as well as faults at Hokkaido Electric Power Co.'s Tomari plant in Hokkaido and the Japan Atomic Power Co.-operated Tsuruga plant in Fukui Prefecture, were suspended following the nuclear accident at the Fukushima No. 1 nuclear power plant.
Tohoku Electric has insisted that the faults "would not trigger seismic activity in the future" and pose no safety risks to the single reactor plant, which is currently suspended for routine maintenance, but some experts at NISA are unconvinced.
In response to a study that showed displacement of geological layers, possible evidence of an active fault, the utility said it resulted from localized and rapid swelling of clay. The firm launched a fresh excavation last year to back its claim.
That excavation showed geological layers pushing upward at the fault, a typical feature of many active faults, but the company restated its position that the faults are inactive to NISA in January.
Shinji Toda, associate professor of earthquake geology at Kyoto University, said: "There is no theory that swelling of clay causes such a displacement. Even if they are not active faults that might be directly responsible for an earthquake, compelling research on them is needed."
If the faults are found to be active, Tohoku Electric would be required to revise costly analysis of the plant's tolerance of ground movement due to an earthquake. The analysis would be required to clear the government's "stress test," which must be passed before reopening.
Similar displacement of geological layers has also been discovered at Tokyo Electric Power Co.'s Higashidori plant, which neighbors Tohoku Electric's plant, and at the Electric Power Development Co.'s (J-Power) Oma plant in the same prefecture. TEPCO and J-Power's facilities are under construction.
Tohoku Electric's survey was conducted in response to revisions to the government's quake resistance guidelines in 2006. Those guidelines urged plant operators to prepare for the movement of active faults around their plants and for more powerful earthquakes.
Tohoku Electric, TEPCO and J-Power released an interim report each in March 2008, saying that at least one reactor at each plant was not threatened by safety problems.
NISA has yet to examine the operators' estimates of the threat from tsunami to their plants, a subject that has seen significant changes in standards over the past 30 years.
Operators of nuclear plants built prior to the late 1980s tended to claim that no tsunami were expected at their plants or that tsunami could not reach their premises. In the late 1980s, a methodology for predicting the height of possible tsunami striking some plants was introduced and, in 2002, utilities reviewed their tsunami preparedness assessments at their initiative.
NISA has not, however, verified those reassessments. And, since preparation for powerful earthquakes took precedence over the tsunami threat under the 2006 guidelines, it has only completed the examination of tsunami preparedness at some plants.
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