Electric utilities and the nation’s nuclear watchdog are at odds over the “science” used to calculate the risks of underground faults at nuclear power plants.
Electric utilities and the nation’s nuclear watchdog are at odds over the “science” used to calculate the risks of underground faults at nuclear power plants.
The Nuclear Regulation Authority sticks to the principle that has been applied since before the 2011 Fukushima nuclear disaster: Faults should be considered “active” unless they are scientifically confirmed to be inactive.
Utilities are banned from operating or constructing reactor buildings and other important facilities directly above active faults.
But the companies, which have applied for NRA approval to restart reactors across the country, say the NRA’s methods are “unscientific” and have provided no solid evidence to back its claims about active faults. The utilities said the authority should listen to the opinions of other experts.
Indeed, the limited amount of reliable data in surveying faults beneath nuclear power facilities has prolonged the periods for the NRA’s safety screenings of the idle reactors.
But a senior NRA official said the utilities tend to deny the existence of active faults and underestimate the effects of such structures.
“They are still trying to only gather data favorable to them and present to authorities the minimum possible impact,” the official said.
In May, an expert panel of the NRA said the possibility of the Shika nuclear power plant in Ishikawa Prefecture standing directly above active faults was real.
The judgment effectively bans Hokuriku Electric Power Co. from restarting the No. 1 reactor at the plant. If the company cannot overturn the NRA’s decision, it will have to decommission the reactor.
Under stricter safety standards introduced in July 2013, faults that have possibly shifted during the past 120,000 to 130,000 years are defined as active, the same as in the older criteria.
The new regulations also require utilities to estimate the largest possible shaking that could hit their facilities based on data concerning active faults and other geological features, and reflect the results in their earthquake-resistance measures.
But there is no clear proof that the geological faults beneath the Shika plant are active. The four panel members said it is most “reasonable” to consider the faults as active.
The committee is expected to soon compile its draft report.
One focus of the debate surrounded a sketch of the geological structure under the nuclear plant made at the time of its construction.
The NRA noted that the cross-section of the underground soil shows a wavy stratum on bedrock--a structure typical of an active fault.
The panel attempted to examine whether newer strata were affected by the fault to make a more accurate decision. But the newer layers were scraped away for the construction of the plant. And a photo taken before they were removed was too blurry to be of much help.
The committee examined the geologic formation around the removed layers but could not find any strata clearly showing when the fault moved.
Hokuriku Electric is arguing that the drawing provides zero evidence of the existence of an active fault.
For Tohoku Electric Power Co.’s Higashidori nuclear power plant in Aomori Prefecture, a report by an expert panel of the NRA said it is impossible to make a clear judgment on whether some faults running under the plant are active.
The NRA also said in its draft report released in May that faults beneath the Mihama plant in Fukui Prefecture, operated by Kansai Electric Power Co., are highly likely inactive. But the NRA said it will continue studying the relation between the faults under the facility and nearby active faults, as well as other factors.
The authority worked out a draft examination report in May concerning the safety screening of Shikoku Electric Power Co.’s Ikata power plant in Ehime Prefecture.
The Median tectonic line, a huge active fault, runs near the facility. A 480-kilometer section of the line would likely affect the nuclear plant during seismic activity, experts say. But there is no established method to estimate the effects of such a huge active fault.
Before compiling the draft report, the NRA and Shikoku Electric spent more than a year discussing the likely impact on the nuclear plant in a possible earthquake based on two scenarios: smaller faults move separately or they shift as a large, single structure.
Kazuki Koketsu, a seismology professor at the University of Tokyo’s Earthquake Research Institute who used to be involved in safety screenings of nuclear facilities, said that making a definitive decision on the activeness of faults is a tricky business.
“There are many factors that make it difficult to accurately estimate fault activities, so a prediction could later prove wrong,” Koketsu said. “The related parties should recognize that meeting the minimum standards does not ensure safety.”