Safety experts issued warnings--and expressed utter bewilderment--during discussions on possible active geological faults running near nuclear reactors in this earthquake-prone country.
Safety experts issued warnings--and expressed utter bewilderment--during discussions on possible active geological faults running near nuclear reactors in this earthquake-prone country.
Not only did they decide that further studies were needed on potential seismic activity at certain nuclear plants, they also criticized past reports by electric power companies as well as central government appraisals that had ruled out the existence of active faults around the reactors.
"Experts on active faults would be left speechless if they were shown these results," a member of the expert panel said at the July 17 meeting convened by the Nuclear and Industrial Safety Agency (NISA).
Based on the panel’s recommendations, NISA will ask Kansai Electric Power Co. (KEPCO) to re-examine a fault running through the grounds of the Oi nuclear power plant in Fukui Prefecture. Panel members said documents presented by KEPCO cannot rule out the possibility the fault is an active one.
NISA will also ask Hokuriku Electric Power Co. to look into a fault located directly under a reactor building at the Shika nuclear power plant in Ishikawa Prefecture. Panel members said this fault was a "typical active fault."
NISA officials said they would make an early decision on what instructions to give the electric power companies for their re-examinations of the faults. But the agency said that during the new studies, it will not ask KEPCO to shut down the No. 3 reactor at the Oi plant, the first reactor to resume operations following the accident at the Fukushima No. 1 nuclear plant.
However, future operations of the reactor would have to be halted if the studies find that major reinforcement work is necessary.
And if the fault at the Shika plant is found to be active, the reactor may have to be decommissioned under central government guidelines that prohibit important nuclear plant facilities from being built directly above active faults.
In both cases, the re-examinations of the faults will take several months to complete. And additional studies could be conducted on faults near other nuclear reactors in Japan.
The magnitude-9.0 Great East Japan Earthquake and tsunami that caused the Fukushima nuclear crisis on March 11 last year has shattered trust in the safety of nuclear power facilities. Appraisals for nuclear plants’ ability to withstand natural disasters have since been strengthened.
The panel members, for example, decided to continue considering the possibility that a fault found at the Tomari nuclear plant operated by Hokkaido Electric Power Co. is active.
The fault at the Oi plant runs in a north-south direction between the No. 2 and No. 3 reactors. Although it is not directly under any reactor, the fault cuts across an intake that brings coolant water to the reactors during emergencies.
Clay has been found on the fault surface, indicating movement. Since the sediment above the fault cannot be dated, experts have warned that movement of other active faults in the area could trigger movement of the fault at the Oi plant.
The fault at the Shika plant is several hundred meters long and passes directly under the No. 1 reactor.
In 1987, when Hokuriku Electric applied for permission to construct the No. 1 reactor, the utility explained that the fault was created through erosion and not active.
However, after the Great East Japan Earthquake, NISA officials took another look at past documents and found that the fault may have moved 120,000 to 130,000 years ago, a relatively short period in geological terms.
Previous assessments of the fault at the Shika plant drew confusion among panel members at the July 17 meeting, where they criticized the laxity of the appraisals of anti-quake safety measures at nuclear plants.
For example, the panel was shown a document about the results of drilling when the application was submitted to construct the No. 1 reactor at the Shika plant.
"This is a representative example of a typical active fault running under the reactor,” Toshifumi Imaizumi, a geology professor at Tohoku University, said. “I am flabbergasted that this could have passed the appraisal."
Although techniques had advanced by the 1980s to determine active faults through drilling, the application submitted for the Shika plant in 1987 said the fault was not active.
Anti-quake guidelines at that time also limited the definition of active faults to those that had moved much more recently than under the current definition.
When an application to construct the No. 3 and No. 4 reactors at the Oi plant was submitted in 1985, photos were attached of the drilling studies conducted on the fault that is now in question. But the entire fault was not shown, and some panel members questioned the accuracy of the document itself because of the poor quality of the photos.
Experts in the past have pointed to the possibility of an active fault under the Oi plant, but KEPCO officials have insisted that there were no potential problems.
Even in June this year, when the central government approved the resumption of operations at the Oi plant, industry minister Yukio Edano said, "At the present time, we have not come across new knowledge."
Mitsuhisa Watanabe, a professor of tectonic geomorphology at Toyo University, has also argued that a fault within the grounds of the Tsuruga nuclear power plant operated by Japan Atomic Power Co. was active.
"This has revealed the sloppiness of the reports submitted by plant operators as well as the appraisal by the central government," Watanabe said. "The responsibility of not only the plant operator, but also NISA and related experts is extremely large."
The catalyst for re-examining faults near Japan’s nuclear plants was the magnitude-7.0 quake that was triggered when a number of faults moved in southern Fukushima Prefecture in April 2011. The central government and electric power companies had long insisted that one of the faults that moved was not active.
After that quake, the central government issued instructions to take another look at the faults.
The central government revised its anti-quake guidelines in 2006, providing an opportunity to confirm and reappraise the safety of nuclear plants.
But NISA never set a clear deadline for electric power companies to submit their reports, leading to delays in the work.
In addition, priority was given to evaluating faults with the potential to cause large swaying that could affect reactor design. Examinations of smaller faults within plant grounds were placed on the back burner.
Before the reappraisal work was completed, the Great East Japan Earthquake struck.
Changes in rules and definitions have also contributed to the need to re-examine the faults.
The rule that reactors should not be constructed directly above a fault was only clearly delineated in December 2010, three months before the Great East Japan Earthquake. The central government had previously said reactors would not be built above active faults, but the definition of an active fault was left vague.
One scholar who was involved in compiling a handbook for appraising the relationship between faults and nuclear plants was Kojiro Irikura, professor emeritus of seismology at Kyoto University.
"There has been no major change in the members of the appraisal committee from before last year's disasters," Irikura said. "However, after 3/11, those members have become much more careful in their evaluations."
The heightened scrutiny extends beyond the Oi and Shika nuclear plants.
Japan Atomic Power is re-examining faults around the Tsuruga plant. The Urazoko active fault is within the plant grounds, and experts have pointed to the possibility that a fault running north-south directly under the No. 2 reactor could move in conjunction with movement in nearby faults.
The active fault known as Shiraki-Nyu is located about 1 kilometer east of the Mihama nuclear plant operated by KEPCO and about 500 meters west of the Monju prototype fast breeder reactor run by the Japan Atomic Energy Agency (JAEA).
There are also nine faults within the grounds of both the Mihama plant and Monju reactor.
While both KEPCO and the JAEA have said there are no signs of recent activity among those faults, NISA officials have suggested further studies into whether those faults could be affected by movements of other faults.
Hokkaido Electric had also indicated that the 11 faults within the grounds of the Tomari plant were very old, and that there were no problems with anti-quake safety measures. NISA officials have asked the utility to confirm the safety under the current conditions.
NISA officials have also asked Tohoku Electric Power Co. to take another look at faults within its Higashidori nuclear plant. While company officials insist there are no problems with safety, experts have pointed to the possibility that some of the faults could be active.
(This article was written by Ryuta Koike, Jin Nishikawa and Toshio Kawada.)