18 Juillet 2012
A reactor building of the Shika nuclear power plant in Ishikawa Prefecture may have been built over an active fault, which could unleash an earthquake, the Nuclear and Industrial Safety Agency has disclosed.
The existence of an active fault below Hokuriku Electric Plant Co.'s Shika complex has been ruled out twice by the government during nuclear safety screenings.
However, NISA says the possibility of the fault being active may have been overlooked in both of the geological surveys at the plant's site.
The first of the government screenings was conducted in 1988 prior to the government's granting of permission to Hokuriku Electric to build the plant's No. 1 reactor, which became fully operational in 1993. The government gave the green light for the construction of the No. 2 reactor in 1999.
The building housing the No. 1 reactor has been built above what is now suspected to be an active fault.
NISA, the nuclear watchdog of the Economy, Trade and Industry Ministry, is scheduled to hear the views of experts Tuesday on the matter before issuing instructions to Hokuriku Electric to carry out a detailed survey on the potential fault, officials said.
As the survey will take several months, resumption of the Shika plant's operations will be delayed significantly, forcing the utility to secure alternative power sources to meet the surge in demand in the coming winter.
This is the second time a reactor building has been constructed above what is suspected to be an active fault. In April, a potentially active fault was found at the Japan Atomic Power Co.'s Tsuruga complex in Fukui Prefecture. If the existence of an active fault is confirmed, it will amount to a violation of government-set standards prohibiting nuclear plants and other important facilities from being built in locations where active faults exist.
A shallow layer of earth covers a 300-meter-long depression running beneath the southwestern corner of the Shika plant's No. 1 reactor building. The depression is about 250 meters deep, NISA officials said.
When it applied for government permission to build the No. 1 reactor, Hokuriku Electric said the depression was not an active fault but a fissure that might have been created by water erosion. Government officials did not question the utility's explanation, according to NISA.
In the wake of the Great East Japan Earthquake last year, NISA examined drawings of drilling surveys Hokuriku Electric submitted to the ministry before it received permission for building the No. 1 reactor. The examination was conducted as part of an ongoing review of possible active faults in and around nuclear plants across the country.
The findings at the Shika plant showed that the fissure was most likely a reverse fault, one that was created when the bedrock was forced upward an estimated 120,000 to 130,000 years ago.
In light of this, NISA concluded the fault should have been detected before the Shika plant was built, the officials said.
NISA is now trying to determine why the potential fault was not discovered during government screenings, but no conclusions have been reached.
A Hokuriku Electric official said, "We don't believe there is a major problem [with what is suspected to be an active fault], but we are ready to respond appropriately when NISA gives us instructions."
Mitsuhisa Watanabe, a professor of geomorphogeny at Toyo University, said: "By examining the drilling survey drawings, the fissure looks like an active fault to me.
"I wonder how the government carried out the screenings before it issued permission for the reactors' construction. The NISA must carry out investigations [of active faults] as quickly as possible at other nuclear power plants."
Regarding the possibility that active faults exist in the compounds of other nuclear plants, NISA has pointed to the immediate need for reexamining other facilities such as Kansai Electric Power Co.'s Mihama and Takahama plants in Fukui Prefecture and Tohoku Electric Power Co.'s Higashidori plant in Aomori Prefecture.
The Nos. 1 and 2 reactors of the Shika plant, with outputs of 540,000 kilowatts and 1,358,000 kilowatts, respectively, are both boiling water-type reactors. They account for 20 percent to 30 percent of Hokuriku Electric's power generation capacity.
Experts on a Nuclear and Industrial Safety Agency (NISA) panel have pointed out that a fault situated just below the No. 1 reactor at the Shika Nuclear Power Plant in Ishikawa Prefecture is highly likely to be an active fault.
Moreover, many of them called for studies of a fault fracture zone, which lies below the premises of the Oi Nuclear Power Plant in Fukui Prefecture.
NISA is taking the opinions seriously and is poised to decide by the end of this month to conduct follow-up surveys on the possible impact that the faults will have on the respective power plants.
Economy, Trade and Industry Minister Yukio Edano told a news conference following a regular Cabinet meeting on July 17, "If expert opinions show it's likely to be an active fault, we'll take prompt action."
NISA held a meeting of 16 experts, including those in the fields of quake resistance and faults, on July 17, in which many attendees pointed to the possibility that the so-called "S-1" fault below Shika plant's No. 1 reactor building is an active fault that moved around 120,000 to 130,000 years ago, or possibly later.
The reactor could be forcibly decommissioned because the government's guidelines for the quake-resistance of nuclear plants do not permit the construction of any nuclear reactors just above an active fault that moved anytime from 120,000 to 130,000 years ago.
An official of Hokuriku Electric Power Co. that operates the Shika plant reiterated its assertion that the fault was made as a result of corrosion by waves and will not trigger a powerful earthquake.
However, many of the experts countered by showing a drawing of the geographical structure of the area based on a survey and asserted that it is a typical active fault. They then called for a detailed survey on the site.
None of the experts ruled out the possibility that S-1 is an active fault, and some attendees criticized the government for approving the results of Hokuriku Electric Power's survey on the fault without sufficiently confirming the details.
The utility has already submitted its primary assessment of its safety evaluation of the Shika plant's No. 1 and 2 reactors, which is a prerequisite for reactivating them. NISA's inspection on the No. 2 reactors has entered a final stage.
However, since an additional survey is expected to take several months, there is a possibility that the resumption of operations at the reactor will be delayed.
Kansai Electric Power Co. submitted a new photo showing parts of a fault fracture zone situated below its Oi Nuclear Power Plant to the meeting of experts, and explained the condition of the fault. However, it failed to convince the experts.
"The entire picture of the fault remains unclear," one of them said.
"We can't expect to receive any more convincing materials. A survey needs to be conducted to judge whether it's an active fault," another pointed out.
The No. 3 reactor at the power station has already been reactivated and operations at the No. 4 reactor are scheduled to be resumed on July 18.
Shinichi Kuroki, deputy director general for nuclear power at NISA, denied that the experts' opinions will affect operations at the Oi nuclear plant.
"I don't think the safety of the plant has been denied. There'll be no immediate impact on its operations," he told reporters.
NISA is set to work out criteria on judging whether the faults below these power plants are active faults as well as details of follow-up surveys.
A new atomic power regulatory body to be launched in September is expected to determine whether these faults are active.