Reversing an earlier assessment, the Nuclear and Industrial Safety Agency admitted an active and dangerous fault may be lurking directly beneath one of the two reactors of the Tsuruga nuclear plant in Fukui Prefecture.
The surprising judgement by NISA, announced Tuesday night, would force Japan Atomic Power Co. to decommission the plant’s reactor 2 if the fault is confirmed as active.
NISA would also find it difficult to restart reactor 1. Both reactors were shut down for regular checks, but have not been restarted in light of the meltdown crisis that hit the Fukushima No. 1 plant in March 2011. In light of the Fukushima crisis, all reactors are subject to stress tests to gauge their disaster survivability.
The government’s quake safety standards do not allow any nuclear reactor to be built right above an active fault or one that could move when a nearby fault causes an earthquake.
On Tuesday, based on results of the latest surveys by outside experts, NISA requested Japan Atomic Power to make a detailed assessment of the two faults located beneath the plant’s compound.
According to NISA, a 35-km-long fault named Urazoko runs beneath the compound of the Tsuruga power plant, and experts believe it caused an earthquake there around 4,500 years ago or later.
Another unnamed fault runs right beneath reactor 2, and the experts now believe the fault could slip and critically damage that reactor if the Urazoko fault causes an earthquake there.
Regulatory standards define a fault as being “active,” or likely to cause an earthquake, if it is believed to have caused a quake about 120,000 to 130,000 years ago or later.
The new findings at Tsuruga could also affect safety debates over reactors nationwide, because whether a nuclear plant is built on a location safe from an active fault has often been a contentious issue between pronuclear factions in the government and antinuclear activists.
Japan Atomic Power will compile an assessment for NISA as soon as possible and conduct a detailed survey of the site, spokesman Mitsuru Marutani told The Japan Times on Wednesday.
“The four experts who conducted the on-site survey were all geologists, but no geomorphology experts were included. So we need to discuss the issue further at a panel meeting that includes their views,” Yukinari Nakagawa, an official at the Ministry of Economy, Trade and Industry, said Wednesday.
NISA is under the jurisdiction of METI.
The Tsuruga nuclear plant, which started operations in 1970, is home to one of only two reactors in the country that are over 40 years old. Reactor 2 began operating in 1987.
A government-sponsored bill submitted to the current Diet session would limit the life span of reactors to up to 40 years in principal, meaning reactor 1 is likely to be decommissioned if the bill is enacted.
Last August, Japan Atomic Power reported that there was no evidence the fault in question would move together with the Urazoko fault.
Electricity from the plant has been sent to Kansai Electric Power Co., Hokuriku Electric Power Co and Chubu Electric Power Co.
The on-site survey was conducted Tuesday by 10 panel members and geologists. The group studied geological layers at four points at the compound.
Information from Jiji and Kyodo added