Nuclear Regulation Authority unable to decide on active fault findings at Tsuruga nuclear power plant

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A four member team of experts from the Nuclear Regulation Authority wrapped up a survey at the Tsuruga nuclear power plant in Fukui Prefecture on Sunday, investigating the status of active faults under the nuclear complex. The investigative team was lead by NRA Commissioner Kunihiko Shimazaki, a seismologist and four other nuclear experts recommended by the academic community.

The Tsuruga plant is located in the same prefecture as the Ohi nuclear power plant operated by Kansai Electric (KEPCO).  In November, the NRA sent a team of experts to inspect the Ohi plant for possible active earthquake faults underneath the site, but has been unable as of yet to agree upon a conclusion as to whether the faults detected were active.  The experts did agree that there is a strong possibility that the crush zone under the Ohi plant is located in different places than previously claimed by the operating utility KEPCO.

As no final decision was able to be made regarding the active fault status, the NRA asked KEPCO to conduct further investigation at the Ohi nuclear power plant, and delayed final judgment on whether the crush zone is an active fault until after next spring, but will allow operations to continue at the Oi nuclear plant for the time being.

After determining that the one day investigation of the Ohi plant in November did not allow enough time for inspectors, the NRA extended the inspection at the Tsuruga plant to two days.

The Tsuruga plant’s No. 1 unit started operations in 1970, making it the oldest of Japan’s 50 commercial reactors, but it was not until 2008 that Japan Atomic Power confirmed that the Urazoko fault, part of which runs beneath the facility, was active.  Previously, The Japan Atomic Power Company had insisted in past research that the crush zones have not moved in the past 130,000 years.

A crush zone is an underground fault which is formed by rocks that become weakened after being pressed between strata and turn into clay or pebbles which form a beltlike formation.  The presence of a crush zone has been confirmed directly beneath the Nos. 1 and 2 reactor buildings at the Tsuruga plant.

Government regulations for building nuclear plants do not allow construction of reactors or other important facilities above active faults, if the experts determine that the fissure would move with the fault, the plant cannot be put back online and may have to be scrapped.

The team dug the ground near the fault and a nearby fissure called D-1 that runs directly below the Number 2 reactor.  The fault is believed to have moved 4,000 years ago.   By definition a fault is “active”, when it is believed to have caused a quake about 120,000 to 130,000 years ago or later.

Team member Kunihiko Shimazaki said they confirmed that the ground right above the D-1 fissure has changed shape. But he said the team found no evidence that the fault and the fissure would move together.  Shimazaki indicated that the nuclear regulator may ask the Tsuruga plant operator to conduct additional inspections or his team may visit the plant again.

The information collected during the investigation will be analyzed at a meeting Dec. 10 in Tokyo.

“The existence of the active (Urazoko) fault raises serious concerns. And I can’t imagine what kind of measures could be taken (by the company) for the safe operation of the two reactors” if it is found to be connected to faults running directly underneath them, NRA Chairman Shunichi Tanaka said.

When asked how long Japan Atomic Power could be prohibited from resuming operations at the plant in such an event, Tanaka said it might have to wait “until the active fault is gone” — suggesting the reactors would probably have to be scrapped.

Source: NHK

Source: The Japan Times

Source: The Daily Yomiuri


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