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The Kanazawa District Court in March 2006 ordered Hokuriku Electric Power Co. to shut down the No. 2 reactor at its Shika nuclear power station in Ishikawa Prefecture — the nation’s second largest power reactor. The unprecedented ruling said the reactor lacked sufficient earthquake resistance and could expose residents to radiation in the event of a major quake. The lawsuit was filed in May 2005 by 128 people from 16 prefectures. The Nagoya High Court’s Kanazawa branch overturned the ruling March 18, saying the reactor’s resistance to earthquakes was adequate.

Still, the power industry should not lower its guard against quakes. The conditions under consideration at the appellate trial differed from those of the first trial. The design of the Shika No. 2 reactor was based on 1981 quake-resistance guidelines, the defects of which had been exposed. The lower court ruling apparently prompted the Nuclear Safety Commission to revise the guidelines. It came up with new, stricter guidelines in September 2006. Based on the new guidelines, Hokuriku Electric Power reinforced the quake resistance of the reactor.

The high court ruling said that since the company utilized the latest knowledge in assessing geological faults, its assumption of possible maximum seismic tremors is appropriate. Although the plaintiffs said the maximum magnitude of quakes caused by unknown fault activity should be assumed to be 7.3, the ruling said the company’s assumption of magnitude 6.8 was appropriate.

In the March 2007 Noto Peninsula quake, the reactor was hit by tremors that were nearly twice as strong as its design was made to withstand. In the July 2007 Chuetsu quake, the seven reactors at Tokyo Electric Power Co.’s Kashiwazaki Kariwa nuclear power station in Niigata Prefecture were struck by that were much stronger than it was designed to withstand. The reactors are still inoperative.

The ruling said that even the experience from these quakes does not demonstrate a danger of residents being exposed to radiation. But the possibility of stronger-than-expected earthquakes cannot be ruled out. The power industry should not slacken its efforts to strengthen the quake resistance of the nation’s nuclear reactors based on the latest knowledge.

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