In July 2007, a magnitude 6.8 earthquake hit Tokyo Electric Power Co.’s Kashiwazaki-Kariwa nuclear complex in Niigata Prefecture, leading to suspension of all its seven reactors. With a combined output of 8.212 million kW, the complex is the world’s largest nuclear power generation complex. On May 9, the No. 7 reactor, an advanced boiling-water reactor with an output of 1.356 million kW, was restarted on a trial basis after one year and 11 months’ suspension.

On May 11, a glitch occurred in the reactor’s core isolation cooling system. Although it was no hazard to the environment, it is hoped that the power company will proceed operations with the utmost caution. It also should make the reactor operations fully transparent to local and central governments as well as to the public.

The original operating plan for the No. 7 reactor called for its turbine to begin operating about a week after it is restarted, and then gradually raising the output to full capacity. It is expected that the No. 6 reactor will be able to be reactivated by the end of this year. But it remains unknown when the operations of the No. 1 to 5 reactors will resume.

The July 2007 earthquake taught us that quake tremors bigger than the reactors were originally designed to withstand do happen. Local residents are therefore still concerned about the nuclear power complex’s quake resistance. TEPCO has since raised the assumed maximum tremor strength from the original 450 gals up to 2,300 gals. This is because the length of the undersea fault that is believed to have caused the earthquake has been re-estimated from the original 8 km to 36 km. It is now assumed that the fault can cause a magnitude 7 quake.

Some experts, however, believe that there is a possibility that the fault is 60 km long and can cause a magnitude 7.5 quake. When even the experts have different views on assumed magnitudes of quakes, priority must be given to safety over profitability. Power companies need to pay attention to any new revelations concerning earthquakes and make efforts to enable nuclear power plants to endure the worst possible scenarios.

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