Tremors spotlight nuclear plants

The earthquake that hit Niigata and Nagano prefectures on Monday brought to light safety problems that could arise at nuclear-power plants during a powerful earthquake. The magnitude-6.8 quake occurred in the Sea of Japan, only 9 km north of Tokyo Electric Power Co.'s Kashiwazaki-Kariwa nuclear-power plant, causing four of its seven reactors to shut down automatically. The three other reactors were undergoing regular checks.

In one of the three reactors, jolts from the quake caused water to overflow from a cooling pool for storing used nuclear fuel. An estimated 1.2 cubic meters of water containing a tiny amount of radioactive material entered the sea via a conduit — the first-ever such leakage into Japan's environment due to an earthquake.

Meanwhile, insulation oil in a transformer supplying electricity to one of the reactors that automatically shut down caught fire — the first-ever at a Japanese nuclear-power plant due to an earthquake — that took about two hours to quell. And a small amount of radioactive material was emitted into the air from another reactor's exhaust stack.

The biggest concern is that the tremors were stronger than the strongest tremors assumed in the plant design. For example, the No. 1 reactor recorded tremors of 680 gals (acceleration of 680 cm per second per second) in the east-west direction, more than twice the 273 gals anticipated in the design. Industry minister Akira Amari rightfully told TEPCO not to resume the plant's operation until sufficient quake resistance is confirmed.

When the Nuclear Safety Commission revised guidelines for nuclear-power plant designs in September 2006, uniform assumptions applied to all nuclear-power plants regarding possible quakes were scrapped. The new guidelines call for examining geological features and records of past earthquakes near a particular nuclear-power plant and revising the anticipated strength of the biggest possible earthquake. The latest earthquake has underscored the need for power companies to review in earnest their nuclear-power plants' ability to withstand seismic tremors. Their reviews must proceed with public transparency.