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.