Tokyo Denryoku said the incident occurred at the sixth reactor of the Kashiwazaki-Kariwa nuclear power plant hit by the first of three waves of the earthquake measuring 6.8 on Richter scale, which also left over 800 people injured. The discovery was made during regular maintenance checks of the reactor. The company suggested the water could have leaked from a special storage facility for used uranium rods.
Shortly after the first tremors, the company said four reactors that were operating at the time had automatically shut down by the safety system. But after a while, reports emerged that a transformer had caught fire on the external side of the third reactor. The fire was put out, and the company said there had been no radiation leak.
Japan's NHK television channel said about 1.2 cubic meters of radioactive water had leaked from the reactor, adding this amount was not dangerous for the environment.
The rescue effort in the areas affected by the quake is continuing, and more than 10,000 have been evacuated.