Heavy rain from Wednesday's massive typhoon may have caused the generator to fail and led to other problems at the crisis-hit facility, the utility said, although it could not confirm the exact cause of the generator failing and the tank overflowing.
TEPCO said it is trying to confirm the exact amount of toxic water that has leaked from the tank, its level of radioactivity and the environmental impact of the leak.
Following the typhoon, high levels of radiation were also detected in a ditch leading to the Pacific Ocean, TEPCO said.
Officials from the utility said that due to the high volume of rainfall Wednesday, contaminated soil flowed into the ditch causing radioactivity to spike to 70 times higher than that measured on Tuesday.
TEPCO detected 1,400 becquerels per litre of beta ray-emitting radioactive material at the ditch which is located just 150 metres from the Pacific Ocean. The utility said that the radioactive materials had likely flowed into the ocean.
Tests are under way to determine the exact extent of the toxicity in the surrounding sea after the mishap at the facility -- the latest in a string of other leaks and accidents caused by faulty equipment and human error.
TEPCO was urged by Japan's Nuclear Regulatory Authority (NRA) to take pre-emptive measures to guard against the typhoon well in advance of it lashing Japan's eastern seaboard Wednesday.
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