Some of the tanks were earlier found to be leaking contaminated water.

"Workers measured the radioactive levels of the water collected in the enclosure walls, pumping it back into tanks when the levels were high," said a TEPCO official.

"Once finding it was mostly rain water they released it from the enclosure, because there is a limit on how much water we can store."

The utility said about 1,130 tons of water with low levels of radiation -- below the 30 becquerels of strontium per litre safety limit imposed by Japanese authorities -- were released into the ground.

But the company also said at one site where water was found contaminated beyond the safety limit workers could not start the water pump quick enough in the torrential rain, and toxic water had leaked from the enclosure for several minutes.

Strontium is a potentially cancer-causing substance that accumulates in bones if consumed.