Highly radioactive water flowed into the ground from a storage tank at the crippled Fukushima nuclear power plant on Wednesday, in what officials said was the largest contamination leak in roughly six months.

The water, reading high levels of radiation, overflowed from a large storage tank after a valve that had been mistakenly left open allowed excessive amounts of contaminated water into the tank.

The overflow was discovered Wednesday night, but before the valve was closed the tank is estimated to have spilled 100 metric tons of water containing 230 million becquerels per liter of "beta-emitting radioactive isotopes, including strontium 90."

The water quickly seeped into the ground, TEPCO officials said, but claimed that it was "unlikely to have reached the ocean."

Last week, TEPCO, which has been criticized repeatedly for its mishandling of the nuclear catastrophe, announced that samples taken from a groundwater well near the plant contained a record-breaking 54,000 becquerels per liter of the radioactive substance cesium, a number which doubles all previous records of cesium in groundwater near the plant.

It was also recently revealed that the company was withholding vital data that showed surging levels of the highly radioactive strontium-90 in a separate groundwater well.

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