Cesium-137 readings near the plant's Reactor 2 were 830 becquerels per litre, compared with the regulatory limit of 90 per litre. Both Cesium-134 and Cesium-137 are harmful to humans.
The latest water samples contain "high density of tritium and strontium," a TEPCO spokesperson said, adding that he had no new information on the condition of six Fukushima workers who were doused with radioactive water as a result of human error.
Wednesday's mishap, which saw nearly 10 tonnes of radioactive water spilled, was the fourth attributed to human error in less than a week at Fukushima, devastated in March 2011 by a massive earthquake and ensuing tsunami.
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