According to the NRA and other sources, Tepco first said it had detected 900,000 becquerels of beta ray-emitting radioactive materials, including strontium-90, from the groundwater last July.
When the firm later measured levels of strontium-90 alone, it found that the groundwater contained 5 million becquerels.
But Tepco concluded at the time that the data were inaccurate, citing the huge difference in the two measurements, despite knowing that its method of measuring beta ray-emitting materials could show lower-than-actual levels, the sources said.
Because the utility has repeatedly changed its explanations, the NRA secretariat has asked it for more detailed information.
Tepco claimed that although the company was aware of the data last July, it released them only recently after its accuracy was finally confirmed this month.
Previous reports said Tepco belatedly revealed last year that some 300 tons of groundwater was flowing into the sea daily, after mixing with radioactive water used to cool the plant’s three stricken reactors.
Read more at Tepco hid record-level radiation data last July
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