(Reuters) The operator of Japan’s wrecked Fukushima nuclear plant knew about record high measurements of a dangerous isotope in groundwater at the plant for five months before telling the country’s nuclear watchdog, a regulatory official told Reuters.

Tokyo Electric Power Co (Tepco) said late on Wednesday it detected 5 million becquerels per liter of radioactive strontium-90 in a sample from a groundwater well about 25 meters from the ocean last September. That reading was more than five times the broader all-beta radiation reading taken at the same well two months earlier.

A Tepco spokesman said there was uncertainty about the reliability and accuracy of the September strontium reading, so the utility decided to re-examine the data.

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