Tokyo Electric Power (Tepco) knew about record-high levels of a dangerous isotope in groundwater near the doomed Fukushima nuclear plant for five months before reporting the information to authorities, Reuters reports.
Tepco said it detected 5 million becquerels per liter of radioactive strontium-90 in a sample from a groundwater well about 25 meters from the ocean in September 2013.
That reading was more than five times the broader all-beta radiation reading taken at the same well two months earlier.
A Tepco spokesman told Reuters uncertainty about the reliability and accuracy of the September strontium reading caused the utility decided to re-examine the data.
Strontium-90, which has a half-life of around 29 years, is estimated to be twice as harmful to the human body as cesium-137, another isotope that was released in large quantities during the meltdowns at the Fukushima Daiichi plant in March 2011. The legal limit for releasing strontium into the ocean is 30 becquerels per liter.
In August 2013, Tepco admitted that some 300 metric tons of radioactive water had leaked from a storage tank on the site. Two months later, Tepco said another tank holding highly contaminated water overflowed, likely sending the liquid into the Pacific Ocean.