Radioactive cesium more than 10 times the standard for waste disposal has been detected on lawn covers that had been placed in a schoolyard from mid-March until early April at Horinouchi Elementary School in Tokyo's Suginami Ward.
Radioactive cesium more than 10 times the standard for waste disposal has been detected on lawn covers that had been placed in a schoolyard from mid-March until early April at Horinouchi Elementary School in Tokyo's Suginami Ward.
The measured value, 90,600 becquerels of radioactive cesium per kilogram, was more than triple the 26,400 becquerels contained in sludge at a sewage treatment facility in Koriyama, Fukushima Prefecture, about 60 kilometers from the disabled Fukushima No. 1 nuclear power plant.
"The cover, with its wide surface area, became contaminated with large amounts of radioactive substances (shortly after the onset of the nuclear accident)," said a Suginami Ward official. "We suspect the light weight of the covers has led to radioactive cesium being detected at such high levels, given that the concentration levels of the substance are calculated on a per kilogram basis."
The Environment Ministry said on the night of Dec. 12, "That level of radioactive cesium can be adequately diluted by mixing one ton of other waste material per kilogram of the contaminated covers before incinerating it," which indirectly gives consent to incinerating the cover.
With the implied consent in hand, the ward government has started planning the incineration process.
According to the ward government, the school started covering the lawn area of its schoolyard with nine sheets of the polyethylene covers only during nighttime in January. The school, however, left the covers on the schoolyard for the entire period from March 18 to April 6, about the same length as the spring break. Later on, the school placed the covers, which are now safely stored at a ward-run facility, in a pile beside its gymnastics hall.
At the request of students' parents, the ward government measured the levels of airborne radiation above the covers in early November, resulting in the detection of 3.95 microsieverts of radiation per hour 1 centimeter above the covers.
Further investigation into the covers themselves revealed on Dec. 6 that they contained a combined 90,600 becquerels of cesium-134 and cesium-137 per kilogram.
The ministry said this was the first time a municipality has sought its guidance regarding contaminated covers on a schoolyard, although it has seen cases in which radiation levels above the disposal standard of 8,000 becquerels or less per kilogram were found in sludge and burned ash.
"Substantial amounts of radiation had been spread from the Fukushima plant until early April after the Fukushima accident took place, and non-negligible airborne radiation amounts had likely been present in Tokyo around that time as well," said Kunikazu Noguchi, a lecturer of radiation protection at Nihon University.
He warned, "the contaminated covers should be kept out of reach from anyone until the disposal is implemented, although people do not need to be overly concerned because the covers are not something you put in your mouth."