Airborne radioactive materials released during debris-clearing work at the Fukushima No. 1 nuclear power plant were found in a town 60 kilometers away on seven occasions since December 2011.
Led by Teruyuki Nakajima, a professor of atmospheric physics at the University of Tokyo’s Atmosphere and Ocean Research Institute, the team noted a surge in concentration of airborne radioactive cesium during clean-up activities that reached the town of Marumori in neighboring Miyagi Prefecture.
The researchers said the findings show that radioactive materials were repeatedly released into the environment and reached extensive areas during debris-clearing operations.
They called on Tokyo Electric Power Co., the operator of the Fukushima plant, to take more care to prevent the spread of radioactive materials during debris-clearing operations, even if it requires implementing more costly methods.
In conducting its research, the team placed a device to collect airborne dust at the town office of Marumori, 59 kilometers north-northwest of the stricken Fukushima plant. The device collected the samples at four- or five-day intervals between December 2011 and December 2013.
The team determined that there were eight cases in which the amount of radioactive cesium in the samples were at least 10 times higher than normal levels and the material likely originated from the Fukushima plant because of wind direction and speed.
The highest level of contamination was recorded in a sample collected between Aug. 16-20, 2013, reaching 50 to 100 times higher than normal levels.
TEPCO conducted large-scale debris-clearing work at the plant on Aug. 19, 2013. Previous research by the farm ministry and Kyoto University also showed that radioactive dust released during the work reached locations 27 km and 48 km from the plant.
In seven other cases, the amount of radioactive materials in the samples was about 10 times higher than normal. The research team reported the results of its findings to the farm ministry in May.
According to TEPCO, seven of the eight cases were recorded during the same period when the utility was doing debris-clearing work at the No. 3 reactor building.
The remaining case involved samples collected between Nov. 16-20, 2012, coinciding with an accidental water leak from a vent pipe of a cesium-absorption device at the plant.
A TEPCO official said it was unlikely that the accident caused a major release of radioactive materials like the August 2013 incident.
The utility had planned to dismantle a shroud over the No. 1 reactor building this month to start full-scale debris-clearing work around the reactor, but postponed the plan in order to strengthen measures to prevent the spread of radioactive materials during clean-up activities.
A worker at the Fukushima plant said that TEPCO has not discussed any drastic measures, such as covering the reactor with a container.
“It will likely resume debris cleanup when criticism calms down,” the worker said.