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The agriculture ministry earlier raised the likelihood that debris-removal operations on Aug. 19, 2013, led to cesium levels exceeding the safety standard detected in rice harvested more than 20 km from the plant.
Akio Koizumi, a health and environment science professor at Kyoto University’s Graduate School of Medicine, and four other scientists discovered that the wind likely carried the cesium more than twice that distance.
The researchers set up air sampling instruments at three points in residential areas of Fukushima Prefecture and have measured radioactive cesium concentrations every week since September 2012 to estimate residents’ exposure to radiation.
From samples collected between Aug. 15 and 22 last year, they found a reading of 1.28 millibecquerels per cubic meter at a location in Soma, 48 km northwest of the plant. That radioactivity level was more than six times higher than usual.
Radioactivity levels were 20 to 30 times higher than normal in Minami-Soma, 27 km north-northwest of the Fukushima plant. And there were almost no changes in cesium concentrations in Kawauchi, 22 km west-southwest of the plant, the researchers said.
Based on the wind’s speed and direction at the time, as well as size of the collected particles, Koizumi and his colleagues concluded that the radioactive cesium came from the Fukushima No. 1 plant as the result of the Aug. 19, 2013, clearance work at the No. 3 reactor.
The team also found that cesium levels at the measuring point in Minami-Soma surged in both May and June 2013. They attributed the increase to debris-clearing operations at the facility.
The research results indicate that future rubble removal at the nuclear plant could disperse radioactive materials over much broader areas surrounding the facility.
In March this year, the scientists presented their findings to the Environment Ministry. It has also been reported that the agriculture ministry instructed Tokyo Electric Power Co., operator of the nuclear plant, to take measures to prevent the release of radioactive substances in the debris-removal work at the site.
TEPCO currently plans to resume debris-removal efforts by the end of July, starting with the dismantling of a cover installed on the No. 1 reactor building, where highly contaminated rubble remains to be removed.
The utility acknowledged that the Aug. 19 operations released a maximum 4 trillion becquerels--more than 10,000 times the usual levels at the site--over four hours, and apologized to residents for “causing trouble.”
However, TEPCO argued that it is unclear whether the increase in cesium readings was related to debris-clearing work.