Radioactive cesium from the Fukushima No. 1 nuclear power plant has spread more than 250 kilometers toward the southwest, reaching as far as Gunma Prefecture, the science ministry said.
The Ministry of Education, Culture, Sports, Science and Technology has been measuring contamination levels in each prefecture in eastern Japan, including Gunma Prefecture from Aug. 23 to Sept. 8 using prefectural government helicopters.
According to the measurements released Sept. 27, most of the radioactive cesium first spread about 60 km northwest from the Fukushima nuclear plant, then changed course and spread to Tochigi Prefecture and further to Gunma Prefecture.
A plume of radioactive materials was carried by winds along the mountain range and then fell to the ground, according to ministry officials.
In Gunma Prefecture, the largest amount of cesium-137 accumulated in the northern part of the prefecture. Cesium-137's half-life, the period it takes for materials to be reduced by 50 percent, is 30 years.
The radioactive accumulation reached a range between 100,000 and 300,000 becquerels per square meter in a mountainous area encompassing the cities of Midori and Kiryu in the eastern part of the prefecture, about 180 km from the Fukushima nuclear plant.
Even in areas in the western part of the prefecture, which are located about 250 km from the Fukushima plant and border Nagano Prefecture, the accumulated radioactivity exceeded 30,000 becquerels.
In the aftermath of the 1986 nuclear accident in Chernobyl, areas where radioactivity levels exceeded 37,000 becquerels were designated as contamination zones.
In mountainous areas of Gunma Prefecture, where accumulated cesium amounts were quite large, the radiation levels were 0.5 to 1.0 microsieverts per hour. In other areas of the prefecture, the radiation levels were less than 0.5 microsieverts.
The criterion that requires removal of radioactive materials from schoolyards is more than 1.0 microsieverts.
The ministry has released the results of measurements in each prefecture on its Japanese website: http://radioactivity.mext.go.jp/ja/1910/2011/09/1910_092714.pdf.