Some 8 percent of Japan's land area, or more than 30,000 square kilometers, has been contaminated with radioactive cesium from the crippled Fukushima No. 1 nuclear power plant.
Spanning 13 prefectures, the affected area has accumulated more than 10,000 becquerels of cesium 134 and 137 per square meter, according to the science ministry.
The ministry has released the latest version of its cesium contamination map, covering 18 prefectures.
Radioactive plumes from the Fukushima No. 1 plant reached no farther than the border between Gunma and Nagano prefectures in the west and southern Iwate Prefecture in the north.
Ministry officials said the plumes flowed mainly via four routes between March 14 and 22 after the plant was damaged by the Great East Japan Earthquake and tsunami on March 11.
The first plume headed westward from late March 14 to early March 15, when huge amounts of radioactive materials were released following a meltdown at the No. 2 reactor.
It moved clockwise over a wide area in the Kanto region. Radioactive materials fell with rain and snow, particularly in the northern parts of Tochigi and Gunma prefectures.
A branch of the plume moved southward from Gunma Prefecture, passing through western Saitama Prefecture, eastern Nagano Prefecture and western Tokyo.
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