The levels of radiation accumulated in soil near the crippled nuclear power plant in northeastern Japan far exceeded the level of radiation the then-Soviet Union had used as a criterion for urging people to evacuate at the time of the 1986 Chernobyl nuclear disaster, threatening to plague local residents for a lengthy period.
Using aircraft, the Ministry of Education, Culture, Sports, Science and Technology checked the cesium-137 (half life of about 30 years) and cesium-134 (half life of about two years) accumulated in soil in collaboration with the U.S. Department of Energy in April.
Cesium-137 that has longer effects, ranging from 3 million to 14.7 million becquerels per square meter, was detected in Namie, Futaba, Minamisoma, Iitate and Katsurao, northwest of the Fukushima No. 1 Nuclear Power Plant, in Fukushima Prefecture. The levels far exceeded 550,000 bacquerels per square meter, the level the then-Soviet Union had used as a criterion for urging people to evacuate at the time of the 1986 Chernobyl nuclear disaster.
Based on recommendations from the International Commission on Radiological Protection (ICRP), the Japanese government used 20 millisieverts per year of radiation in the atmosphere as the criterion to designate evacuation areas in the wake of the nuclear accident in Fukushima. Therefore, there are areas that have not been designated as evacuation zones although they have larger amounts of accumulated radiation.
The Ministry of Education, Culture, Sports, Science and Technology says, "Radioactive substances in soil do not enter human bodies immediately." On the other hand, when authorities try to decide whether to allow local residents to return to their homes or resume farming, levels of soil contamination could be one of the hot topics of debate.
Hiromi Yamazawa, professor of environmental radiology at Nagoya University, said, "The problem with soil contamination is external exposure through gamma rays emitted from cesium adhered to soil." He said that replacing soil with non-contaminated soil is an effective way of reducing the concentration of radiation. He also said, "Replacing soil in lower layers with that from upper layers is also effective."
http://mdn.mainichi.jp/mdnnews/news/20110511p2a00m0na018000c.html
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