Plutonium contamination from the Fukushima nuclear disaster remains localized and has not spread since the last survey on the problem in September 2011, the science ministry said.
It released a map Aug. 21 showing areas of soil contamination by plutonium that was spewed into the environment during last year's crisis at the Fukushima No. 1 nuclear power plant.
The farthest point from the stricken plant was in Iitate, Fukushima Prefecture, at a distance of 32.5 kilometers.
In the survey released Sept. 30 last year, six months after the disaster, plutonium was detected up to 40 km from the Fukushima plant.
"The amount of plutonium deposits is considerably smaller than those of cesium, and the radiation doses are accordingly much smaller," a ministry official said.
In the latest survey, researchers analyzed soil samples from 62 locations in 23 municipalities in Fukushima, Miyagi, Ibaraki and Tochigi prefectures. Those sites are in addition to the 100 locations sampled in last year's survey.
Plutonium deposits directly related to the nuclear crisis were identified at 10 of the 62 locations in the four municipalities of Iitate, Namie, Okuma and Minami-Soma.
The maximum concentration of plutonium-238 was 11 becquerels per square meter at one location in Namie. That level is far lower than the 41 becquerels per square meter in Fukui Prefecture in 1978, the maximum recorded in Japan in the pre-1980 period, when the United States and Russia carried out numerous atmospheric nuclear tests.
The location in Namie is also the only site where the plutonium-238 reading exceeded the 8 becquerels per square meter recorded in Kumamoto Prefecture in 2001, the maximum reading during the more recent 11-year period (1999-2009).
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To read The Asahi Shimbun story on the earlier survey, visit: (ajw.asahi.com/article/0311disaster/AJ2011100112896).
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