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November 22, 2011

Soil radiation maps around Fukushima

Nature - Independent studies map Fukushima radiation hot spots

One of the radionuclides released was caesium-137, which has a half-life of 30 years and poses a long-term health risk if absorbed by the soil in significant quantities.

At the end of August, Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) released a map (see 'Directly comparing Fukushima to Chernobyl') showing caesium-137 concentrations in the soil from measurements at more than 2,000 locations in and around Fukushima. The map is helping the government to determine which areas can be returned to farming, which need their topsoil removed and which should be declared unfit for food production or even left uninhabited.

Two studies have now used different methods to map the fallout over larger areas. One1, led by Teppei Yasunari of the Universities Space Research Association in Columbia, Maryland, maps caesium-137 concentrations throughout the nation.

In eastern Fukushima prefecture, caesium-137 concentrations exceeded the government limit of 2,500 becquerels per kilogram (Bq/kg), which the researchers say is enough to “severely impair” food production. In several neighbouring prefectures, levels exceeded 250 Bq/kg, enough to “partially impact” farming.

PNAS - Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan

A tremendous amount of radioactivity was discharged because of the damage to cooling systems of nuclear reactors in the Fukushima No. 1 nuclear power plant in March 2011. Fukushima and its adjacent prefectures were contaminated with fission products from the accident. Here, we show a geographical distribution of radioactive iodine, tellurium, and cesium in the surface soils of central-east Japan as determined by gamma-ray spectrometry. Especially in Fukushima prefecture, contaminated area spreads around Iitate and Naka-Dori for all the radionuclides we measured. Distributions of the radionuclides were affected by the physical state of each nuclide as well as geographical features. Considering meteorological conditions, it is concluded that the radioactive material transported on March 15 was the major contributor to contamination in Fukushima prefecture, whereas the radioactive material transported on March 21 was the major source in Ibaraki, Tochigi, Saitama, and Chiba prefectures and in Tokyo.




Rain played a major role in flushing the radionuclides out of the atmosphere, say the researchers. The team used a computer simulation to show that the soil contamination in the areas they studied was caused largely by downpours on 15 March in Fukushima and on 21 March in the other four prefectures.

Kinoshita’s group calculated that the annual radiation dose would be about 40 millisievertsin Iitate and around 10 mSv in Naka-Dori, compared with the government limit of 20 mSv. Kinoshita says experts in radiation health maintain that there shouldn’t be health problems below about 100 mSv, but cautions that the long-term effects of low doses still aren't fully understood.

PNAS - Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident

The largest concern on the cesium-137 (137Cs) deposition and its soil contamination due to the emission from the Fukushima Daiichi Nuclear Power Plant (NPP) showed up after a massive quake on March 11, 2011. Cesium-137 (137Cs) with a half-life of 30.1 y causes the largest concerns because of its deleterious effect on agriculture and stock farming, and, thus, human life for decades. Removal of 137Cs contaminated soils or land use limitations in areas where removal is not possible is, therefore, an urgent issue. A challenge lies in the fact that estimates of 137Cs emissions from the Fukushima NPP are extremely uncertain, therefore, the distribution of 137Cs in the environment is poorly constrained. Here, we estimate total 137Cs deposition by integrating daily observations of 137Cs deposition in each prefecture in Japan with relative deposition distribution patterns from a Lagrangian particle dispersion model, FLEXPART. We show that 137Cs strongly contaminated the soils in large areas of eastern and northeastern Japan, whereas western Japan was sheltered by mountain ranges. The soils around Fukushima NPP and neighboring prefectures have been extensively contaminated with depositions of more than 100,000 and 10,000 MBq km-2, respectively. Total 137Cs depositions over two domains: (i) the Japan Islands and the surrounding ocean (130–150 °E and 30–46 °N) and, (ii) the Japan Islands, were estimated to be more than 5.6 and 1.0 PBq, respectively. We hope our 137Cs deposition maps will help to coordinate decontamination efforts and plan regulatory measures in Japan.

Fukushima prefecture was a major producer of fruit.

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