The Yomiuri Shimbun
Authorities trying to decontaminate radioactive soil in the aftermath of the accident at the Fukushima No. 1 nuclear power plant have found that sunflowers, despite their reputation for absorbing radioactive cesium, have little effect, an experiment has shown.
The experiment was conducted by the Agriculture, Forestry and Fisheries Ministry. Following the outbreak of the nuclear crisis, a large amount of radioactive substances were released, contaminating the areas around the plant.
Although the experiment found that scraping away the surface of the contaminated soil is effective to reduce the density of radioactive cesium, this method leaves the problem of how to dispose of a large quantity of contaminated soil.
The experiment on removing radioactive cesium started in May in farmland totaling 7,000 square meters in Iitatemura and other locations in Fukushima Prefecture.
In the experiment, the effects of the following four methods were examined: scraping away surface soil; washing contaminated soil with water and removing the water; burying topsoil and replacing it with subsoil; and using sunflowers and other plants to absorb radioactive cesium in soil.
The results showed the least effective of the four methods was the use of plants.
Sunflowers, which were planted in May and bloomed in August, absorbed only 520 becquerels, or about 0.05 percent, of about 1.07 million becquerels of radioactive cesium per square meter of contaminated soil.
When four centimeters of topsoil was scraped away, the density of radioactive cesium was reduced by up to 75 percent. When three centimeters was scraped away from pastures with green grass, the density was reduced by up to 97 percent.
While switching topsoil and subsoil reduced the density of cesium on the surface, the amount of radioactive cesium remained unchanged.
The central government restricts rice growing in paddy fields with soil containing more than 5,000 becquerels of radioactive cesium per kilogram.
In Fukushima Prefecture alone, there are about 8,300 hectares of rice and vegetable fields in which radiation levels were found to exceed the government-set limit. If the topsoil of all the fields is scraped away, about 3 million to 4 million tons of contaminated soil is expected to be removed.
Based on the experiment's results, the ministry intends to finance the removal of cesium-contaminated soil in the prefecture with reserve funds and the third supplementary budget for this fiscal year.
As it is an enormous task to scrape away such a large amount of soil, the ministry is expected to coordinate with local governments on decontamination efforts.
(Sep. 16, 2011)