Up to 28 million cubic meters of soil contaminated by radioactive substances may have to be removed in Fukushima Prefecture, according to the Environment Ministry.
In a simulation, the ministry worked out nine patterns according to the rates of exposure to and decontamination of radioactive materials in soil, mainly in forests.
The ministry found if all the areas which were exposed to 5 millisieverts or more per year were to be decontaminated, 27.97 million cubic meters of contaminated soil would have to be removed. The calculation covered 13 percent of the prefecture’s area.
These figures indicate the size of the temporary facilities that will be needed to store the soil, and the capacity of intermediate storage facilities where the soil will be taken later.
The assumptions were made using three categories according to yearly radiation doses in soil–20 millisieverts or more; 5 millisieverts or more; and 5 millisieverts or more plus some areas with contamination of from 1 to 5 millisieverts.
The three categories were divided further according to possible decontamination rates in forests–100 percent, 50 percent and 10 percent. The resulting nine patterns were broken down further to include “houses and gardens,” “schools and child care centers” and “farmland.”
The ministry calculated that the largest amount of contaminated soil was 28.08 million cubic meters in the case of 100 percent decontamination in forests in the category of 5 millisieverts or more plus some areas with contamination of from 1 to 5 millisieverts.
The smallest amount was 5.08 million cubic meters if 10 percent decontamination is carried out in forests with radiation doses of 20 millisieverts or more.
In the breakdown of areas with yearly radiation doses of 5 millisieverts or more, it was found 1.02 million cubic meters of soil should be removed from houses and gardens, 560,000 cubic meters from schools and child care centers and 17.42 million cubic meters from farmland.
The total amount of contaminated soil with a yearly radiation dose of 5 millisieverts or more is 27.97 million cubic meters in the case of 100 percent decontamination in forests that cover an area of 1,777 square kilometers.
The figures will be submitted Tuesday to a ministry study group that decides on the nation’s decontamination policy.
The ministry made its calculation based on an aerial survey by the Education, Culture, Sports, Science and Technology Ministry and a land use survey by the Land, Infrastructure, Transport and Tourism Ministry.
A senior Environment Ministry official said, “The standard we basically agreed on at a study meeting is decontamination in areas with yearly radiation doses of 5 millisieverts or more.”
Even though forests occupy about 70 percent of contaminated areas in the prefecture, the ministry does not believe it will be necessary to remove all contaminated soil, as long as the government restricts the entry of residents in mountainous areas and recovers cut branches and fallen leaves, according to the official.
The usual practice is to remove soil up to a depth of five centimeters. However, a senior official said this depends on the location of the contaminated soil.
The Environment Ministry will draw up plans based on a law concerning special measures on dealing with environmental contamination by radioactive substances as early as the end of November and start full-scale decontamination in January.
But the government still has not procured sufficient storage sites for contaminated soil, which has been temporarily buried in school yards or piled on vacant lots.
According to the central government, contaminated soil should be collected at temporary storage sites by local governments. The government recommends placing impermeable sheets under the soil at locations far from living areas.
The government also has no prospect of setting up intermediate storage facilities. Shortly before he stepped down, former Prime Minister Naoto Kan called on the Fukushima Gov. Yuhei Sato to set up facilities in the prefecture. The request was rejected.
Contaminated soil must be removed as soon as possible to allow evacuees to return to their houses within no-entry and evacuation zones.
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