Gov't to decontaminate areas with radiation exposure of 5 millisieverts or more per year

SOURCE: The Mainichi Daily News
DATE: 28 SEPT 2011

Original Article Japanese Version

The Environment Ministry has decided to decontaminate areas where people could be exposed to radiation of 5 millisieverts or more per year by removing up to 28.78 million cubic meters of radioactively contaminated soil in Fukushima and four other adjacent prefectures affected by the ongoing crisis at the Fukushima No. 1 Nuclear Power Plant.

The areas subject to the decontamination project are in Fukushima, Miyagi, Yamagata, Ibaraki and Tochigi prefectures. A huge temporary storage facility for contaminated materials needs to be built, and therefore the government is likely to have tough talks with local municipalities on selecting a space for such a facility.

Under the special measures bill passed into law in August to deal with radiation contamination and debris, the environment minister designates highly contaminated areas as "special decontamination areas" and the central government becomes directly in charge of decontaminating such areas. The central government also designates other areas with certain levels of contamination as "priority inspection areas for contamination" and local governments are to check the levels of contamination in those areas to determine locations to be decontaminated and actually decontaminate them.

A government map displaying radiation levels in the area around the Fukushima No. 1 Nuclear Power Plant.

In mapping out the decontamination program for those areas, the Environment Ministry decided that areas where people could be exposed to radiation of 5 millisieverts or more per year should be decontaminated. On the reason to set the levels of radiation at 5 millisieverts or more per year, the Environment Ministry said, "If the dose of radiation exposure is less than 5 millisieverts, the dose of additional radiation exposure will fall below the official dose limit of 1 millisiervert as it decreases with the passage of time and is spread by rain and wind."

The amounts of soil to be removed are calculated on these assumptions, with 5 centimeters of topsoil being removed from contaminated areas. For calculating the amounts of soil to be decontaminated in forests, the areas are divided into three sections of 10 percent of the whole areas, 50 percent and 100 percent.

Workers measure the ground near a rain water outlet in Minamisoma, Fukushima Prefecture, on June 12. (Mainichi)

As a result, if 100 percent of the designated areas were to be decontaminated, the amounts of radioactive soil and other materials to be removed would reach 28.785 million cubic meters, which would fill the Tokyo Dome stadium 23 times. Of the estimated total amounts of soil and other materials to be removed, 1.02 million cubic meters comes from residential and urban areas, 17.42 million cubic meters from farmland, and 8.75 million cubic meters from forests. The total area is 2,419 square kilometers, which is equal to about 17 percent of Fukushima Prefecture.

The Environment Ministry said it would try to work out specific decontamination plans by the end of this year.