Ministry sets radiation levels requiring decontamination

September 28, 2011

By HARUFUMI MORI / Staff Writer

The Environment Ministry will decontaminate areas with an estimated annual dose of 5 millisieverts or more from the accident at the Fukushima No. 1 nuclear plant, which the government will shoulder as its burden in cleaning up the mess.

All the areas recording 5 millisieverts or more are in Fukushima Prefecture and encompass 1,778 square kilometers, or 13 percent of the prefecture's area.

Five millisieverts in an annual dose is equivalent to about 1 microsievert per hour based on the assumption that people spend eight hours outdoors and the rest of the time indoors.

The latest standards are translated into an exposure of about 1 microsievert of radiation per hour.

The cleanup work is aimed at removing soil up to about 5 centimeters down from the ground surface, a point where cesium is concentrated.

The decontamination will also clean up so-called "hot spots" in urban areas where unusually high levels of radioactive substances were measured, such as side ditches and gutters, if the projected annual reading is 1 millisievert or more.

The ministry set decontamination standards for hot spots taking into account the impact on the livelihood of residents in the neighborhood.

As for cleanup of forests, it has decided to remove radioactive materials by scooping up fallen leaves rather than scraping off the topsoil.

The maximum amount of soil, fallen leaves and other debris to be removed under the decontamination plan is set at 29 million cubic meters, the equivalent of 23 Tokyo Domes, according to the ministry's calculation.

But critics said no matter how much the government emphasizes the legitimacy of the latest figure, setting the government standards will be meaningless unless municipalities and residents are convinced of their safety.

Most of the decontamination operations are expected to get under way next year.

The ministry presented results of its estimate for decontamination work to a meeting of experts on Sept. 27.

Hisaki Mori, executive managing director at the Radioactive Waste Management and Nuclear Facility Decomissioning Technology Center and one of the experts sitting in the ministry meeting, called for the government to explain its radiation standards to help ease public concern.

"The government is accountable (for the standards)," Mori said.

By HARUFUMI MORI / Staff Writer
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Workers decontaminate soil with high-pressure water nozzles in Minami-Soma in Fukushima Prefecture. (Asahi Shimbun file photo)

Workers decontaminate soil with high-pressure water nozzles in Minami-Soma in Fukushima Prefecture. (Asahi Shimbun file photo)

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  • Workers decontaminate soil with high-pressure water nozzles in Minami-Soma in Fukushima Prefecture. (Asahi Shimbun file photo)

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