A veritable mountain of soil will have to be stripped from the crippled Fukushima No. 1 nuclear power plant and surrounding areas to remove radioactive materials.
Yuichi Moriguchi, a professor of environmental systems engineering at the University of Tokyo, estimated it could amount to 100 million cubic meters of soil.
That is enough to fill the 55,000-capacity Tokyo Dome, home of the Yomiuri Giants baseball team and a popular concert venue, 80 times.
Moriguchi sits on the Environment Ministry panel tasked with developing standards for removing radioactive materials released from the stricken facility.
The government is hoping to reduce the volume by narrowing down areas to be decontaminated mainly to residential districts and farmland.
It also plans to develop technologies to separate radioactive materials from soil.
Moriguchi estimated that radioactive materials must be removed from up to 2,000 square kilometers of land, or one-seventh of Fukushima Prefecture.
The area includes the no-entry zone and the planned evacuation zone, where residents have been advised to evacuate, totaling 1,100 square kilometers.
The estimates are based on the assumption that areas with radiation levels of 1 microsievert or more per hour need to be decontaminated to contain an additional radiation dose to 1 millisievert or less a year.
Moriguchi identified those areas from a map showing radiation levels measured in late June.
Scientists believe that cesium can be removed almost completely if the first 5 centimeters of top soil are stripped.
Cleaning up the entire 2,000-square-kilometer area will require removing 100 million cubic meters of soil.
Moriguchi presented his estimates at a study session in which Environment Ministry officials participated.
The estimates did not take into account how the land is used.
Woodland accounts for about 60 to 70 percent of the total area. The volume of soil to be stripped may be reduced to several tens of millions of cubic meters if mountainous areas away from residential districts are excluded.
"I don't think that the whole (2,000-square-kilometer) area will be decontaminated," Moriguchi said. "But I hope the calculations will be of help when people think about how difficult it will be to remove radioactive materials and who will shoulder the burden."
The scope of decontamination and the volume of soil to be stripped will directly affect the size and construction cost of a temporary storage facility for radioactive soil and waste, to be built in Fukushima Prefecture.
It cost about 160 billion yen ($2 billion) to build the Low-level Radioactive Waste Disposal Center in Rokkasho, Aomori Prefecture, a final disposal site for 200,000 cubic meters of contaminated metal parts and working uniforms from nuclear power plants.
If the planned temporary storage facility has the same structure as the Rokkasho facility, the construction cost will reach about 80 trillion yen.
The Environment Ministry panel, which met for the first time on Sept. 14, will draft standards for removing radioactive materials by the end of November so that the removal work can start in earnest from January.
Environment Ministry officials plan to present various calculations about the scope of decontamination and the volume of soil to be stripped, using Moriguchi's estimates as one reference.
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