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.
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.
The government said in its provisional policy for decontamination released in August that it aims to bring the projected annual dose of radiation to 1 millisievert in the long run.
But the environment ministry decided on the 5 millisieverts a year level for now based on a projection that the cleanup operation would not generate a significant effect in areas with a reading of lower radiation levels even if the work was carried out.
A senior ministry official said the ministry's conclusion was arrived at weighing the pros and cons of cleanup and labor costs against the predicted results.
"It would be ideal to set it at 1 millisievert, but we would not be able to complete decontamination if we insisted on that," the official said.
The ministry expects to eventually bring down radiation exposure levels to 1 millisievert in part as some of the radioactive cesium will have a half-life of two years.
But such cleanup work will not be done in forests.
The environment ministry said instead retrieving fallen leaves and trimming tree branches would suffice, citing a study by the education ministry.
Environment ministry officials said that the removal of leaves and branches would reduce the quantity to be eliminated by one-fifth or one-sixth, compared with the amount of soil that would need to be scraped off in the comparable area.
Another advantage of ridding leaves and branches instead of soil, the ministry said, is to allow officials to further reduce the volume of tainted materials through incineration.
Forests account for about 70 percent of the areas to be covered in the government's decontamination operation.
As for hot spots in urban areas, the government will need to decontaminate about 640 square kilometers in Fukushima and four neighboring prefectures alone.
Officials are expected to use high-pressure water nozzles and other methods to clean up those areas.
About 400,000 cubic meters of soil are expected to be removed as a result.
While the government sets its standards for radiation levels requiring decontamination work, municipalities will likely decide on their own figures.
If they seek to carry out cleanup in areas with radiation levels lower than the government standards, they are required to pick up the tab for the decontamination work.
Meanwhile, the Fukushima city government announced its decontamination plan on Sept. 27, covering all the 110,000 housing units, schools, parks, roads and public facilities in the city.
Its goal is to lower radiation levels in the air to 1 microsievert per hour or lower in all areas where residents live, by the end of fiscal 2012.
It will start the work next month, beginning with areas with a reading of about 3 microsieverts per hour.