Radioactive materials from the damaged Fukushima No. 1 nuclear plant reached the Kanto region mainly via two routes, but they largely skirted the heavily populated areas of Tokyo and Kanagawa Prefecture, an expert said.
Relatively high levels of radioactive cesium were detected in soil in northern Gunma and Tochigi prefectures and southern Ibaraki Prefecture after the Fukushima No. 1 nuclear power plant was damaged by the March 11 Great East Japan Earthquake and tsunami. But contamination was limited in Tokyo and Kanagawa Prefecture, where 22 million people live.
Hiromi Yamazawa, a professor of environmental radiology at Nagoya University, said the first radioactive plume moved through Ibaraki Prefecture and turned northward to Gunma Prefecture between late March 14 and the afternoon of March 15.
Large amounts of radioactive materials were released during that period partly because the core of the No. 2 reactor at the Fukushima No. 1 plant was exposed.
"The soil was likely contaminated after the plume fell to the ground with rain or snow," Yamazawa said, adding that western Saitama Prefecture and western Tokyo may have been also contaminated.
Rain fell in Fukushima, Tochigi and Gunma prefectures from the night of March 15 to the early morning of March 16, according to the Meteorological Agency.
The second plume moved off Ibaraki Prefecture and passed through Chiba Prefecture between the night of March 21 and the early morning of March 22, when rain fell in a wide area of the Kanto region, according to Yamazawa's estimates.
He said the plume may have created radiation hot spots in coastal and southern areas of Ibaraki Prefecture as well as around Kashiwa, Chiba Prefecture.
Yamazawa said the plume continued to move southward, without approaching Tokyo or Kanagawa Prefecture, probably because winds flowed toward a low-pressure system south of the Boso Peninsula.
"It rained slightly because the low-pressure system was not strong," said Takehiko Mikami, a professor of climatology at Teikyo University. "Contamination in central Tokyo might have been more serious if (the plume) had approached more inland areas."
According to calculations by The Asahi Shimbun, about 13,000 square kilometers, or about 3 percent of Japan's land area, including about 8,000 square kilometers in Fukushima Prefecture, have annual exposure levels of 1 millisievert or more.
Gunma and Tochigi prefectures have a combined 3,800 square kilometers with an annual exposure of 1 millisievert or more.
Among Tokyo's 23 wards, Katsushika Ward had the highest radiation level of 0.33 microsievert per hour, according to a science ministry map showing radioactive contamination for 12 prefectures.
The ward government has been measuring radiation levels in seven locations once a week since late May. It plans to take measurements at about 500 public facilities, such as schools and parks, in response to residents' demands for detailed surveys.
The Gunma prefectural government has measured radiation levels in 149 locations since September and has identified six northern mountainous municipalities with an annual exposure of 1 millisievert or more.
Earlier this month, the prefectural government asked 35 municipalities to decide whether radioactive materials will be removed.
High radiation levels were detected in Minakami, Gunma Prefecture, known as a hot spring resort.
Mayor Yoshimasa Kishi said the town could be mistaken as a risky place if it decides to have radioactive materials removed.
The science ministry's map showed that 0.2 to 0.5 microsievert was detected in some locations in Niigata Prefecture.
Niigata Governor Hirohiko Izumida said the figures were likely mistaken, noting that these locations have high natural radiation levels because of granite containing radioactive materials.
The prefectural government plans to conduct its own surveys of airborne radiation levels and soil contamination.
Many municipalities are calling for financial support for removing radioactive materials.
In Kashiwa and five other cities in northern Chiba Prefecture, radioactive materials need to be removed over an estimated 180 square kilometers of mainly residential areas.
The Kashiwa city government is providing up to 200,000 yen ($2,620) to kindergartens and nursery schools for removal work.
But some facilities have asked children's parents to help pay the costs because they cannot be covered by the municipal assistance.
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