Some 8 percent of Japan's land area, or more than 30,000 square kilometers, has been contaminated with radioactive cesium from the crippled Fukushima No. 1 nuclear power plant.

Spanning 13 prefectures, the affected area has accumulated more than 10,000 becquerels of cesium 134 and 137 per square meter, according to the science ministry. The ministry has released the latest version of its cesium contamination map, covering 18 prefectures.

Radioactive plumes from the Fukushima No. 1 plant reached no farther than the border between Gunma and Nagano prefectures in the west and southern Iwate Prefecture in the north.

Ministry officials said the plumes flowed mainly via four routes between March 14 and 22 after the plant was damaged by the Great East Japan Earthquake and tsunami on March 11.

The first plume headed westward from late March 14 to early March 15, when huge amounts of radioactive materials were released following a meltdown at the No. 2 reactor. It moved clockwise over a wide area in the Kanto region. Radioactive materials fell with rain and snow, particularly in the northern parts of Tochigi and Gunma prefectures.

A branch of the plume moved southward from Gunma Prefecture, passing through western Saitama Prefecture, eastern Nagano Prefecture and western Tokyo. It stopped in western Kanagawa Prefecture, where the Tanzawa mountain range rises up.

The second plume headed northwest in the afternoon of March 15, heavily contaminating parts of Namie and other municipalities in Fukushima Prefecture.

The third plume headed northward, apparently in the afternoon of March 20. Areas in northern Miyagi Prefecture and southern Iwate Prefecture were probably contaminated when it rained between the late afternoon of March 20 and early March 21.

The fourth plume headed southward from the night of March 21 and early March 22. It passed through northern Chiba Prefecture but largely skirted central Tokyo due to a pressure pattern, limiting contamination in Tokyo and Kanagawa Prefecture.

It is believed that the amount of radioactive materials released from the Fukushima No. 1 plant increased between March 20 and 23, but the reasons are not yet known.

In Fukushima and seven other prefectures, 11,600 square kilometers, or 3 percent of Japan's land area, have annual additional radiation levels of at least 1 millisievert, according to Environment Ministry estimates.

The government has said it will remove radioactive materials if annual additional radiation levels reach 1 millisievert or more.

The science ministry has been carrying out aerial monitoring of radioactive materials since April. Helicopters fly at relatively low speeds, allowing monitoring of levels of radiation released from the ground at a height of 1 meter.

Cesium accumulations in soil and radiation levels are also measured separately at selected sites on the ground.

Officials estimate cesium accumulations at other locations using correlations between radiation levels 1 meter above the ground monitored from helicopters and the actual cesium accumulations at the selected sites.

Cesium 137 will have a long-term impact on the environment because it has a half-life of 30 years. It was detected even before the Fukushima accident, apparently as a result of nuclear testing conducted by other nations.

Still, the maximum amount found in nationwide surveys since fiscal 1999 was 4,700 becquerels in Nagano Prefecture. The science ministry's cesium contamination map excludes the effects of pre-disaster contamination.

The map will cover 22 prefectures when it is completed by the end of the year. Data for Aomori, Ishikawa, Fukui and Aichi prefectures will be added.

(By Hiroshi Ishizuka, The Asahi Shimbun, 21/11/2011)