The map, released May 6, was compiled from data from a joint aircraft survey undertaken by the Ministry of Education, Culture, Sports, Science and Technology and the U.S. Department of Energy.
It showed that a belt of contamination, with 3 million to 14.7 million becquerels of cesium-137 per square meter, spread to the northwest of the nuclear plant.
After the 1986 Chernobyl disaster, those living in areas with more than 555,000 becquerels of cesium-137 per square meter were forced to relocate. However, the latest map shows that accumulated radioactivity exceeded this level at some locations outside the official evacuation zones, including the village of Iitate and the town of Namie.
"I am surprised by the extent of the contamination and the vast area it covers," said Tetsuji Imanaka, assistant professor of nuclear engineering at the Kyoto University Research Reactor Institute. "This (map) will be useful in planning evacuation zones as well as the decontamination of roads and public facilities."
The map was made with data showing the amount of accumulated radioactive substances across a grid comprising 1 to 2 km boxes. The measurements were taken between April 6 and April 29.
The U.S. Department of Energy used an airplane and a helicopter to survey the area within a 60-km radius of the plant, while the ministry hired a private helicopter to scan the area within 60 to 80 km.
Dosimeters were mounted on the aircraft, which flew at altitudes of about 150 to 700 meters to survey the ground surface.
The difference in energies between various radioactive substances allowed them to estimate the accumulation of cesium-137 and cesium-134, which have half-lives of about 30 years and two years, respectively.
While the measurements were all taken from the air, the ministry plans to add more data measured on the ground to help fine-tune the delineation of the evacuation zones.
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