Relatively high levels of radioactive cesium have been detected in parts of Niigata Prefecture, according to the science ministry.
Relatively high levels of radioactive cesium have been detected in parts of Niigata Prefecture, according to the science ministry.
The ministry on Oct. 12 released a map showing accumulations of cesium 134 and 137 for Niigata and Akita prefectures.
In aerial surveys between Aug. 30 and Sept. 28, the ministry measured radiation levels from the land surface with highly sensitive detectors. It also measured actual concentrations in soil.
Radioactive cesium, released from the crippled Fukushima No. 1 nuclear power plant, fell to the ground, apparently with rain or snow, after it was carried away by wind.
In Niigata Prefecture, 30,000 to 60,000 becquerels of cesium 137 were detected per square meter in parts of Uonuma and Aga, which border on Fukushima prefecture, and in parts of northern municipalities, including Sekikawa and Murakami.
Cesium 137 will have a long-term impact on the environment because its half-life period is 30 years, compared with two years for cesium 134.
The areas where 30,000 to 60,000 becquerels were detected per square meter are estimated to have radiation levels exceeding 1 millisievert a year, or 0.2-0.5 microsievert per hour.
The government will be responsible for removing radioactive materials if annual additional radiation levels reach 1 millisievert or more, according to an Environment Ministry plan.
The science ministry said it has not found serious cesium contamination in Akita Prefecture.
The highest accumulation levels of cesium 137 were about 20,000 becquerels per square meter in southern parts of Yuzawa, eastern parts of Akita and other locations.
Radiation levels in these areas were mostly in the range of 0.1 microsievert per hour.
The science ministry plans to carry out aerial monitoring of radioactive cesium in 22 prefectures. With Niigata and Akita prefectures, results for 12 prefectures have been published.
After the 1986 Chernobyl nuclear power plant accident in the former Soviet Union, areas with 37,000 becquerels of cesium 137 or more per square meter were designated as contaminated zones.
Residents were forced to evacuate from areas with 550,000 becquerels or more.