2012 Volume 61 Issue 7 Pages 373-378
After the emission of radioactive plumes from the Fukushima Daiichi Nuclear Power Plant in mid March, 2011, predominantly Cs nuclei have been responsible for radiation doses since June. The data of air dose rates at 155 points in central and eastern Fukushima prefecture acquired by the central and local governments through the period of June 10th and December 5th are analyzed to investigate their temporal and spatial variations. The calculated average rate of reduction is 22±2%, being twice as large as 11%, which is based on the radioactive decay process of 134Cs and 137Cs during the same period. Therefore, weathering effects are found comparable to those of radioactive decay. Owing to the strong weathering effects, the reduction rate of air dose rate tends to depend on the season. It is also found that some areas are still being contaminated through principally westward winds blowing from more contaminated areas like central-Namie and its vicinity.