Characteristics of the Distributions of 131I and Radioactive Cesium in the Soil of the Tokyo Metropolitan Area After the FDNPP Accident

The national and local governments measured the temporal changes in 131I in the atmosphere after the FDNPP accident. These results indicated that the radioactive plume released from the FDNPP spread from Fukushima to the Tokyo metropolitan area and Tokyo itself. The Tokyo metropolitan area was exposed in highly contaminated plumes on March 15–16 and 20–22 immediately after the accident. In central Tokyo, there was a single day with 0.0 mm of light rain on March 16 and a reasonably heavy rainfall of 33.5 mm on March 21–22. Coincidentally, on these rainy days, highly contaminated radioactive plumes had reached central Tokyo, causing radionuclides in the plumes to be washed out and precipitated and leading to radioactive contamination of the ground surface. The radioactive plume spread over in the Tokyo metropolitan area due to the FDNPP accident, significant heterogeneity in radioactive iodine and radioactive cesium was observed on the ground surface in a narrow area. Such non uniformity of the radioactive nuclides precipitated at the surface indicates that the plume, including radionuclides, was advancing with this uneven distribution. In this chapter, the characteristics of radioactive plumes that were transported into the Tokyo metropolitan area are evaluated from the geographic distribution of 131I and radioactive cesium.