Spatiotemporal Distribution of Radionuclides in Soil in the Tokyo Metropolitan Area

In the first stage after the FDNPP accident, radioactive contamination on the ground surface in the Tokyo metropolitan area was controlled by weather conditions such as wind direction and rainfall. Many radionuclides in the radioactive plume released from the FDNPP reactors were advected in the atmosphere without falling to the surface. However, some of them precipitated to the ground surface, causing environmental radiation contamination in Tokyo. These radionuclides diffused, which is believed to have caused temporary but significant external and internal exposure to the Japanese people. However, given the destruction of infrastructure due to the massive earthquake, there are almost no measurement data regarding environmental radioactive contamination immediately after the FDNPP nuclear accident. The collapse of infrastructure occurred not only in Fukushima Prefecture where the FDNPP operated but also in the capital Tokyo more than 200 km from the FDNPP. However, the status of environmental radioactive contamination immediately after the accident can be estimated by measuring the contamination by radioactive material recorded in the geosphere or hydrosphere. In this chapter, the geographic distribution of radioactive contamination of soil in the Tokyo metropolitan area during the initial phase of the FDNPP accident is discussed. The results of field sample measurements show that severe environmental radioactive contamination continued for approximately 5 years after the accident. However, after more than 9 years had passed, the radioactive contamination seemed to have changed as radionuclides migrated and were redistributed from the initial contamination situation.