Preliminary Estimation of Release Amounts of 131 I and 137 Cs Accidentally Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere

The Fukushima Daiichi nuclear power plant accident in Japan triggered by a magnitude 9.0 earthquake and resulting tsunami on March 11, 2011 caused the month-long discharge of radioactive materials into the atmosphere. It is urgent to assess the radiological dose to the public resulting from this release, using both environmental monitoring data and computer simulation based on atmospheric dispersion modeling of radioactive materials. However, the source term essential to computer simulations, e.g., nuclides, release rates, and duration, was not available, although it was expected to be provided from a stack monitor and/or a reactor behavior analysis. Thus, in cooperation with the Nuclear Safety Commission of Japan (NSC), the Japan Atomic Energy Agency (JAEA) has been trying to estimate the source term of iodine and cesium discharged from the Fukushima Daiichi nuclear power plant into the atmosphere. The method applied in this estimation is a reverse estimation of source term by coupling environmental monitoring data with atmospheric dispersion simulations under the assumption of unit release rate (1 Bq h ). The authors have already applied this method to the Chernobyl accident, JCO accident, and other incidents, and have considerable experience. This paper estimated the release rates and total amounts of I and Cs discharged into the atmosphere from March 12 to April 5, 2011.