Source Term Estimation of 131I and 137Cs Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere

It is urgent to assess the radiological dose to the public resulting from the month-long discharge of radioactive materials into the atmosphere from the Fukushima Daiichi Nuclear Power Plant accident in Japan in March 2011. To do this task, computer simulations on the dispersion of radioactive materials in the environment are useful. However, the source term essential to computer simulations was not available. Thus, the Japan Atomic Energy Agency has been trying to estimate the source term of iodine and cesium discharged to the atmosphere. As the first step, the source term was preliminary estimated by coupling environmental monitoring data with atmospheric dispersion simulations. The release rates and total amounts of 131I and 137Cs discharged into the atmosphere were estimated for the period from 12 March to 5 April, 2011. Then, detailed analysis on the local atmospheric dispersion around the Fukushima Daiichi Nuclear Power Plant was carried out, revealing the formation process of high dose rate zones in a northwest direction from the plant. With this and further analyses for the early phase of the accident, the release rates and total amounts of 131I and 137Cs discharged into the atmosphere were reestimated for the period from 12 to 15 March 2011. Finally, the validity of the revised source term was examined by comparing daily and monthly surface deposition (fallout) over land in eastern Japan between measurements and outputs from the regional-scale atmospheric dispersion simulation.