Great East Japan Earthquake occurred at14:46 JST (05:46 UTC) on 11 March 2011 with the epicenter about 70 km east of the east coast of Miyagi prefecture of Tohoku District (Northeastern Japan). Its magnitude of 9.0 is one of the five most powerful earthquakes since the start of modern quake record in 1900. Severe tsunami waves caused by the quake hit the east coast of East Japan. The quake and tsunami resulted in 15,867 deaths and 2,909 people missing, according to a Japanese National Police Agency report,. Fukushima Dai-Ichi Nuclear Power Plant of Tokyo Electric Power Company (TEPCO) lost all of electric power supplies due to the quake and tsunami and fell into meltdown at three reactors. Hydrogen explosions and reactor ventilations released a lot of radioactive materials into the atmosphere during a successive few weak. Total releases are estimated indirectly from the atmospheric contaminations at 500,000 T Bq for Iodine 131 and 10,000 T Bq for Cesium 137 by TEPCO. This is approximately five times greater than the Three Mile Island accident, but 10 percents of the Chernobyl accident.
2. Atmospheric diffusion and soil contamination
Radioactive materials flowed following the wind and gradually diluted through turbulent mixing. They fell down into the ground surface due to dry and wet depositions. In particular, the wet deposition collects all of radioactive materials below the cloud and sometimes forms hotspots. Radiations come from areal and surface materials. Aerial materials rapidly pass over, so that people should stay inside for a while. More serious are long-lasting radiations from the surface materials. It is very hard to remove them from the environment. In Fukushima, many people had to leave their home towns because of strong radiations.
3. Dissemination of warning to the public
The government took warning of radiation exposure based on the circle centerd at the source position. Actual distributions of areal and surface materials have strong directional dependences, reflecting wind direction and precipitation. The distance is determined considering only isotropic diffusions, but not considering wind direction or wet deposition. Numerical dispersion models are able to predict the atmospheric diffusions and depositions. The government prepared a dispersion model, so-called SPEEDI (System for Prediction of Environmental Emergency Dose Information), but did not open its result to the public. It was afraid that people get into a panic. People, however, complained to the government of the negligence of disclosure.
4. How to use dispersion models for mitigating hazards of radiation exposure
Predictions by dispersion models are subject to the uncertainty of prediction originated from the chaotic nature of the atmosphere. That was another reason that the government hesitated to disclose predictions by dispersion models. Of course, dispersion models provide much better information on areal density and depositions than the concentric circles for the waning. Thus, it is very important to disseminate model products, including their predictability, so that people can use them for mitigating disaster properly. Inthis conference, we would like to discuss how to use model products for emergency cases.