Impacts of Fukushima Daiichi NPP accident through atmospheric environment. First step toward perspective grasp of environmental influence

Environmental influence of the discharge of a large amount of radioactive materials to the atmosphere caused by Fukushima Daiichi NPP accident was overviewed. Monitoring data of air gamma dose rate around at site boundary of NPP suggested discharge was not significant until the evening of March 14, maximum was observed on March 15 and then gradually decreased 3 or 4 decades less until the end of March. Discharge source information was obtained by combining the dust sampling results of radioactive materials concentration in the environment with diffusion simulations by the System for Prediction of Environmental Emergency Dose Information (SPEEDI), such as Iodine 131 release rate was 10¹⁴ Bq/h until March 14, 10¹⁶ Bq/h on March 15 and decreased to 10¹⁴ Bq/h by March 24 and then 10¹³ Bq/h by March 27. As for state of air diffusion, environmental influence was almost decided by the state of air diffusion by around March 25 having a large discharge source. Contamination within 20 km region due to diffusion and deposition, contamination within several ten km in the northwest direction from NPP on March 15 and influence of wide area contamination in Kantou region were discussed. (T. Tanaka)