Estimation of transport and deposition processes of radionuclides emitted from the Fukushima accident using a regional chemical transport model

A powerful tsunami generated by the earthquake occurred at eastside of Japan on March 11, 2011 caused serious damage to several reactors of the Fukushima Nuclear Power Plant #1 (37.42N, 141.04E) on their cooling system. Explosive release of radionuclides, such as iodine 131 and caesium 137 was occurred for several days. The Nuclear Safety Commission of Japan estimated that 150 PBq of iodine 131 was released into the atmosphere by this accident. This value is expected to be about 8% of iodine 131 released at Chernobyl accident. To estimate the budget of total deposition of radionuclides into the land and ocean, we have implemented wet deposition, dry deposition, and the radioactive decay of iodine 131 and caesium 137 into a regional chemical transport model WRF/Chem (Weather Research and Forecast model / Chemistry) version 3.4. The horizontal resolution is 3km, and it has 35 layers up to 100 hPa. The initial and lateral boundary of meteorological field is taken from the operational reanalysis data of mesoscale model (MSM) by Japan Meteorological Agency (JMA). Meteorological fields are also assimilated with the JMA ground-based observations at every 10 minutes by using nudging. As the release of radionuclides was caused by the complex of each reactor's explosion, 12 tagged-tracers were introduced to evaluate each event. The results demonstrate that radionuclides have been mainly depositted over the land via the wet deposition process. Most of the deposition over the land was occured on 15 and 21 March, because easterly or northerly wind were dominant on these days following to the passage of low pressure. The ammount of total deposition has been estimated to be 0.14 TBq into the ocean, and 0.19 TBq into the land. The impact of iodine 131 on human health via inhalation was also estimated, and it was estimated that the horizontal distribution of highly polluted area for inhalation was quite different from where highly deposition was observed.