The national mapping projects were implemented with the collaboration of many organizations in order to obtain reliable and detailed information on radiological conditions due to the Fukushima Daiichi Nuclear Power Plant accident. In the projects, repeated large-scale environmental monitoring has clarified the distributions of radionuclide deposition densities and air dose rates over wide areas and their time-dependent tendencies. In parallel, migration of radiocesium was investigated in the test sites having different environmental conditions to draw a comprehensive picture on the movement of radiocesium in the environment. It turned out that the migration velocity of radiocesium drastically varies depending on conditions and the variation was reflected in air dose rates. The radiocesium migration was slow in undisturbed fields and forest; while monitoring data in urban and water areas implied fast migration velocity of radiocesium in these areas. It was confirmed that radiocesium movement through rivers plays an essential role in long-distance migration of radiocesium. In order to securely store the obtained environmental data and open them to the public, a database was constructed and has been maintained. In the latter half of the projects, we started construction of a numerical model to predict contamination conditions according to the statistical analysis of accumulated monitoring data, as well as numerical models to simulate migration of radiocesium.
Keywords: Database; Fukushima Daiichi NPP accident; Large-scale environmental monitoring; Migration study; National mapping projects; Prediction models.
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