2011 Fukushima Dai-ichi nuclear power plant accident: summary of regional radioactive deposition monitoring results.

After the Great East Japan Earthquake and resulting Tsunami on March 11, 2011, serious accident of the Fukushima Dai-ichi Nuclear Power Plant has been occurred. Huge amounts of radionuclides were released in atmosphere and ocean. Japanese prefectural governments have carried out environmental radioactivity monitoring; external dose rate, radioactivity measurements in environmental samples and others. Since March 18, 2011, daily and monthly deposition samples were collected in 45 stations covering Japanese Islands and radionuclides in the deposition samples were determined. We summarize radioactive deposition data reported by Japanese Government and study the depositional behaviors of the Fukushima-derived radionuclides. The results revealed that Fukushima-derived radioactive cloud dominantly affected in the central and eastern part of Honshu-Island, although it affected all of Japanese land area and also western North Pacific. The temporal change of the Fukushima-derived (137)Cs revealed that the apparent atmospheric residence time of the Fukushima-derived (137)Cs in sites within 300 km from the Fukushima Dai-ichi NPPis about 10 d.