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 137Cs revealed that the apparent atmospheric residence time of the Fukushima-derived 137Cs in sites within 300 km from the Fukushima Dai-ichi NPPis about 10 d.

► We summarize regional effects of radioactivity fallout from Fukushima Dai-ichi NPP.
► High 131I, and 137Cs depositions occurred in central and northeast Honshu Island.
► The maximum deposition occurred in March within 300 km area from the site.
► In May, the Fukushima fallout decreased rapidly in all of sites in Japan.
► The apparent residence time of the Fukushima-derived 137Cs was about 10 d.