Abstract

Monthly depositions of anthropogenic radionuclides, which directly reflect atmospheric processes including emission from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), are an important probe to re-construct FDNPP accident sequence. Japanese radioactivity survey network monitored monthly ⁹⁰Sr deposition after the FDNPP accident. The highest monthly ⁹⁰Sr deposition (470±17 Bq m^-2) was observed in March 2011 at Futaba near the FDNPP site, which is the same order of magnitude as that due to global fallout observed in 1963. The higher ⁹⁰Sr deposition in March 2011 occurred in the Kanto and inland of the Tohoku areas. The monthly ⁹⁰Sr deposition decreased with time, as did ¹³⁷Cs. Atmospheric effect of the FDNPP-derived ⁹⁰Sr continued more than 1 year. ⁹⁰Sr/¹³⁷Cs activity ratios in March 2011, which were in the range from 0.99 × 10^-4 to 3 × 10^-4, showed no large spatial variation. This result allowed us to estimate total atmospheric emission of ⁹⁰Sr from the FDNPP accident from the total atmospheric release of ¹³⁷Cs. The total release amount of ⁹⁰Sr at an early stage of accident was calculated to be 2.7 – 3.6TBq.