Geochemical Journal, Vol. 46 (No. 4), pp. 271-278, 2012
H. Haba, J. Kanaya, H. Mukai, T. Kambara and M. Kase
Nishina Center for Accelerator-Based Science, RIKEN, Wako, Saitama 351-0198, Japan
(Received April 6, 2012; Accepted June 3, 2012)
Airborne radionuclides from the accident of Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011 were measured at the RIKEN Wako Institute, Japan, about 220 km to the southwest of FDNPP, from March 15, 2011 to March 16, 2012. Radioactivity concentrations of 140Ba, 137Cs, 136gCs, 134gCs, 133gI, 132Te, 131I, 129mTe, 110mAg, 99Mo, and 95gNb (m: metastable state; g: ground state) were determined by γ-ray spectrometry with a germanium detector. The time variations of the radioactivity concentrations and their ratios in Wako are discussed by referring to those of the ambient effective dose rate as well as the amount of rainfall. The pronounced peaks of the radioactivity concentrations were observed on March 15, 20-21, and 29-31, 2011. The highest concentrations for typical radionuclides in unit Bq m-3 were 8.8 ± 0.2 for 137Cs, 1.2 ± 0.2 for 136gCs, 8.5 ± 0.2 for 134gCs, 4.7 ± 0.3 for 133gI, 58 ± 2 for 132Te, 8.0 ± 1.2 for 129mTe, and 35 ± 1 for 131I in the sample collected in the period of March 15, 2011, 11:15-11:45 JST (Japan Standard Time). The 134gCs/137Cs radioactivity ratio was determined to be 1.0 ± 0.1 for March 11, 2011, and this value was consistent with other observations related to the FDNPP accident. The environmental radioactive contamination in Wako occurred mainly on March 21, 2011, due to the first rainfall after the accident.
Key words: Fukushima Dai-ichi nuclear power plant accident, airborne radionuclide, 131I, 137Cs, 134gCs