ABSTRACT

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.