Concentration and size-distribution of airborne radionuclides at Nagano city after the accident at the Fukushima 1 nuclear power plant

Concentrations of radionuclides released from the Fukushima 1 nuclear power plant have been measured for surface air, rain and soil at Nagano City (36°39'N, 138°10a). By using γ-ray spectrometry of collected samples we have detected and determined fission products of ¹³¹I, ¹³²Te, ¹³⁴Cs and ¹³⁷Cs. Concentrations of relatively short-lived nuclides, ¹³¹I and ¹³²Te, in surface air rapidly decreased with increasing elapsed time from the accident mainly due to radioactive decay and dilution. Concentrations of long-lived nuclides, ¹³⁴Cs and ¹³⁷Cs, in wet precipitation (rain and snow) gradually decreased due to diffusion and dilution of airborne species in air, and due to so-called washout every rainfall. The particle size distribution of radioactive aerosols has been observed using the Andersen-type classifier combined with the high volume air sampler. Aerosol particles were classified and collected on four glass fiber filters, and on a back-up filter for aerosols smaller than 1.1 μm in diameter. About 80% of collected activities was found in the back-up filter, which means that the majority of radioactivity would attached to aerosols with the diameters less than 1.1 μm. Using three data corresponding to particle-size ranges of 3.3-7.0, 2.0-3.3 and 1.1-2.0 μm, Activity Median Aerodynamic Diameter (AMAD) was estimated under the assumption that the particle size distribution of radioactive aerosols in surface air is in a lognormal and unimodal distribution. Estimated AMADs ¹³¹I-, ¹³⁴Cs- and ¹³⁷Cs-aerosols were ranging from 0.56 to 0.60 μm, which were greater than that of ⁷Be-aerosols, about 0.2 μm. (author)