Sizes and the Transport Media of Airborne Radiocesium Released from the Fukushima Nuclear Accident

The accident of Fukushima Dai-ichi nuclear power plant discharged a large amount of radioactive materials into the environment. Although the radioactive particles have been collected by use of High-Volume air samplers at many research institutions, the activity size of radionuclides and their mixing state with other aerosol components have not yet been reported. We thus started the sampling of size-segregated aerosol at Tsukuba City, Japan, located 170 km south of the plant on April 28, 2011, by use of a low-pressure cascade impactor. The sampling continued until October 26, 2011, and eight samples in total were collected by that time. The radioactivity of ¹³⁴Cs and ¹³⁷Cs in aerosols collected at each stage were determined by gamma-ray with a high sensitivity Germanic detector. After the gamma-ray spectrometry analysis, the chemical species in the aerosol samples were analyzed.

The activity size distributions of ¹³⁴Cs and ¹³⁷Cs in aerosols measured from the first sample (LPI-AIST-1) reside mostly in the accumulation mode size range and showed the double peak structure. These activity size distributions almost overlapped with the mass size distribution of non-sea-salt sulfate aerosol. We further conducted the successive extraction experiment of radiocesium from the aerosol deposits on the aluminum sheet substrate (8th stage of LPI-AIST-1, 0.5–0.7 µm in aerodynamic diameter) with water, 0.1M HCl, and 1M HCl. In contrast to the relatively insoluble property of Chernobyl radionuclides, those in in the accumulation mode aerosols collected at Tsukuba are completely water-soluble (100%).

From the results, we regard that sulfate is the main transport medium of these radionuclides, and re-suspended soil particles that attached radionuclides were not the major airborne radioactive substances by the end of May, 2011 (Kaneyasu et al. ES&T, 2012). The changes in the activity size distributions of radiocesium and the mass size distribution of major aerosol components for aerosol samples collected by the end of August, 2011, will further be discussed in the presentation.