Geochemical studies on the Chernobyl radioactivity in environmental samples

Artificial radionuclides in deposition and airborne dust samples in 1986 were measured at Tsukuba and 11 stations in Japan. Ln May 1986, the Chernobyl radioactivity was observed in rain and air samples in Japan. The Chernobyl-derived Pu isotopes, which are characterized by higher Pu-238/Pu-239,Pu-240 (85) and Pu-241/Pu-238 (0.5) activity ratios than those of the nuclear test-derived Pu and Sr-90, were detected in deposition and airborne dust samples in Japan, as well as volatile radionuclides such as I-131 and Cs-137. However, the activities of Pu isotopes and Sr-90 observed in Japan were about two and three orders of magnitude lower than those expected from the activity ratios in the total release at Chernobyl, which means that the residence time of Pu in the air was shorter than that of Cs-137. In order to understand the fractionation between the Chernobyl radionuclides we studied about individual wet and dry deposition. The results suggest that this cause is due to the difference of the panicle size of radionuclide-bearing particles, which may be related to the release process of Chernobyl radionuclides.