[M-AG33] Basic study on the estimation of the "Black Rain" area caused by the atomic bomb

[MAG33-P06] Study on distribution and dispersion of radionuclide fallout in Hiroshima District

*Yayoi Inomata¹, Yasuhito Igarashi², WG Meteorology and Soil (1.Institute of Nature and Environmental Technology, Kanazawa University, 2.Institute for Integrated Radiation and Nuclear Science, Kyoto University)

Keywords:137Cs, ex210Pb, inventory, atomic bomb, large scale nuclear weaponds test , soil

Dominant origin of radiocaesium in soil were large scale nuclear weapon tests by United States and Soviet Union in the 1950-1960s, nuclear power plant accident such as Chernobyl in 1986 and Fukushima in 2011, direct discharge from nuclear reprocessing plants in England and France in the 1970s-1980s. However, before the large scale nuclear test experiment, it was well known that there are local fallout of radionuclides by atomic bomb in Hiroshima and Nagasaki in 1945. The purpose of this study was to determine extent of the effects of radionuclides from the atomic bomb in Hiroshima. Soil samples were collected to a depth of 30 cm by the scraper plate method, in which the soil layer was thinly stripped by dividing it into 120 mesh every 5 km. The concentration distribution of radionuclides in unaltered soil, which has not been covered with soil or excavated by humans, was investigated. The target nuclides are ¹³⁷Cs and ²¹⁰Pb (excess ²¹⁰Pb; ex²¹⁰Pb), which is a decay product of ²²²Rn diffused from the ground surface. These nuclides have been used as tracers in studies of forest soil erosion because they are supplied to the surface from the atmosphere by wet and dry deposition. The ¹³⁷Cs radioactivity concentration in the soil was greatest in the 30-100 mm layer, which may correspond to the highest ¹³⁷Cs deposition from the large scale nuclear weapon tests in 1960-1963. The ¹³⁷Cs from the atomic bomb in 1945 was not clearly discernible in most cases because it was hidden by the signal of the ¹³⁷Cs from the large-scale atmospheric nuclear tests, although some samples showed a small peak in the lower layer of the large-scale nuclear test peak. Inventories of ¹³⁷Cs and ex²¹⁰Pb were higher in areas northwest of the hypocenter. These inventories were correlated with precipitation, suggesting that wet deposition is the dominant process for removal of these nuclides from the atmosphere. The ¹³⁷Cs/ex²¹⁰Pb ratio was also higher in areas with higher inventories. Soils in areas with large ratios were mainly brown forest soils, while soils in areas with small ratios were immature soils, suggesting that the differences in the distribution of ¹³⁷Cs and ex²¹⁰Pb were caused not only by the effect of precipitation but also by differences in soil properties. In the future, we will add new data to the mesh of the target area to elucidate the fallout areas and fallout amounts of radionuclides derived from the atomic bombs. Furthermore, we plan to identify the layer corresponding to 1945 and estimate the horizontal distribution of atomic bomb derived ¹³⁷Cs.

#The analysis of radionuclides in soil was done with the help of many researchers and companies. For the sake of space, we have listed the group as the Soil and Meteorological Working Group.