[H-CG22] Monitoring the Comprehensive Nuclear-Test-Ban Treaty: Status, operations, and scientific application

[HCG22-P07] Study on distribution of radionuclide fallout in Hiroshima District

*Yayoi Inomata¹, Yasuhito Igarashi², Satoru Endo³, Koichi Takamiya², Hiroshi Yashima², Satoshi Fukutani², Maiko Ikegami², Tadashi Mukai², Tomonori Adachi², Jun Inoue⁴, WG Meteorology and Soil (1.Kanazawa University, Institute of Nature and Environmental Technology, 2.Institute for Integrated Radiation and Nuclear Science, Kyoto University, 3.Graduate School of Advanced Science and Engineering, Hiroshima University, 4.Osaka Metropolitan University )

Keywords:137Cs, ex210Pb, Soil inventory, nuclear bombs

It is known that Hiroshima and Nagasaki were devastated by radionuclides from the nuclear bombs dropped on them in 1945 prior to the large-scale nuclear tests, and that there was a localized fallout (black rain). In this study, Hiroshima (latitude 34.3333-34.833333°N; longitude 132.125-132.75°E) was divided into 120 meshes (every 5 km) and soil samples in each mesh were collected to a depth of 30 cm. The concentration distribution of radionuclides in the unaltered soil, which had not been covered by human intervention such as soil covering or soil excavation, was investigated. The target radionuclides were ¹³⁷Cs and ²¹⁰Pb (excess ²¹⁰Pb; ex²¹⁰Pb), which are decay products of ²²²Rn dissipated from the ground surface and naturally occurring radionuclides supplied to the ground surface from the atmosphere by precipitation and dry deposition, and have been used as tracers in studies on forest soil erosion. The purpose of this study is to display the distribution of these radionuclides by compiling soil radioactivity data, etc., and to comprehensively estimate the black rain deposition area by illustrating the past measurement data and experiences.
¹³⁷Cs and ex²¹⁰Pb inventories tended to be higher in the north-northwest region away from the hypocenter. Soil depths in 1945 dated by ex²¹⁰Pb ranged from 17-282 cm (median 56 cm). ²¹⁰Pb inventory tended to be deeper in the 1945 equivalent layer than in the vicinity of the hypocenter. The ¹³⁷Cs inventories within 30 km from the hypocenter reported in the survey of residual radioactivity in Hiroshima and Nagasaki conducted by the Ministry of Health and Welfare (FY1951 and FY1953) were almost in the same range as the values obtained in this study.
The radionuclides in soil are controlled by atmospheric concentrations and deposition by precipitation. Precipitation in Hiroshima was large in the winter-spring season, and areas with large precipitation corresponded to areas with large ¹³⁷Cs and ex²¹⁰Pb inventories. The variation of ¹³⁷Cs concentrations in precipitation in Hiroshima confirms that the fallout of radionuclides originating from large-scale nuclear tests was large during the winter-spring season. In particular, the annual fallout of ¹³⁷Cs in 1963-1964 was clearly larger than that in other years, suggesting that the high inventory layer found in the soil was a trace of large-scale nuclear tests.
Based on the experiences of people who were actually in Hiroshima at the time of the atomic bombing, we estimated the area of black rainfall in the vicinity of the hypocenter (132.375-132.5°E, 34.3-34.5°N). Heavy rain fell west-north of the hypocenter, and “no rain or no comment” was noted in east or south of the hypocenter. Some sites had records of heavy rain or rain despite the low ¹³⁷Cs inventory obtained in this study, indicating that rainfall areas were sparse and rain intensity may have varied greatly from place to place.