1:45 PM - 2:00 PM
[MAG33-01] Fundamental study of assessing the area of the "Black Rain" caused by the atomic bombing of Hiroshima and Nagasaki
Keywords:The "Black Rain" caused by A-bomb explosion, Radioactive fallout, Model simulation, Soil survey, Cs-137, Excess Pb-210
This presentation provides an overview of the research study being conducted from January 2021 to FY2023 to accurately estimate the deposition area of radioactive fallout from the atomic bombings of Hiroshima and Nagasaki in 1945, the so-called "Black Rain". The study has two primary components: (1) area reconstruction calculations using a simulation model and (2) soil survey to obtain validation data for the simulation model. This presentation is an overview of the study.
(1) Simulation model construction
(a) Explosion reproduction calculation: To provide the meteorological model with information on bomb-derived materials in the explosion cloud, we conducted reproduction calculations from about 0.1 second after the explosion to the formation of the explosion cloud using a commercially available computed fluid dynamics model using explosion simulation technology.
(b) Calculation of cloud formation and rainfall reproduction using a meteorological model: Calculations were divided into multiple processes, the feasibility of individual calculations for the reproduction of atomic clouds and their temporal evolution was examined, and full-scale calculations were conducted for the case of Hiroshima. We also considered the evaluation of uncertainties in the model calculations. As meteorological data, we selected ensemble-averaged 20th century reanalysis data (20CRv3) produced by the U.S. National Oceanic and Atmospheric Administration (NOAA). The computational domain was a triple nested structure: the area including the Korean Peninsula (3.6 km grid) > western Japan (1.2 km grid) > Hiroshima area (0.4 km grid). Finally, we introduced explosive clouds, urban fires, and shock dust materials into the meteorological model and performed deposition calculations.
(2) Soil survey
A soil survey was conducted to obtain data to verify the model results. That is, topsoil samples were collected in a wide area of Hiroshima (about 110 grids on a 5 km grid), including areas where precipitation events and deposition of radioactive materials were estimated to have occurred, and were analyzed for 137Cs, excess 210Pb and particulate carbon (fine charcoal) to investigate the diffusion status of radioactive fallout. In the survey, we adopted a forest sampling method (scraper-plate method), focusing on the existing survey in Nagasaki City and examples of studies on forest soil runoff.
In the former sediment sample, the common increase of concentration peaks of heavy metals such as Cu and 137Cs, 241Am, fine-grained charcoal, and preserved in the stratigraphy in the bottom sediment confirmed the traces as close-in fallout of the Nagasaki atomic bomb. These are evidences of the "Black Rain" deposition - (1) radioactive fission products such as 137Cs, (2) fine-grained coal originating from city fires caused by heat irradiation were transported through a common process while mixing with each other, and probably (3) shock dust-radioactive materials that were blown up from the ground surface by shock waves were also transported through a common process. In the latter case, soil columns in forest environments show that global fallout-derived 137Cs and excess 210Pb of atmospheric origin are cleanly stratified and persistent at undisturbed soil sites. We thought that such results could be expected for the undisturbed topsoil of Hiroshima and Nagasaki, and conducted a review and investigation.
An overview will be given in this presentation, but details on individual items will be given in individual presentations.
Acknowledgments: This study was funded by the Ministry of Health, Labour and Welfare of Japan, to which we express our gratitude.
(1) Simulation model construction
(a) Explosion reproduction calculation: To provide the meteorological model with information on bomb-derived materials in the explosion cloud, we conducted reproduction calculations from about 0.1 second after the explosion to the formation of the explosion cloud using a commercially available computed fluid dynamics model using explosion simulation technology.
(b) Calculation of cloud formation and rainfall reproduction using a meteorological model: Calculations were divided into multiple processes, the feasibility of individual calculations for the reproduction of atomic clouds and their temporal evolution was examined, and full-scale calculations were conducted for the case of Hiroshima. We also considered the evaluation of uncertainties in the model calculations. As meteorological data, we selected ensemble-averaged 20th century reanalysis data (20CRv3) produced by the U.S. National Oceanic and Atmospheric Administration (NOAA). The computational domain was a triple nested structure: the area including the Korean Peninsula (3.6 km grid) > western Japan (1.2 km grid) > Hiroshima area (0.4 km grid). Finally, we introduced explosive clouds, urban fires, and shock dust materials into the meteorological model and performed deposition calculations.
(2) Soil survey
A soil survey was conducted to obtain data to verify the model results. That is, topsoil samples were collected in a wide area of Hiroshima (about 110 grids on a 5 km grid), including areas where precipitation events and deposition of radioactive materials were estimated to have occurred, and were analyzed for 137Cs, excess 210Pb and particulate carbon (fine charcoal) to investigate the diffusion status of radioactive fallout. In the survey, we adopted a forest sampling method (scraper-plate method), focusing on the existing survey in Nagasaki City and examples of studies on forest soil runoff.
In the former sediment sample, the common increase of concentration peaks of heavy metals such as Cu and 137Cs, 241Am, fine-grained charcoal, and preserved in the stratigraphy in the bottom sediment confirmed the traces as close-in fallout of the Nagasaki atomic bomb. These are evidences of the "Black Rain" deposition - (1) radioactive fission products such as 137Cs, (2) fine-grained coal originating from city fires caused by heat irradiation were transported through a common process while mixing with each other, and probably (3) shock dust-radioactive materials that were blown up from the ground surface by shock waves were also transported through a common process. In the latter case, soil columns in forest environments show that global fallout-derived 137Cs and excess 210Pb of atmospheric origin are cleanly stratified and persistent at undisturbed soil sites. We thought that such results could be expected for the undisturbed topsoil of Hiroshima and Nagasaki, and conducted a review and investigation.
An overview will be given in this presentation, but details on individual items will be given in individual presentations.
Acknowledgments: This study was funded by the Ministry of Health, Labour and Welfare of Japan, to which we express our gratitude.