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

[HCG22-07] Fundamental Study on the Estimation of the "Black Rain" Area Caused by the A-Bombing: Progress

*Yasuhito Igarashi¹, Hirohiko Ishikawa¹, Masayuki Takigawa², Akiyo Yatagai³, Satoru Endo⁴, Sumi Yokoyama⁵, Naoko FUKUDA⁵, Jun Inoue⁶, Yayoi Inomata⁷, TAKAMIYA Koichi¹, Hiroshi Yashima¹, Satoshi Fukutani¹, Maiko Ikegami¹, Tadashi Mukai¹, Tomonori Adachi¹, WG Meteorology and Soil (1.Institute for Integrated Radiation and Nuclear Science, Kyoto University, 2.Japan Agency for Marine-Earth Science and Technology, 3.Graduate School of Science and Technology, Hirosaki University, 4.Graduate School of Advanced Science and Engineering, Hiroshima University, 5.Atomic Bomb Disease Institute, Nagasaki University, 6.Osaka Metropolitan University, 7.Institute of Nature and Environmental Technology, Kanazawa University)

Keywords:The "Black Rain" caused by A-bomb explosion, Radioactive fallout, Model simulation, Soil survey

[Introduction]
This presentation will outline the progress of the basic study on the estimation of the depositional area of radioactive fallout from the 1945 atomic bombing of Hiroshima and Nagasaki, the so-called "Black Rain". The project was started in January 2021 and the brief introduction was made JpGU2024. In this presentation, we will briefly introduce the progress of the research study since then.
[Methods]
(1) Development of a meteorological simulation model
In order to reproduce the deposition of radioactive materials, taking into account the meteorological conditions at the time of the atomic bomb explosion, the subsequent diffusion of radioactive materials, and the effects of a large-scale fire on transport and diffusion, (1) a meteorological simulation model was constructed and a series of work was conducted as follows.
(a) Explosion reproduction calculation: To provide the meteorological model with information on bomb-derived materials in the explosion cloud, a commercially available computed fluid dynamics model was used to reproduce the fireball condition from about 0.1 second after the explosion to the formation of the explosion cloud.
(b) Reproduction of cloud formation and rainfall by a meteorological model: The calculations were divided into multiple processes, and full-scale calculations were conducted for the case of Hiroshima. We also paid attention to the evaluation of uncertainties in the model calculations. Comparing the calculation results with available meteorological data from Japan, the U.S., and Europe, it was found that the meteorological simulation using the reanalysis data (ERA5) produced by the European Research and Analysis Center for Medium-term Prediction (ERCP) gave the "Black Rain" evaluation with the smallest uncertainty. Therefore, we introduced explosive clouds, city-area fires, and impact dust-derived materials into the meteorological model using ERA5 as input, and conducted deposition calculations using FLEXPART as the transport and diffusion model.
(2) Validation of Model Calculations - Soil Survey
Next, we describe the progress of the soil survey that is being conducted to obtain data to validate the model results. Topsoil samples were collected from a wide area of Hiroshima (approximately 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 radioactive materials and particulate carbon (fine charcoal; 20 μm~) to investigate the diffusion status of radioactive fallout. In the survey, a forest sampling method (scraper-plate method) was adopted, focusing on the existing survey in Nagasaki City and examples of studies on forest soil runoff.
The soil survey showed evidence of "Black Rain" deposition - (a) radioactive materials such as ¹³⁷Cs derived from nuclear explosions, (b) fine-grained charcoal derived from city fires caused by thermal irradiation, transported through a common process while mixing with each other, and possibly (c) shock dust - which was blown up from the ground surface by shock waves. The evidence suggests that these complicated mixture were deposited on the ground surface by precipitation and dry processes together with radioactive materials. In the soil column in the forest environment at the site determined to be undisturbed by the soil survey so far, ¹³⁷Cs from global fallout (GF) and excess ²¹⁰Pb from atmospheric sources are cleanly stratified and remain. Therefore, the depositional age index estimation based on excess ²¹⁰Pb indicates that ¹³⁷Cs from the A-bomb and several cases were found in undisturbed soil columns in Hiroshima and Nagasaki. In such cases the presumed A-bomb derived ¹³⁷Cs concentration maxima or increases were found below the GF-derived ¹³⁷Cs concentration maxima, and in which the stratigraphic sequence of the A-bomb derived ¹³⁷Cs concentration maxima was nearly isotopic to the fine-grained charcoal number concentration maxima.

Acknowledgments: This study was commissioned by the Ministry of Health, Labour and Welfare of Japan, to which the authors thanks are due.