Spatial distribution of radon and thoron in the soil around active faults as a contribution to the elevated indoor radon

Research Project

Project/Area Number	21K12228
Research Category	Grant-in-Aid for Scientific Research (C)
Allocation Type	Multi-year Fund
Section	一般
Review Section	Basic Section 63010:Environmental dynamic analysis-related
Research Institution	National Institutes for Quantum Science and Technology
Principal Investigator	Janik Miroslaw 国立研究開発法人量子科学技術研究開発機構, 放射線医学研究所計測・線量評価部, 研究員 (30833933)
Co-Investigator(Kenkyū-buntansha)	小平聡国立研究開発法人量子科学技術研究開発機構, 放射線医学研究所計測・線量評価部, グループリーダー (00434324) ゴメスクリストファー神戸大学, 海事科学研究科, 教授 (20800577)
Project Period (FY)	2021-04-01 – 2024-03-31
Project Status	Granted (Fiscal Year 2022)
Budget Amount *help	¥4,160,000 (Direct Cost: ¥3,200,000、Indirect Cost: ¥960,000) Fiscal Year 2023: ¥1,040,000 (Direct Cost: ¥800,000、Indirect Cost: ¥240,000) Fiscal Year 2022: ¥1,690,000 (Direct Cost: ¥1,300,000、Indirect Cost: ¥390,000) Fiscal Year 2021: ¥1,430,000 (Direct Cost: ¥1,100,000、Indirect Cost: ¥330,000)
Keywords	radon / thoron / active fault / permeability / carbon dioxide / radon transport / radon in soil
Outline of Research at the Start	Soil is a major source of the natural radioactive gases radon (Rn) and thoron (Tn). However, while proving very useful for monitoring faults and volcanic activity, Rn emissions can be severely detrimental to human health. Previous investigation of indoor Rn near Fugendake volcano showed elevated Rn concentration value. Since Rn elevation does not necessarily occur in all faults, and the mechanism of this enhancement is still debated, it seems reasonable to investigate how closely there is a relationship between Rn in soil near the tectonic faults and Rn exhaled at the surface.
Outline of Annual Research Achievements	We analysed the data from the first measurements taken near the Fugendake volcano (U1). The radon concentration was lower than expected with low variation of horizonal nor vertical distribution. We therefore chose another active fault site, with a different geological structure near Takayama city (T1). Radon (Rn) and thoron (Tn) concentrations in soil gas with an active device together with radon exhalation rate, soil permeability and CO2 were measured. Measurements of Ra and Th in the collected soil samples were carried out using HPGe detector. Bulk density and porosity of soil samples were determined. Geophysical methods combined with the standard geotechnical cone penetrometer were used to recognize the geological structure and soil characteristics of the area and locate the predicted faults positions and routes. Obtained data were prepared as an input for the radon diffusion-convection transport model.
Current Status of Research Progress	Current Status of Research Progress 2: Research has progressed on the whole more than it was originally planned. Reason The scope of activities set for this year has been fulfilled. As a result of measurement, at point T1 a significant increase in the concentration of radon and thoron was observed in the vicinity of the geological fault. In contrast, at the second point (U1), changes in the radon and thoron concentration near the fault were small. At site T1, radon concentrations range from 5 to 75 kBq/m3, with the highest value occurring on the fault, and thoron from 11 to 37 kBq/m3. At the U1 site radon concentration ranged from 4-5.5 kBq/m3, and thoron from 13 to 23 kBq/m3. At both sites we found a moderate/strong correlation between radon and CO2 concentrations whereas the correlation between permeability and radon is weak. We can assume that such a large discrepancy between measured radon value cannot be explained by the soil formation alone, and one possible explanation is the difference between the types of faults.
Strategy for Future Research Activity	A third measurement campaign is in the pipeline for the final year of the project. The collected data will be used to improve the results of the model. The model output will be both the vertical distribution of radon in soil and the simulation of radon entry rate into a typical (hypothetical) house. The algorithms for evaluating the location of faults will also be improved. The results of the project will be presented at international workshops and conferences and will be published in international scientific journals.