Elucidation and modeling of the effects of forest management on air dose rates

The Fukushima Daiichi Nuclear Power Plant accident, triggered by the Great East Japan Earthquake on March 11, 2011, caused large-scale radioactive contamination across terrestrial areas. Among the released radionuclides, Cesium-137 remains a significant source of radiation due to its long half-life, resulting in persistently high ambient dose rates in contaminated forests. To reduce these dose rates, forest thinning is being implemented as a radiation countermeasure. However, its effectiveness and the specific mechanisms through which thinning influences dose rates remain unclear. This study focuses on the relationship between forest thinning, throughfall, soil moisture, and ambient dose rates. Previous research has shown that rainfall temporarily reduces dose rates by increasing soil moisture, which attenuates gamma radiation from Cesium-137 in the soil. Building on these findings, we investigated the impact of thinning on rainfall reaching the forest floor and its subsequent effect on dose rates. Field studies were conducted at two sites, Iitoi and Fuyuzumi, located approximately 40 km northwest of the FDNPP. Thinning was implemented from October to December 2022, and monitoring devices were installed in April 2024. Results show that thinning increases throughfall and soil moisture, reducing dose rates over time. Soil moisture in thinned plots rose from 33.1% to 35.0%, while control plots decreased from 28.6% to 25.5%. Correspondingly, ambient dose rates dropped from 0.9 μSv/h to 0.75 μSv/h in thinned plots, compared to 0.85 μSv/h in control plots. Based on these observations, we developed a long-term predictive model to estimate ambient dose rates from rainfall and soil moisture data, providing a generalized framework for assessing the long-term impact of forest management on radiation levels in contaminated areas.