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Ma X, Kobayashi H. Impacts of vertical variations in soil properties on H*(10) simulations for 137Cs deposition. JOURNAL OF ENVIRONMENTAL RADIOACTIVITY 2024; 279:107524. [PMID: 39197304 DOI: 10.1016/j.jenvrad.2024.107524] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2024] [Revised: 08/17/2024] [Accepted: 08/18/2024] [Indexed: 09/01/2024]
Abstract
Photon transport simulations based on the Monte Carlo method have played a crucial role in assessing and estimating the ambient dose equivalent rates H*(10), resulting from the deposition of 137Cs in soil following the nuclear power plant accident in Fukushima. However, a comprehensive examination of the effect of vertical variations in soil properties on the simulation outcomes has not yet been performed. Disregarding the vertical distribution of soil properties not only leads to potential inaccuracies in the shielding responses of soil layers but also in the determination of the radioactive source inventory, particularly when using the concentration data in Bq/kg. These oversights diminish the reliability of the simulation results. This study addresses several soil property factors that could potentially influence the simulation results, including variations in chemical composition induced by water content, bulk density profile, and estimated inventory profile, all evaluated through an examined simulation model. The results show that inappropriate assignment of the soil density profile can cause considerable errors in the H*(10) simulation outcomes. Furthermore, the sensitivity of H*(10) to variations in soil vertical density is analyzed, with the results indicating that H*(10) can be highly sensitive to changes in the bulk density of the top 0-5 cm soil layers. These results should facilitate the establishment of appropriate simulation strategies and support the reassessment of past simulation results.
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Affiliation(s)
- Xibo Ma
- School of Engineering, Tohoku University, 6-6-11-1201 Aramaki Aoba, Aoba-ku, Sendai, Japan
| | - Hikaru Kobayashi
- School of Engineering, Tohoku University, 6-6-11-1201 Aramaki Aoba, Aoba-ku, Sendai, Japan.
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Malins A, Imamura N, Niizato T, Takahashi J, Kim M, Sakuma K, Shinomiya Y, Miura S, Machida M. Calculations for ambient dose equivalent rates in nine forests in eastern Japan from 134Cs and 137Cs radioactivity measurements. JOURNAL OF ENVIRONMENTAL RADIOACTIVITY 2021; 226:106456. [PMID: 33217723 DOI: 10.1016/j.jenvrad.2020.106456] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/19/2020] [Revised: 10/29/2020] [Accepted: 11/04/2020] [Indexed: 06/11/2023]
Abstract
Understanding the relationship between the distribution of radioactive 134Cs and 137Cs in forests and ambient dose equivalent rates (H˙∗(10)) in the air is important for researching forests in eastern Japan affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This study used a large number of measurements from forest samples, including 134Cs and 137Cs radioactivity concentrations, densities and moisture contents, to perform Monte Carlo radiation transport simulations for H˙∗(10) between 2011 and 2017. Calculated H˙∗(10) at 0.1 and 1 m above the ground had mean residual errors of 19% and 16%, respectively, from measurements taken with handheld NaI(Tl) scintillator survey meters. Setting aside the contributions from natural background radiation, 134Cs and 137Cs in the organic layer and the top 5 cm of forest soil generally made the largest contributions to calculated H˙∗(10). The contributions from 134Cs and 137Cs in the forest canopy were calculated to be largest in the first two years following the accident. Uncertainties were evaluated in the simulation results due to the measurement uncertainties in the model inputs by assuming Gaussian measurement errors. The mean uncertainty (relative standard deviation) of the simulated H˙∗(10) at 1 m height was 11%. The main contributors to the total uncertainty in the simulation results were the accuracies to which the 134Cs and 137Cs radioactivities of the organic layer and top 5 cm of soil, and the vertical distribution of 134Cs and 137Cs within the 5 cm soil layers, were known. Radioactive cesium located in the top 5 cm of soil was the main contributor to H˙∗(10) at 1 m by 2016 or 2017 in the calculation results for all sites. Studies on the 137Cs distribution within forest soil will therefore help explain radiation levels henceforth in forests affected by the FDNPP accident. The merits of this study are that it modelled multiple forests for a long time period, with the important model inputs being informed by field measurements, and it quantified how the measurement uncertainties in these inputs affected the calculation results.
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Affiliation(s)
- Alex Malins
- Japan Atomic Energy Agency, Center for Computational Science and e-Systems, 178-4-4 Wakashiba, Kashiwa, Chiba, 277-0871, Japan.
| | - Naohiro Imamura
- Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan
| | - Tadafumi Niizato
- Japan Atomic Energy Agency, Collaborative Laboratories for Advanced Decommissioning Science (CLADS) Environmental Research Group, 10-2 Fukasaku, Miharu-machi, Tamura-gun, Fukushima, 963-7700, Japan
| | - Junko Takahashi
- Center for Research in Isotopes and Environmental Dynamic, University of Tsukuba, Tennodai 1-1-1, Tsukuba-shi, Ibaraki, 305-8572, Japan
| | - Minsik Kim
- Japan Atomic Energy Agency, Center for Computational Science and e-Systems, 178-4-4 Wakashiba, Kashiwa, Chiba, 277-0871, Japan
| | - Kazuyuki Sakuma
- Japan Atomic Energy Agency, Collaborative Laboratories for Advanced Decommissioning Science (CLADS) Environmental Research Group, 10-2 Fukasaku, Miharu-machi, Tamura-gun, Fukushima, 963-7700, Japan
| | - Yoshiki Shinomiya
- Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan
| | - Satoru Miura
- Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan
| | - Masahiko Machida
- Japan Atomic Energy Agency, Center for Computational Science and e-Systems, 178-4-4 Wakashiba, Kashiwa, Chiba, 277-0871, Japan
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