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3次元輸送計算コードMCNPを用いた森林除染による空間線量率の低減効果の検討

Analysis on the effect of forest decontamination on reducing the air dose rate using the three-dimensional transport code MCNP

邉見 光; 山口 徹治; 武田 聖司; 木村 英雄

Hemmi, Ko; Yamaguchi, Tetsuji; Takeda, Seiji; Kimura, Hideo

福島第一原子力発電所の事故起源の放射性セシウムにより汚染された森林の除染に関して、居住区域における空間線量率の低減が顕著になる汚染源の条件や除染の範囲を感度解析によって検討した。汚染源を$$^{134}$$Csおよび$$^{137}$$Csを含む堆積有機物層(A$$_{0}$$層)と表層土(A$$_{1}$$層)とし、モンテカルロ法による3次元輸送計算コードMCNPを用いて空間線量率を算出した。森林斜面の数、角度、汚染の分布状態、森林土壌中の放射性セシウムの量、除染範囲、林縁から評価点までの距離、評価点の高さをパラメーターとした。その結果、汚染の分布が均一の場合、林縁から20mまでのA$$_{0}$$層の除染が、空間線量率の低減に効果的であることがわかった。一方、林縁から20m以遠の汚染が20m以内よりも高いような、汚染の分布が不均一の場合、A$$_{1}$$層に比べA$$_{0}$$層に含まれる放射性セシウムの量が多い条件においてのみ、林縁から40mまでのA$$_{0}$$層の除染により、空間線量率が顕著に低減した。

Conditions of contaminated sources and ranges of forest decontamination that significantly reduce the air dose rate in residential areas were investigated by means of a sensitivity analysis related to the decontamination of the forest contaminated by radiocesium deriving from the accident at Fukushima Daiichi Nuclear Power Station. The contaminated sources including $$^{134}$$Cs and $$^{137}$$Cs were assumed to be a layer of sedimented organic matter (the A$$_{0}$$ layer) and surface soils (the A$$_{1}$$ layer). The air dose rates were calculated using the three-dimensional Monte Carlo transport code MCNP. A slope number of the forest, angles, state of contaminant distribution, radiocesium content in the forest soils, decontamination ranges, distance from the forest boundary to an evaluation point, and height at the evaluation point were adopted as the parameters. The decontamination of a litter (A$$_{0}$$) layer within the distance of 20 m from the forest boundary was revealed to be effective in reducing the air dose rate when the source distribution was homogeneous. The air dose rates were significantly reduced by the decontamination of the A$$_{0}$$ layer within a distance of 40 m from the forest boundary on condition that the radiocesium content of the A$$_{0}$$ layer was larger than that of the A$$_{1}$$ layer and the source distribution was non-homogeneous, such as the forest areas beyond 20 m from the forest boundary, which were more heavily contaminated than those within 20 m.

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