福島第一原子力発電所事故により放出された放射性セシウムの群馬県域での分布  [in Japanese] Distribution of Radiocesium in Gunma Prefecture Released by the Fukushima Daiichi Nuclear Power Plant Accident  [in Japanese]

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Abstract

群馬県域の土壌について,東京電力福島第一原子力発電所事故により放出された放射性セシウムの分布状態をガンマ線スペクトロメトリーによって調査した.事故当日(2011年3月11日)に減衰補正した,群馬県一帯に降下した放射性セシウムのCs-134/Cs-137放射能比は平均で0.996±0.0014であり,2号機あるいは3号機の原子炉で生成したとされる放射性セシウムの同比に近い.事故により群馬県一帯の土壌にもたらされたCs-134及びCs-137の合計放射能濃度は2011年3月25日頃の時点で約1 kBq kg<sup>−1</sup>(幾何平均値)であるが,平野部及び山間部ではそれぞれ0.54及び1.4 kBq kg<sup>−1</sup>(同平均値)であり,山間部で平野部よりも2,3倍高い.土壌の垂直下部方向への放射性セシウムの移動は極めて少なく,事故後5年が経過しても表層4 cm以内に放射性セシウムの95% 以上が保持されている.文部科学省が航空機モニタリングで求めた放射性セシウムの沈着量と今回の土壌分析から求めた沈着量は平野部の10 kBq m<sup>−2</sup>以下の区域を除くとほぼ一致した.

We investigated the distribution of radiocesium in soils of Gunma Prefecture released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The arithmetic mean of the Cs-134/Cs-137 radioactivity ratio of soil samples (<i>n</i> = 253) from Gunma Pref. is 0.996±0.0014 (decay corrected to March 11, 2011), which may indicate that the radiocesium precipitated in Gunma Pref. is mainly derived from the Reactors 2 and 3 in the FDNPP. The geometric mean of the concentrations (decay corrected to March 25, 2011) of radiocesium (Cs-134 + Cs-137) in soil samples (<i>n</i> = 267) collected from Gunma Pref. is about 1 kBq kg<sup>−1</sup>. However, the geometric mean (1.4 kBq kg<sup>−1</sup>, <i>n</i> = 171) of mountainous soil samples of Gunma Pref. is two or three-times higher than that (0.54 kBq kg<sup>−1</sup>, <i>n</i> = 96) of plain soil ones of the same pref. The profiles of vertical distribution of radiocesium in soil samples collected from 3 different environmental situations are quite similar despite the difference in the kinds and amount ratios of clay minerals included. Moreover, more than 95 % of radiocesium is retained within the upper part (0-4 cm), even though 5 years after elapsed. The relationship between the spatial radiation dose rate of sampling points (1 m height) and the concentrations of radiocesium in soil samples is positive but weak (<i>r</i> = 0.65), probably because the concentration of radiocesium in soil samples changes widely even in a small area. The distribution of radiocesium deposition obtained in this work is substantially consistent with that by the MEXT (Ministry of Education, Culture, Sports, Science and Technology) airborne radiation monitoring, except the area (≤10 kBq m<sup>−2</sup>) of the plain region of Gunma Pref.

Journal

  • BUNSEKI KAGAKU

    BUNSEKI KAGAKU 66(4), 281-297, 2017

    The Japan Society for Analytical Chemistry

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