つくばと飯舘における福島第一原発事故由来の大気中放射性セシウム濃度の変化と高濃度現象の要因

Taeko Doi; 土井妙子; Mai Takagi; 高木麻衣; Atsushi Tanaka; 田中敦; Muneo Kanno; 菅野宗夫; Yukiko Dokiya; 土器屋由紀子; Yoichi Tao; 田尾陽一; Kazuyoshi Masumoto; 桝本和義

doi:10.3769/radioisotopes.68.83

つくばと飯舘における福島第一原発事故由来の大気中放射性セシウム濃度の変化と高濃度現象の要因The Variation of Atmospheric Radioactive Caesium Concentration from Fukushima Dai-ichi Nuclear Power Plant Accident in Tsukuba and Iitate, and Factors Controlling Its High Concentration Events

土井妙子¹，高木麻衣²，田中敦³，菅野宗夫⁴，土器屋由紀子⁴，田尾陽一⁴，桝本和義⁵Taeko Doi¹, Mai Takagi², Atsushi Tanaka³, Muneo Kanno⁴, Yukiko Dokiya⁴, Yoichi Tao⁴, Kazuyoshi Masumoto⁵

¹ 国立環境研究所資源循環・廃棄物研究センターNational Institute for Environmental Studies, Material Cycles and Waste Management Research ◇ 305–8506　茨城県つくば市小野川16–2 ◇ 16–2 Onogawa, Tsukuba, Ibaraki 305–8506, Japan

² 国立環境研究所福島支部Fukushima Branch, National Institute for Environmental Studies ◇ 963–7700　福島県田村郡三春町深作10–2 ◇ 10–2 Fukasaku, Miharu-town, Fukushima 963–7700, Japan

³ 国立環境研究所環境計測研究センター National Institute for Environmental Studies, Environmental Measurement and Analysis ◇ 305–8506　茨城県つくば市小野川16–2 ◇ 16–2 Onogawa, Tsukuba, Ibaraki 305–8506, Japan

⁴ 認定NPO法人ふくしま再生の会Resurrection of Fukushima ◇ 960–1815　福島県相馬郡飯舘村佐須滑87–1 ◇ 87–1 Sasusuberi, Iiate-village, Souma-gun, Fukushima 960–1815, Japan

⁵ 高エネルギー加速器研究機構放射線科学センターHigh Energy Accelerator Research Organization, Radiation Science Center ◇ 305–0801　茨城県つくば市大穂1–1 ◇ 1–1 Oho, Tsukuba, Ibaraki 305–0801, Japan

受付日：2018年5月9日Received: May 9, 2018

受理日：2018年10月17日Accepted: October 17, 2018

発行日：2019年3月15日Published: March 15, 2019

東京電力福島第一原発事故一年後のつくばと飯舘の地表大気中の放射性Cs（¹³⁴Cs+¹³⁷Cs）濃度を測定して，それぞれ10⁻⁵ Bq m⁻³，10⁻⁴ Bq m⁻³のレベルであった。飯舘では強風や局地的な除染時の土壌の再浮遊により時々高濃度を示した。2013年8月の高濃度現象は福島第一原発での瓦礫の撤去が原因と推定される。放射性Csエアロゾルの粒径は1.2 µmより大きく呼吸により肺胞まで達するが，その後は排出されると考えられる。

Radiocaesium (¹³⁴Cs+¹³⁷Cs) concentrations in surface air of Tsukuba (Ibaraki prefecture) and Iitate (Fukushima prefecture) about one year after Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident were levels of 10⁻⁵ Bq m⁻³ and 10⁻⁴ Bq m⁻³, respectively. The atmospheric radiocaesium at two sites of Iitate occasionally showed high concentrations due to resuspension of soil particles accompanied with strong wind and operation of local decontamination. A marked high concentration event on August 2013 seems to occur due to the removal of the debris at FDNPP. The activity median aerodynamic diameters of ¹³⁴Cs- and ¹³⁷Cs-bearing particles were larger than 1.2 µm. Radiocaesium-bearing aerosols with particle size of larger than 1 µm may partly be reached into the alveoli by respiration, although they will be excreted from lung thereafter. It is thought that the influence of annual internal radiation exposure by inhalation estimated from the atmospheric concentrations of radiocaesium was low.

Key words: Fukushima Dai-ichi Nuclear Power Plant Accident; radioactive caesium; resuspension; trajectory analysis; aerosol particle size; Tsukuba in Ibaraki; Iitate in Fukushima

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