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東電福島第一原発事故後の東海再処理施設周辺における降下じん中放射性物質濃度

Radionuclide concentrations in fallout around Tokai Reprocessing Plant after Fukushima Nuclear Power Plant incident

永岡 美佳  ; 藤田 博喜; 中野 政尚 ; 渡辺 均; 住谷 秀一 

Nagaoka, Mika; Fujita, Hiroki; Nakano, Masanao; Watanabe, Hitoshi; Sumiya, Shuichi

福島第一原子力発電所から南約115km離れている核燃料サイクル工学研究所で降下じんを採取し、放射性核種の分析・測定を実施した。福島原発事故当時は$$gamma$$線放出核種として$$^{131}$$I, $$^{129}$$Te, $$^{129m}$$Te, $$^{132}$$Te等のさまざまな短半減期の放射性核種が検出されたが、現在では$$^{134}$$Cs及び$$^{137}$$Csのみが検出されている。事故から約1年間(平成23年3月$$sim$$平成24年2月)の間の年間降下量を算出すると、$$^{134}$$Cs: 19,000Bq/m$$^{2}$$, $$^{137}$$Cs: 17,000Bq/m$$^{2}$$であった。$$^{137}$$Cs濃度の高い一部の試料からは$$^{90}$$Srが検出され、平成23年3月の$$^{90}$$Sr降下量は5.1Bq/m$$^{2}$$であり、その後濃度は減少していった。一方、Puにおいては、過去のフォールアウトと同様であった。

Monthly deposition samples were collected in the NCL which is located at about 115 km south of the Fukushima Dai-ichi NPP, and the radioactive nuclides were analyzed and measured. Although radionuclides such as $$^{131}$$I (T$$_{1/2}$$=8.02 days), $$^{129m}$$Te-$$^{129}$$Te (T$$_{1/2}$$=33.6 days) and $$^{132}$$Te-$$^{132}$$I (T$$_{1/2}$$=3.2 days) were observed by $$gamma$$ spectrometry at the early time of the Fukushima Dai-ichi NPP incident, only $$^{134}$$Cs and $$^{137}$$Cs are detected now. The annual deposition of $$^{134}$$Cs and $$^{137}$$Cs were about 19,000 and 17,000 Bq/m$$^{2}$$, respectively in March 2011 - February 2012 after the incident. $$^{90}$$Sr was detected in some samples with $$^{137}$$Cs concentration (100-14,000 Bq/m$$^{2}$$). Monthly $$^{90}$$Sr deposition in March 2011 was 5.1 Bq/m$$^{2}$$ and then decreased after that. On the other hand, $$^{239,240}$$Pu concentration was the same level to the concentration before the incident.

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