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Simplified oxygen combustion process in OBT analysis

OBT分析における迅速燃焼装置を用いた前処理工程の簡素化

藤原 健壮 ; 桑田 遥*; 御園生 敏治   ; 北村 哲浩   

Fujiwara, Kenso; Kuwata, Haruka*; Misono, Toshiharu; Kitamura, Akihiro

一般的にトリチウムが環境中に排出されたり、生成した場合は、生体半減期が長いため、有機結合型のトリチウムを確認する必要がある。しかしながら、さまざまな形態で存在するトリチウムを分析するための前処理が複雑であるためん、組織内自由水のトリチウムで1ヶ月程度、有機結合型トリチウムでさらに数週間かかってしまう。特に事故時の緊急時における環境モニタリングにおいては、より多くのデータが必要であるため、迅速分析が必要となる。本課室では、組織内自由水では凍結乾燥工程において、有機結合型トリチウムに関しては、燃焼工程において作業を最適化した。組織内自由水については、凍結乾燥のセルに加温乾燥装置を取り付け、組織内自由水の90%を凍結乾燥で、のこり10%を加温乾燥で回収できるように改良した。また、凍結乾燥の試料を小分けし、1つの試料量を減らすことにより乾燥時間の短縮を図った。試料を4つに分けた場合、乾燥時間が3日に短縮された。これまでの工程では14日必要だった工程を3日に減らすことが可能となった。また、真空のセル内で凍結と加温の乾燥を同時に行っているため同位体交換を防ぐと考えられる。有機結合型トリチウムについては、燃焼量を減少させて時間短縮を図り、その検出下限値を評価した。上記の手法で福島沖の魚のトリチウムの濃度を測定した場合、検出下限値が数Bq/kgとなり、実効線量等量は10の-5乗mSvとなった。

Generally, when tritium is produced or released in the environment, it is required to confirm the behavior of organically bound tritium (OBT) due to its longer biological half-life. However, sequential treatment processes are needed to measure the concentration of each type of tritium and it takes a month for measurement of Tissue Free Water Tritium (TFWT) and a few weeks for measurement of OBT. Therefore, simplification of the processes and reduction of the duration were required for the purpose of rapid analysis and obtaining more data on environmental monitoring, especially in case of accident that requires emergency response. We are applying the analytical system composed of the TFWT separation process by means of freeze-drying and the OBT separation process by means of oxygen combustion vessel. In the TFWT separation process, a heater was attached to the outside of the freeze-dried cell, and freeze-drying and heating-drying were continuously performed quickly. The system has been improved that 90% TFWT was removed by freeze-drying and 10% TFWT was removed by heating drying. Then, the freeze-dried samples were divided for reducing the sample volume of one batch. Since the time of freeze-drying depends on the size of the sample, it could be shortened to about 3 days by dividing the sample into four. It successfully leads to reduction of measurement duration, from 14 days to 3 days. And in order to prevent isotope exchange, freeze- drying and heating-drying were performed inside the evacuated cell. In the OBT separation process, the pretreatment time was shortened by reducing the amount of combustion, and the value of detection limit was evaluated. When measured by the above-mentioned method, the tritium concentration of fish samples in Fukushima was below detection limit (several Bq/kg) and committed effective dose was very small (under 10$$^{-5}$$ mSv).

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