Local surface and vertical distribution and isotope ratios for radiocesium

放射性Csの表層及び深度分布と同位体比

島田 亜佐子 ; 塚原 剛彦*; 野村 雅夫*; 武田 聖司 

Shimada, Asako; Tsukahara, Takehiko*; Nomura, Masao*; Takeda, Seiji

東京電力福島第一原子力発電所(1F)の事故により広範囲の東日本が放射性セシウムで汚染された。廃止措置終了確認時において放射性Csが検出された場合に、それが廃止措置施設由来なのか1F事故や過去の核実験等のバックグラウンド由来であるか推定するためには、バックグラウンド由来の汚染についての状況把握が必要である。そこで、東海村において、局所的な平面及び鉛直方向へのCsの汚染分布と共にCs/Cs放射能比とCs/Cs同位体比についても測定した。その結果、地表面における平面方向Cs放射能濃度は地点間のみでなく、同じ地点でもかなりばらついているがCs/Cs放射能比はほぼ一定であることが分かった。しかし、Cs/Cs同位体比にはばらつきが認められ、汚染の度合いが大きい地点では1F近傍の結果と誤差の範囲で一致したが、汚染の度合いが低い地点ではややばらつき、同位体比が高い傾向が認められた。1F事故以前のフォールアウトによるCsが影響していると考えられる。また、深度の異なる試料のCs/Cs同位体比についても比較検討結果を報告する予定である。

Because of the accident at the Fukushima Daiichi Nuclear Power Station (1F), a large area of eastern Japan was contaminated by radiocesium. There are many old nuclear facilities in the area. When the old nuclear facilities will be decommissioned in three decades, the radioactive level of radiocesium from 1F would be more than clearance level. When radiocesium would be detected at the completion of their decommissioning in the site, estimation of the origin of which is from background originated 1F and fallout of atmospheric nuclear test or from decommissioned facilities will be important. Therefore, soils of 3 sites were collected by using U8 containers and by core boring, and Cs radioactivity, Cs/Cs radioactivity ratios, Cs/Cs isotope ratios were determined. Separation method of Cs from soil materials were developed based on the reported methods. The Cs radioactivity concentrations were varied in not only between sites but also in the same site, however, Cs/Cs radioactivity ratio was approximately constant in all sites. In case of highly contaminated soils, Cs/Cs isotope ratio was almost comparable to that for the soil collected at near the 1F. On the other hand, in case of lower contaminated soils, Cs/Cs isotope ratio was slightly higher than that for soil collected at near the 1F. It is possibly that fallout before the 1F accident contained Cs to increase the Cs/Cs isotope ratio. In the presentation, vertical distribution will also be discussed.