マイクロウェーブ加熱分解/ICP-MS分析による土壌中U及びUの同位体比分析と福島第一原子力発電所事故にかかわる広域土壌調査

Isotope ratio analysis of U and U nuclide using a microwave digestion associated with ICP-MS and the large areal soil survey related to Fukushima Daiichi Nuclear Disaster

高貝 慶隆*; 古川 真*; 長橋 良隆*; 高瀬 つぎ子* ; 敷野 修*; 亀尾 裕 

Takagai, Yoshitaka*; Furukawa, Makoto*; Nagahashi, Yoshitaka*; Takase, Tsugiko*; Shikino, Osamu*; Kameo, Yutaka

マイクロウェーブ分解/ICP-MS分析による土壌中のU及びUの同位体比分析法を開発した。マイクロウェーブ加熱分解では、硝酸-過酸化水素の混酸を用いることで、ケイ酸塩中の天然ウランの溶解を抑制した。岩石標準物質中のウラン同位体比を実試料の同位体比の指標とした結果、放射能を含む標準線源を使用せずに、0.37%の精度でウランの同位体比を測定できることがわかった。U及びUの検出下限値はそれぞれ0.010g/kgであった。本法は、従来法である完全酸分解/ICP-MSあるいは線スペクトロメトリーと比較し、迅速な分析が可能である。

Isotope ratio analysis for U and U in soil samples using a microwave digestion procedure associated with ICP-MS was proposed and demonstrated. In the microwave digestion procedure, the dissolution of natural uranium in silicate (in rocks) was reduced by using a mixture of nitric acid and hydrogen peroxide (to measure the radioactive uranium from a disaster). Upon ICP-MS, the isotope ratio of the certificated geochemical reference material was realized to precisely correct the isotope ratio in real soil samples. In addition, cell-pass voltages can be available to make calibrations and/or to correct the mass bias in the mass-spectrometer. By these effects, the isotope ration of uranium can be measured with an accuracy of 0.37% without using a radioactive standard source. U and U were quantitatively determined, and those detection limits were both 0.010 g/kg. In the case of an emergency, like a nuclear hazard, the proposed method is useful to immediately gather a large amount of information in large area as compared with a common method such as a complete dissolution process associated with an -ray spectrometer or ICP-MS. In addition, the survey of how radioactive uranium spreads was conducted from 7 to 80 km around the Fukushima Daiichi Nuclear Power Plant (Fukushima-DNPP) (115 points in Fukushima prefecture). As a result, the values of the uranium isotope ratio for those soils were similar to the natural abundance, although the various concentrations of uranium were detected from sampling points.