廃棄物関連試料の放射性セシウム分析に係る試験所間比較: 土壌, 飛灰, 主灰及び溶融スラグを対象として

Abstract

東北地方太平洋沖地震に起因した福島第一原子力発電所における事故は, 放射性物質の拡散をもたらし, 放射性物質で汚染された廃棄物(放射性物質汚染廃棄物)の処理という困難な問題を生じている. 喫緊の課題である放射性物質汚染廃棄物等の適正処理の確保には, 放射性物質, 特に放射性セシウム({134}Cs, {137}Cs)の精度の高い測定分析が必要である. 本研究では, 分析操作で留意すべき事項や精度改善の必要性等を確認するために, 大量に発生している放射性物質で汚染された土壌と焼却灰に含まれる放射性セシウム分析について, 国内の6試験所による試験所間比較を実施した. 試験所間比較では, 放射能濃度段階別に調製した土壌試料3種(道路脇, 植え込み及び畑地から採取した土壌)及び焼却灰関連試料3種(飛灰, 主灰, 溶融スラグ)の放射性セシウムを, Ge半導体検出器, NaIあるいはLaBr3シンチレーション検出器で測定した. 測定試料の放射能濃度レベルは, 350～64000 Bq kg{−1}({134}Cs＋{137}Cs)程度であり, 放射性物質汚染対処特措法に基づく指定基準(8000 Bq kg{−1}〔{134}Cs＋{137}Cs〕)はこの範囲にはいる. 試験所間比較に参加した試験所で得られた試験所内及び試験所間の測定データは, いずれの相対標準偏差も10% 以下と小さく, 再現性良く取得されていることが示された.

Radiation leaks from the Fukushima Daiichi nuclear power station has occurred after the Japan's Tohoku earthquake on March 11, 2011. Since then, by using validated analytical procedures, the behavior of radioactive materials, especially radiocesium ({134}Cs, {137}Cs), should be investigated for not only government-initiated areawide waste management, but also for routine waste management and recycling. To confirm the necessity to improve the accuracy and precision of the current analytical procedures of radicesium in radiation-contaminated wastes, such as soil and incineration ash samples, we have conducted research for intra- and inter-laboratory comparisons on radiocesium with six participants. During an inter-laboratory study, radiocesium in activity concentration-prepared samples was measured by a Ge semiconductor detector, a NaI(TI) scintillation detector and a LaBr3(Ce) scintillation detector. The mean activity concentrations (n = 14) in roadside soil, plantation soil, agriculture soil, fly ash, bottom ash, and molten slag were 63900, 5610, 356, 21200, 3640, and 670 Bq kg{−1} (the sum of {134}Cs and {137}Cs), respectively. The standard for Special Measures concerning the Handling of Pollution by Radioactive Materials Act in Japan is 8000 Bq kg{−1} (the sum of {134}Cs and {137}Cs) within the obtained activity concentration range. The results demonstrated that the relative standard deviations (RSDs) for intra- and inter-laboratory data from tested samples were less than 10%, indicating participants in this research could obtain reproducible results based on intra- and inter-laboratory comparisons. In this study, small variations in the data were observed as follows: 1) variation in the intra-laboratory data for agriculture soil; the lowest activity concentration of radiocesium tended to be comparably higher than other samples. 2) A detector-dependent variation in the inter-laboratory data was also indicated.