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Characterization of nuclear material by neutron resonance transmission analysis

中性子共鳴透過分析法による核物質の特性評価

Schillebeeckx, P.*; Alaerts, G.*; Becker, B.*; Paradela, C.*; Heyse, J.*; Kopecky, S.*; Vendelbo, D.*; Wynants, R.*; 原田 秀郎  ; 北谷 文人 ; 小泉 光生 ; 土屋 晴文

Schillebeeckx, P.*; Alaerts, G.*; Becker, B.*; Paradela, C.*; Heyse, J.*; Kopecky, S.*; Vendelbo, D.*; Wynants, R.*; Harada, Hideo; Kitatani, Fumito; Koizumi, Mitsuo; Tsuchiya, Harufumi

中性子共鳴反応を利用して非破壊で試料を分析する手法に、中性子共鳴透過分析法(NRTA)がある。NRTAは中性子を試料に照射し、透過してくる中性子を計測することで試料を分析する。我々はJRC-IRMM(共同研究センター/標準物質・測定研究所)の中性子飛行時間施設GELINAにおいて、核物質の特性を評価するためNRTAの研究開発を行っている。その一環として、過酷事故で発生が予測される粒子状溶融燃料デブリに含まれる核物質の定量にNRTAを適用することを目的とした開発に取り組んできた。その開発の中で、デブリ中の核物質の定量精度が中性子吸収材のボロン($$^{10}$$B)の存在量やデブリのさまざまな形や大きさに強く影響を受けることがわかった。こうしたデブリに特徴的な性質に対処するため、解析手法の改良やモデルの開発を行ってきた。本発表では、開発した解析手法やモデルの妥当性を検討するためGELINAで実施したNRTA実験の結果について報告し、中性子吸収材の存在下においても、NRTAでは2%以下の精度で核分裂性物質の量を導出できることを示す。

The appearance of resonance structures in neutron induced reaction cross sections are fingerprints to study properties of materials and objects. Resonance structures are the basis of an analytical technique, i.e. Neutron Resonance Transmission Analysis (NRTA), which is being developed at the time-of-flight facility GELINA of the JRC-IRMM to characterize special nuclear materials. NRTA is based on the analysis of dips in a transmission spectrum that is obtained from a measurement of the attenuation of the neutron beam by a sample. To apply NRTA for the analysis of particle like debris samples of melted fuel produced in a severe nuclear accident is not evident. From this work one concludes that the accuracy of the results is strongly affected by the characteristics of the samples, in particular by the presence of neutron absorbing impurities, e.g. $$^{10}$$B, and the variety in shape and size of the particle like debris samples. To account for these effects, improved data analysis procedures and interpretation models have been developed. These procedures and models will be presented and validated by results of measurements carried out at GELINA. It will be demonstrated that the relative amount of fissile material can be derived absolutely with an accuracy better than 2% without the need of calibration samples, even in the presence of strong neutron absorbing materials.

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