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Measurement of niobium reaction rate for material surveillance tests in fast reactors

高速炉の材料サーベイランス試験におけるニオブ反応率の測定

伊藤 主税; 前田 茂貴; 井上 利彦; 富田 英生*; 井口 哲夫*

Ito, Chikara; Maeda, Shigetaka; Inoue, Toshihiko; Tomita, Hideki*; Iguchi, Tetsuo*

Nbドシメータは、中性子断面積のエネルギー分布が鉄の弾き出し損傷関数と相似であり、高速中性子照射量や鉄のdpa評価に適するとともに、反応生成核種である$$^{93m}$$Nbは半減期が16.4年と長く、材料サーベイランス試験における性子照射量モニタになる利点を有する。本研究では、Nbドシメータを溶解・乾燥固化処理して自己吸収効果を低減させるとともに、誘導結合プラズマ質量分析装置でその重量を精度よく求め、誤差4%の高精度で反応率を測定できた。さらに、Nbドシメータ溶解に非放射性Nbを添加することにより、不純物の放射化物である$$^{182}$$Taに起因する妨害X線の影響を定量評価し、反応率の測定精度低下を抑制した。本手法を「常陽」のドシメトリーに適用して$$^{93}$$Nb(n,n')$$^{93m}$$Nb反応率を測定した結果、Nbドシメータによる測定値と多重放射化箔法による評価値の比は誤差の範囲内で一致し、Nb反応率の高精度測定法を確立した。さらに、レーザー共鳴イオン化質量分析法を用いた$$^{93m}$$Nb収率の精密測定法を提案し、Nb原子の超微細構造を利用して$$^{93m}$$Nb原子核を選択的に共鳴イオン化する手法を開発した。

A highly accurate and precise technique for measurement of the $$^{93}$$Nb(n,n')$$^{93m}$$Nb reaction rate was established for the material surveillance tests, etc. in fast reactors. The self-absorption effect on the measurement of the characteristic X-rays emitted by $$^{93m}$$Nb was decreased by the solution and evaporation to dryness of niobium dosimeter. A highly precise count of the number of $$^{93}$$Nb atoms was obtained by measuring the niobium solution concentration using inductively coupled plasma mass spectrometry. X-rays of $$^{93m}$$Nb were measured accurately by means of comparing the X-ray intensity of irradiated niobium solution with that of the solution in which stable $$^{93}$$Nb was added. The difference between both intensities indicates the effect of $$^{182}$$Ta, which is generated from an impurity tantalum, and the intensity of X-rays from $$^{93m}$$Nb was evaluated. Measurement error of the $$^{93}$$Nb(n,n')$$^{93m}$$Nb reaction rate was reduced to be less than 4%, which was equivalent to the other reaction rate errors of dosimeters used for Joyo dosimetry. In addition, an advanced technique using Resonance Ionization Mass Spectrometry was proposed for the precise measurement of $$^{93m}$$Nb yield, and $$^{93m}$$Nb will be resonance-ionized selectively by discriminating the hyperfine splitting of the atomic energy levels between $$^{93}$$Nb and $$^{93m}$$Nb at high resolution.

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