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NRADにおけるMA照射場の高速スペクトル化のためのB$$_{4}$$Cフィルター厚さの最適化検討

Optimization study of B$$_{4}$$C filter thickness for shifting to fast spectrum at experimental position in NRAD core

大泉 昭人; Hummel, A. J.*; Marshall, M. A.*; Palmiotti, G.*; Youinou, G. J.*

Oizumi, Akito; Hummel, A. J.*; Marshall, M. A.*; Palmiotti, G.*; Youinou, G. J.*

米国アイダホ国立研究所(INL)では、熱中性子炉であるNRAD(Neutron Radiography Reactor)の中で、様々な種類の熱中性子遮蔽フィルタを用いて局所的に幅広いスペクトルインデックスを形成した照射場で核分裂反応率やサンプル反応度価値を測定する実験(ANCESTOR実験計画)を提案している。原子力機構では、MAの核データ検証に資するデータの拡充や不確かさを低減するために、NRADで核分裂計数管を用いた積分実験を行うことを計画している。本研究では、NRADの試験領域の中性子スペクトルをより高速化した照査場を構築できるように、熱中性子を遮へいするB$$_{4}$$Cフィルタの厚さについて最適化検討を行った。その結果、低エネルギー中性子の遮蔽能力、中性子束レベル、製作精度に起因する不確かさの予測を総合的に評価し、3mmが最適であると結論づけた。本研究で構築された最適化炉心を用いた積分実験を行うことで、MA核データの検証に資する積分実験データのさらなる拡充が期待される。

The Idaho National Laboratory (INL) has proposed to perform measurements of the fission rate ratio and sample reactivity worth under a wide range of spectral indices created using several types of thermal neutron shielding filters in an experimental position in the Neutron RADiography (NRAD) reactor. The JAEA has a plan to perform integral experiments with fission chambers in the NRAD to further validate the nuclear data of MAs, and ultimately reduce the uncertainty in the cross-sections. In this study, the optimization of the B$$_{4}$$C filter thickness was conducted to acquire the high-accuracy and the beneficial experimental data. The optimized B$$_{4}$$C filter thickness was determined to be 3 mm based on the comprehensive evaluation from the viewpoints of the shielding ability of low energy neutrons, the level of the total neutron flux, and the influence of fabrication accuracy of the B$$_{4}$$C filter. The adoption of the tentative optimized model would be acquired the experimental data such as fission reaction rate and sample worth with the wider spectral indices because the tentative optimized model would be able to increase the ratio of the fast neutrons by almost completely eliminating the effect of the thermal neutrons.

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