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Impact of revised thermal neutron capture cross section of carbon stored in JENDL-4.0 on HTTR criticality calculation

JENDL-4.0に格納された炭素の熱中性子捕獲断面積がHTTRの臨界計算に及ぼす影響

後藤 実; 島川 聡司; 中尾 安幸*

Goto, Minoru; Shimakawa, Satoshi; Nakao, Yasuyuki*

過去にIAEAにおいてHTTRの臨界近接試験の国際ベンチマーク計算が行われ、その多くは過剰反応度を過大評価した。原子力機構のベンチマーク計算も、他の場合と同様に過剰反応度を過大評価する結果となった。その後、幾何形状モデルの見直しや当時最新の核データライブラリJENDL-3.3の利用により原子力機構の計算結果は改善されたが、過大評価の問題は残った。一方、異なる核データライブラリを用いたHTTRの炉心計算の比較により、ライブラリ間の熱領域における炭素の中性子捕獲断面積のわずかな違いが、計算結果に有意な違いをもたらすことが見いだした。この断面積の値はミリバーンのオーダーで小さいため、原子炉の核計算分野における関心は低く、JEND, ENDF/B、又はJEFFといったメジャーな核データライブラリでさえ、その値は長い間見直されてこなかった。われわれは、HTTRの炉心計算の精度向上の観点から、この断面積を最新の測定値に基づき見直すこと、及び従来に比べて10%程度大きな値に見直すことを提案した。2010年5月、JENDLの最新版JENDL-4.0が原子力機構より公開され、熱領域の炭素の中性子捕獲断面積が最新の測定値に基づき、従来に比べて9%大きな値に見直された。HTTRの臨界近接試験の計算に及ぼす影響を調べた結果、JENDL-3.3を用いた場合に比べて実効増倍率を0.4%$$Delta$$$$k$$-0.9%$$Delta$$$$k$$小さく評価し、その結果、従来の過剰反応度を過大評価する問題を解決できた。

In the past, benchmark calculations of criticality approach for the HTTR, which is a Japanese HTGR, were performed by research institutes in several countries, and almost all of the calculations overestimated the excess reactivity. In Japan, the benchmark calculations performed by JAEA also resulted in overestimation. JAEA improved the calculations by revising the geometric model and replacing the nuclear data library with JENDL-3.3, which was the latest JENDL at that time. However, the overestimation remained and this problem has not been resolved until today. We performed calculations of the HTTR criticality approach with several nuclear data libraries, and found that slight difference in the capture cross section of carbon at thermal energy among the libraries causes significant difference in the $$k$$$$_{eff}$$ values. The cross section value of carbon was not concerned in reactor neutronics calculation because of its small value of the order of 10$$^{-3}$$ burn, and consequently the cross section value was not revised for a long time even in the major nuclear data libraries: JENDL, ENDF/B and JEFF. We thought that the cross section value should be revised based on the latest measurement data in order to improve the accuracy of the neutronics calculations of the HTTR. In April 2010, the latest JENDL;JENDL-4, was released by JAEA, and the capture cross section of carbon was revised. JENDL-4 yielded 0.4%$$Delta$$$$k$$-0.9%$$Delta$$$$k$$ smaller $$k$$$$_{eff}$$ values than JENDL-3.3 in the calculation of the HTTR critical approach, and consequently the problem of the overestimation of the excess reactivity in the HTTR benchmark calculation was resolved.

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