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Impact of capture cross-section of carbon on nuclear design for HTGRs

高温ガス炉の核設計における炭素の捕獲断面積の影響

島川 聡司; 後藤 実; 中川 繁昭; 橘 幸男

Shimakawa, Satoshi; Goto, Minoru; Nakagawa, Shigeaki; Tachibana, Yukio

炭素の熱エネルギー領域の捕獲断面積はわずか3mbと小さいため、その精度は、一般的な原子炉の核設計においては重要視されてこなかったが、高温ガス炉(HTGR)のように黒鉛リッチな原子炉の核設計においては無視できない。5%の炭素の捕獲断面積の違いは、原子力機構の高温工学試験研究炉(HTTR)の臨界近接試験を対象としたモンテカルロ計算において、0.24から0.47%$$Delta$$$$k$$の実効増倍率の違いをもたらす。JENDL-3.3, ENDF/B-VII、又はJEFF-3.1を用いたモンテカルロ計算は、HTTRの実効増倍率を過大評価する。JENDL-3.3を用いた場合、他のライブラリを用いた場合に比べて測定値に近い計算値が得られるものの、HTTRの臨界近接試験結果を、0.4から1.0%$$Delta$$$$k$$過大評価する。JENDLの最新版であるJENDL-4では、われわれのHTTRの核特性解析に関する知見に基づき、熱領域の炭素の捕獲断面積は3.86mbに見直された。この値は、ENDF/B-VII及びJEFF-3.1の3.36mb、JENDL-3.3の3.53mbに比べて10から15%大きい。その結果、JENDL-4をHTTRのモンテカルロ計算に用いることで、従来の実効増倍率の過大評価を予想通り大幅に改善することができた。

Capture cross section of carbon in thermal energy range has been regarded as unimportant in neutronics calculations on general reactor design, because of its infinitesimal value of only 3 mb at 2200 m/s. However, it is not negligible for design works for graphite-rich reactors, such as the High Temperature Gas-cooled Reactors (HTGRs). For the High Temperature Engineering Test Reactor (HTTR) of JAEA, five percent differences in capture cross section of carbon makes 0.24% change in thermal utilization factor of the four factor formula. This impact is for the HTTR with a core configuration of full-loaded core, named the packed core. In this case, change of multiplier factor will be equivalent to a change of thermal utilization factor. The impact of the cross section is dependent on an atomic number ratio of graphite/235-uranimu in reactor core. For more graphite-rich core such as the HTTR with ring core configuration, the five percent change of the cross section value makes a 0.47%$$Delta$$$$k$$ on multiplier factor. From our studies in the HTTR analysis, a value of capture cross section at 2200 m/s has been revised to 3.86 mb in evaluated nuclear data library of JENDL-4. Comparing with the value of JENDL4, the values in other libraries are about 10-15% smaller as 3.36 mb in ENDF/B-VII, 3.36 mb in JEFF-3.1 and 3.53 mb in JENDL-3.3. It was observed that discrepancy of a multiplier factor between former calculation and experiment of the HTTR showed disagreement in the evaluation of the critical approach tests. Monte Carlo calculation results using JENDL3.3 are overestimated about 0.4%$$Delta$$$$k$$ with packed core configuration and 1.0%$$Delta$$$$k$$ with ring core, respectively. In this report, the improvement of excess reactivity calculation for the HTTR with newly JENDL-4 is described.

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