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「常陽」照射MA試料の測定結果を用いたMAの燃焼解析法精度評価 -先行基礎工学分野に関する平成7年度報告書-

Accuracy of MA transmutation calculation with MA specimens irradiated in the experimental fast reactor "JOYO"

横山 賢治*; 竹田 敏一*; 山本 敏久; 北田 孝典; 青山 卓史

not registered; Takeda, Toshikazu*; not registered; Kitada, Takanori; Aoyama, Takafumi

マイナーアクチニド(MA)の燃焼に関する研究の一環として、MAの代表核種である237Npについて、高速実験炉「常陽」で照射された237Np試料の測定結果に基づき燃焼解析法の精度評価を行った。解析方法として、燃焼計算にはORIGEN2を用い、中性子束分布計算には従来から用いられている「常陽」炉心管理コードMAGI、3次元拡散計算コードCITATIONに加えて、照射集合体の内部構造を詳細に取り扱うため連続エネルギーモンテカルロコードMVPを導入した。また、ドシメータの反応率からアンフォールディングした中性子束も用い、これらを用いて計算した237Np変換率を比較した。MVPの計算結果の比較により、今回の解析対象集合体では、集合体内部構造の非均質効果が237Npの燃焼計算に与える影響は小さいことがわかった。MAGI、CITATIONの中性子束を用いた237Np変換率の計算は、中性子捕獲、核分裂反応ともに測定値を約30%過大評価した。また、アンフォールディングした中性子束を用いた場合でも測定値より約12$$sim$$20%大きかった。この原因として、MAGI、CITATIONの中性子束計算法の他にアンフォールディングによる中性子束の誤差や237Np変換率の測定誤差についても検討する必要があることがわかった。本研究により、237Np燃焼特性の解析精度を向上させる上での問題点を摘出した。

The accuracy to calculate the transmutation of minor actinides (MAs) was evaluated using MA specimens irradiated in the Experimental Fast Reactor JOYO. The $$^{237}$$Np transmutation characteristics were analyzed in this study. The transmutation ratio of $$^{237}$$Np was calculated by the one point burnup calculation code "ORIGEN2." The neutron fluxes which were input to "ORIGEN2" were calculated with the JOYO core management code system "MAGI" and "CITATION", both are based on three dimensional diffusion theory with 7 energy group. The continuous energy Monte-Carlo code "MVP" was introduced in order to evaluate the detail flux distribution inside the subassembly. The neutron flux obtained by a radiometric dose analysis without $$^{237}$$Np dosimeter was also used for the transmutation calculation. By comparing the flux distributions by MVP with and without the geometric heterogeneity in a subassembly, it was found that there had little effect on the calculation of $$^{237}$$Np transmutation near the core center of JOYO Mark-II. Calculated $$^{237}$$Np transmutation ratio with neutron fluxes by the MAGI and CITATION were 30% larger than that obtained from the $$gamma$$-ray spectrum measurement, whereas the calculated value with the adjusted neutron flux by a radiometric dose analysis was about 12$$sim$$20% larger than the measured value. It is apparently due to an overestimation of the neutron fluxes by the MAGI and CITATION. As it was also found that there were large uncertainties in the adjusted neutron flux and the measured $$^{237}$$Np transmutation ratio, further investigation is required. As a result of this study, the points to be considered were found to improve the accuracy of the analysis.

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