Analytical Study of Criticality Experiments of Organic and Light Water Moderated Mixed Oxide Fuel Pin Arrays

有機溶液及び軽水減速FTR燃料炉心の臨界安全実験解析

相原 永史*; 福村 信男*; 角谷 浩享*; 小山 智造; M.J.Hai*

not registered; Fukumura, Nobuo*; not registered; Koyama, Tomozo; M.J.Hai*

日米共同臨界実験の中の一項目として、有機溶液及び軽水減速材とMCX燃料ピンの格子からなる炉心の臨界実験が行なわれた。この実験結果に対してベンチマーク解析を実施し、精度評価を行った。 解析コードはATRの解析を通じて高精度の実績を得ているWIMS-CITATION結合システムを用いた。解析手法はエネルギー6群、3次元形状モデルを用い、減速材中の水素の散乱核を計算するために、有機溶液及び軽水共通にネルキンモデルを用いた。 解析の結果、軽水減速炉心はKeff=1に対して、0.2%K以内の解析精度で良い一致を示したが、これに比べて有機溶液減速材炉心は、格子ピッチが大きくなるに従い、僅かな過小評価を示すことが分った。これは、格子ピッチが大きくなると減速材の占める割合が増大することに伴って、有機溶液と軽水の化学結合モデルの違いによる影響が現れてくるものと考えられる。 今後、有機溶液減速材特有の中性子散乱核モデルを作成して、軽水減速材の場合と同様の解析精度を確立させる必要がある。

As part of a joint criticality data development between the power peactor and Nuclear Fuel Development Corporation (PNC) of Japan and the United State Department of Energy (USDOE), critical experiments have been conducted with organic moderated Fast Test Reactor (FTR) mixed oxide fuel pin arrays. The neutronic characteristics of an organic moderator can be examined by comparing the results of these experiments with the results of the same type of experiments performed with light water moderated system. In recent experiments performed at the Battelle Pacific Northwest Laboratories Critical Mass Laboratory, five distances of the lattice pitches were utilized which span from soft to hard neutron energy spectra. Results obtained by benchmark analyses of these experiments are discussed in this paper. The benchmark analysis was performed using the coupled WIMS-CITATION computer code whose accuracy has been demonstrated by applying them to heavy water reactor (ATR) with cluster type fuel. The scattering kernel used in the calculation for hydrogen atoms in light water was based on Nelkin's model. This same scattering kernel was used for hydrogen atoms in the organic moderator. The agreement between calculation and experiment (k-eff=1) for the light water moderated cores are fairly good (less than 0.2% k/k). On the other hand, k-eff's for the organic moderated core show a tendency to diverge from k-eff=1 gradually, as the lattice pitches become larger; the approximation used to calculate the scattering kernels for the organic moderator becomes less accurate. It is theorized that small difference in chemical bonding between light water and organic moderator account for the increasing deviation from k-eff=1 for the organic moderators, the larger the fuel pin lattice pitch. Future work would be necessary to establish a more accurate scattering kernel model of the organic moderator to precisely calculate critical conditions for organic moderated fuel pin ...