3次元六角体系用中性子輸送計算コードの整備,2; MINISTRIコードの改良及び機能拡張

Development of neutron transport calculation codes for 3-D hexagonal geometry, 2; Improvement and enhancement of the MINISTRI code

杉野 和輝; 滝野 一夫 

Sugino, Kazuteru; Takino, Kazuo

実機高速炉炉心やロシアのBFS臨界実験体系の中性子輸送計算を高精度で行うことが可能な決定論的手法に基づく3次元六角体系用三角メッシュ離散座標法(SN法)輸送計算コードMINISTRIコード(Ver.7.0)を整備した。具体的には、これまで整備したMINISTRIコード(Ver.1.1)の非収束性の問題を詳細に分析し、適切な改良を行うことにより、大型炉心体系への適用における収束性を飛躍的に向上させることができた。改良後のMINISTRIを種々の高速炉炉心を対象にして検証を行った結果、同じ断面積を用いた多群モンテカルロ法計算結果に対して、実効増倍率で0.1%以内、出力分布で0.7%以内の一致が見られ、十分な精度を有することを確認した。また、計算時間に関しては、初期拡散計算機能の導入と並列処理化により、従来と比較して約10分の1への計算時間の短縮を図ることができた。更に、セル非等方ストリーミング効果取り扱い機能の導入、摂動計算ツールの整備、六角格子内三角メッシュ毎断面積指定機能の追加、六角メッシュ計算コードMINIHEX統合を行い、汎用性を高めた。

A deterministic discrete ordinates method (SN method) transport calculation code for three-dimensional hexagonal geometry has been developed as the MINISTRI code (Ver. 7.0). MINISTRI is based on the triangle-mesh finite difference method, which can perform neutron transport calculations with high accuracy for cores of fast power reactors and assemblies of the Russian BFS critical facility. The present study has derived a proper scheme for remarkably improving the convergence of MINISTRI by investigating the issue of previous MINISTRI (Ver. 1.1), which sometimes plays a poor convergence performance in calculations for large-scale power reactor cores. The verification test of improved MINISTRI has been carried out for various cores by setting the reference result as the multi-group Monte-Carlo calculation with the same cross-sections as used in MINISTRI. As a result, it is found that the agreements are within 0.1% for eigenvalues and within 0.7% for power distributions. Thus, the satisfying accuracy of MINISTRI has been confirmed. In order to reduce the calculation time, the initial diffusion calculation scheme and the parallel processing have been implemented. As a result, the calculation time is reduced to the approximately one tenth compared with previous MINISTRI. Furthermore, adoption of the treatment of the anisotropic cell streaming effect, preparation of the perturbation calculation tool, implementation of the function for specification of the triangle-mesh-wise material and merging of the hexagonal-mesh calculation code MINIHEX have been carried out. Thus, the versatility of MINISTRI has been enhanced.