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Insufficient self-shielding correction in VITAMIN-B6

VITAMIN-B6の不十分な自己遮蔽補正

今野 力; 落合 謙太郎; 大西 世紀

Konno, Chikara; Ochiai, Kentaro; Onishi, Seiki

ENDF/B-VIから作られた多群ライブラリVITAMIN-B6を使った簡単なベンチマークテストを行った。このテストでは、中心に20MeVの中性子源を置いた直径1mの鉄球内の中性子スペクトルをANISNで計算するというもので、VITAMIN-B6を使ったANISN計算の結果は、FENDLを使ったANISN計算,MCNP計算の結果と最大50%の違いが生じた。この違いの原因を詳細に調べたところ、VITAMIN-B6では、$$^{56}$$Feのバックグランド断面積の最小値が1で、また、1/($$sigma$$$$_{t}$$+$$sigma$$$$_{0}$$)$$^{l+1}$$とすべき加重関数を1/($$sigma$$$$_{t}$$+$$sigma$$$$_{0}$$)の加重関数を採用していた。そのため、VITAMIN-B6では自己遮蔽補正が適切に行われず、その結果、ベンチマークテストでVITAMIN-B6を使ったANISN計算の結果は、FENDLを使ったANISN計算,MCNP計算の結果と大きく異なったことがわかった。

We carried out a simple benchmark calculation test with the multigroup cross-section library VITAMIN-B6 generated from ENDF/B-VI. The model of this test consisted of an iron sphere of 1 m in radius with an isotropic 20 MeV neutron source in the center. Neutron spectra in the sphere were calculated with the Sn code ANISN and VITAMIN-B6 or FENDL/MG-1.1. A calculation with MCNP and FENDL/MC-1.1 was carried out as a reference. The neutron spectra with ANISN and FENDL/MG-1.1 agreed with those with MCNP, while those with ANISN and VITAMIN-B6 were by at most 50% larger than those with MCNP. We examined VITAMIN-B6 in detail in order to investigate causes for the discrepancy. As the result, it was founded out that reasons of the discrepancy were the followings; (1) The smallest background cross section of $$^{56}$$Fe in VITAMIN-B6 is 1. (2) The weighting function used in generating VITAMIN-B6 is not adequate. VITAMIN-B6 should be revised for adequate self-shielding correction.

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