Comparative study between a two–group and a multi–group energy dynamics code

Abstract

The purpose of this study is to evaluate the effects and importance of different cross-section representations and energy group structures for steady state and transient analysis. More energy groups may be more accurate, but the calculation becomes much more expensive, hence a balance between accuracy and calculation effort must be find. This study is aimed at comparing a multi-group energy dynamics code, MGT (Multi-group TINTE) with TINTE (TIme Dependent Neutronics and TEmperatures). TINTE's original version (version 204d) only distinguishes between two energy group structures, namely thermal and fast region with a polynomial reconstruction of cross-sections pre-calculated as a function of different conditions and temperatures. MGT is a TINTE derivative that has been developed, allowing a variable number of broad energy groups. The MGT code will be benchmarked against the OECD PBMR coupled neutronics/thermal hydraulics transient benchmark: the PBMR-400 core design. This comparative study reveals the variations in the results when using two different methods for cross-section generation and multi-group energy structure. Inputs and results received from PBMR (Pty) Ltd. were used to do the comparison. A comparison was done between two-group TINTE and the equivalent two energy groups in MGT as well as between 4, 6 and 8 energy groups in MGT with the different cross-section generation methods, namely inline spectrum- and tabulated cross-section method. The characteristics that are compared are reactor power, moderation- and maximum fuel temperatures and k-effective (only steady state case). This study revealed that a balance between accuracy and calculation effort can be met by using a 4-group energy group structure. A larger part of the available increase in accuracy can be obtained with 4-groups, at the cost of only a small increase in CPU time. The changing of the group structures in the steady state case from 2 to 8 groups has a greater influence on the variation in the results than the cross-section generation method that was used to obtain the results. In the case of a transient calculation, the cross-section generation method has a greater influence on the variation in the results than on the steady state case and has a similar effect to the number of energy groups.