| Summary: | 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. === Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.
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