However, group ordering is the way the cross sections are needed in the codes. The SN equations are solved by sweeping down from group 1 to the lower energy groups, so cross sections for all materials and orders are needed for each group in sequence. The library is ordered with group as the outermost loop, then Legendre order, then material. Of course, the SN codes contain the logic to change the input library from material to group ordering if the entire cross-section set will fit into the available memory. If the group structure is too large for the internal logic, the group-ordered TRANSX output can be requested, thereby allowing very large group structures to be used for transport calculations. It may be noted that fusion-related transport calculations (other than hybrid studies) do not require iteration to converge a fission source, so they characteristically employ larger group structures than are practical with multiplying systems. Thus, this group-ordered option, as well as the adjoint option, may be of particular value in fusion.
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