Proceeding of M&C2007 and SNA 2007, Apr 14-19, 2009
A new method for generating subgroup weights, which are required for the subgroup method
based resonance calculation, is formulated by imposing the shielded cross section conservation
principle, rather than the resonance integral conservation. The inconsistency of the background
cross sections defined in the subgroup parameter generation and usage steps is first identified and
the impact of this inconsistency is quantified. To remove this inconsistency which leads to a large
error that would require nontrivial empirical adjustments of the resonance data, a consistent
procedure employing subgroup level dependent background cross sections to conserve the
shielded cross sections for a few number representative pin cells is derived. This procedure
requires the solution of a constrained minimization problem for which a regularization technique
is used to exclude ill-conditioned calculations. Results indicate that this procedure can improve
significantly the accuracy of resonance treatment in codes involving enforced equivalence after the
solution of the subgroup fixed source problem, such as DeCART.
