Generation of subgroup weights employing shielded cross section conservation principle for representative pin cells

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 regularizaron 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.