The treatment of resonance interference effects in the subgroup method

► In this paper, we propose to first remedy the inaccurate treatment of the interference effect between resonance isotopes in conventional subgroup method. ► In our work we use group averaged cross section instead of resonance integrals in the objective function for generating subgroup weights. This is much more effective since group average cross sections are the parameters which will be used in transport calculation. We have shown that the subgroup group weights generated with this method improves the accuracy of the resonance integration. ► We solved the least squares optimization problem with the penalty method for constraints. The Krylov subspace method, CGNR, was found to be very efficient method for solving this type of optimization problem. ► In addition to U-238, which was used as resonance isotope in previous work on lambda factors, U-235 was also treated as a resonance isotope in the lambda calculation developed here. This improves the accuracy of the subgroup method since we are able to provide lambdas for the groups in which U-238 has no significant resonance.