The treatment of resonance interference effects in the subgroup method

This paper describes the development of a method to treat resonance interference effects within the framework of the subgroup method. The new procedure provides for the treatment of multiple resonance absorbers in which the subgroup weights are determined using a least squares technique and based on the cross sections generated from a mixture of multiple resonance isotopes and a suitably wide range of background cross sections. The method was implemented in the Method of Characteristics code DeCART and validated using MCNP. In order to implement the new method, the NJOY code was used for the calculation of neutron spectra and resonance parameters in for each representative LWR mixture. The resonance parameters, lambda, of the scattering isotopes are computed not just with U-238 as the resonance isotope as in previous applications of the subgroup method, but also with U-235 as resonance isotope for the energy groups in which U-238 has no significant resonances. After developing a procedure for generating lambda factors for scattering isotopes, a method is then described for generating subgroup parameters in a homogeneous system. Again NJOY is used for resonance calculations of a set of mixtures for each resonance isotope at each selected temperature. The group average cross sections instead of the resonance integrals of these mixtures are used to generate subgroup parameters using an optimization algorithm. The generated library is then verified by comparing the solution from DeCART with the solution from MCNP. The method is then extended to a heterogeneous system. The code RMET21 is used for transport calculations for the heterogeneous system. The interference effect from the most important resonance isotopes is treated by generating subgroup weights with resonance cross sections for the cases with two resonance isotopes. The results indicate that the subgroup method can accurately represent resonance interference effects within the framework of the subgroup method. Published by Elsevier Ltd.