[en] A new core design code system, GALAXY/COSMO-S, has been developed. GALAXY is a lattice physics code which is used to generate a cross section data for the PWR core simulator COSMO-S. This paper presents a new robust cross section representation methodology for COSMO-S. The development of a cross section representation methodology has been carried out based on the theoretical and quality engineering approach. The concept of quality engineering to achieve high quality and high productivity at the same time has been incorporated in the cross section representation methodology to establish the robustness and the efficiency in actual applications. In order to validate this cross section representation methodology, direct comparisons with a lattice physics code have been made on various calculation conditions. As a result, it is concluded that a new cross section representation methodology has enough robustness and accuracy to generate a cross section data for a PWR core simulator. (authors)