For stability analyses of the Swiss operating boiling water reactors, the methodology employed and validated so far at the Paul Scherrer Institute (PSI) was based on the RAMONA-3 code with a hybrid upstream static lattice/core analysis approach using CASMO-4 and PRESTO-2. More recently, steps were undertaken toward a new methodology based on the SIMULATE-3K (S3K) code for the dynamical analyses combined with the CMSYS system, which relies on the CASMO/SIMULATE-3 suite of codes and was established at PSI to serve as framework for the development and validation of reference core models of all the Swiss reactors and operated cycles.

This paper presents a first validation of the new methodology on the basis of a benchmark recently organized by a Swiss utility and including the participation of several international organizations with various codes/methods. Now in parallel, a transition from CASMO-4E (C4E) to CASMO-5M (C5M) as basis for the CMSYS core models was also recently initiated at PSI. Consequently, it was considered adequate to address the impact of this transition both for the steady-state core analyses as well as for the stability calculations and to achieve thereby an integral approach for the validation of the new S3K methodology. Therefore, a comparative assessment of C4 versus C5M is also presented in this paper, with particular emphasis on the void coefficients and their impact on the downstream stability analysis results.