Applicability of two-phase CFD to nuclear reactor thermalhydraulics and elaboration of Best Practice Guidelines

Two-phase Computational Fluid Dynamics (CFD) is now increasingly applied to some nuclear reactor thermalhydraulic investigations. A Writing Group of the OECD–NEA on the “Extension of CFD to two-phase safety issues” has identified a list of Nuclear Reactor Safety (NRS) issues for which the use of 2-phase CFD can bring a real benefit and proposed a general multi-step methodology. Various modeling options were identified and classified and some first Best Practice Guidelines (BPG) were proposed in the final report of the WG3. The purpose of this paper is to specify the methodology in more detail for the selection of model options, to discuss the conditions and limits of applicability of the various options. Four main modeling approaches are considered, the porous body approach, the RANS approach for open medium, the filtered methods, and the pseudo-DNS (Direct Numerical Simulation). sification of the modeling approaches is proposed with a nomenclature. The conditions to ensure the consistency between the various choices and steps of the methodology are specified, including the coherence between turbulence and interface filtering, between averaging and formulation of the closure laws. A list of frequent errors is given. A checklist for application of two-phase CFD to reactor thermalhydraulic issues is proposed.