Source terms modeling for spacer grids with mixing vanes for CFD simulations in nuclear reactors

Abstract

In the present study, a numerical method was developed to simulate the presence of spacer grids with mixing vanes in nuclear reactors fuel assemblies. These mixing devices usually have a complex morphology that results in difficult mesh procedures and a high computational cost. Spacers and vanes were simulated using momentum sources to reduce the computational power requirements and to enable the feasibility of a quarter of the full reactor core simulations. Several approaches were tested using RANS models. The starting point is calculation with bodyfitted mesh of one grid span of a fuel assembly with spacer grids featuring dimples, springs and mixing vanes. Velocities and Reynolds stresses are extracted from the solution, then converted to source terms. A new computational domain is created with a coarser mesh and without the presence of dimples, springs, and vanes, but source terms are added to the momentum and Reynolds stress transport equations forcing the solution as the detailed geometry computation. The forcing is imposed only in few mesh elements of the domain corresponding to the original location of dimples, spring and vanes. The approach was numerically stable and relatively implemented in the open source code, Code_Saturne (EDF) and in the commercial code Star-ccm+ (Cd-Adapco). Best practice was defined and grid sensitivity analysis on different quantities was performed. The robustness of the new numerical method was demonstrated. This new methodology creates an intermediate approach that provides higher spatial resolution than subchannel codes and reduced computational cost compared to detailed CFD simulation. Development of the method was based on anisotropic second order closure models, since lower order models are not able to capture swirling and secondary flow generated by the mixing devices; nevertheless, this novel approach applies also to one and two equations turbulence models, without lack of generality.

Initial full core SMR simulations with NekRS

“…There have been several studies in the literature reporting various attempts to explore spacer grid and mixing vanes (SGMV) modeling for either CFD or subchannel analysis. Capone et al (2016) developed a framework to force local flow fields (in RANS simulations) to match higher-fidelity numerical solutions (e.g. LES) where the SGMV is explicitly resolved [1].…”

“…Capone et al (2016) developed a framework to force local flow fields (in RANS simulations) to match higher-fidelity numerical solutions (e.g. LES) where the SGMV is explicitly resolved [1]. Mikuž and Roelofs later proposed a porosity model for the mixing grid that can be used in CFD analysis with very coarse grids [9].…”

“…In the case of fine-scale LES simulations, the grids can be modeled explicitly. In our URANS simulations we used a reduced order methodology based on momentum sources to mimic the mixing of the vanes (see for example Capone et al [1]). The momentum sources have been carefully calibrated with detailed LES simulations of spacer grids performed with Nek5000.…”