Participation in the NEA/OECD MATiS-H Benchmark exercise. Study of spacer grids in a rod bundle

Abstract

Nuclear fuel bundles contain spacers essentially for mechanical stability and to influence the flow dynamic and heat transfer phenomena along the fuel rods. This work presents the analysis of the turbulence effects of a split-type and swirl-type spacer grid geometries on single-phase in a PWR (Pressurized Water Reactor) rod bundle. Various Computational Fluid Dynamics (CFD) calculations have been performed and the results validated with the experiments of the OECD/NEA-KAERI Rod Bundle CFD Benchmark Exercise on Turbulent Mixing in a Rod Bundle with Spacers at the MATiS-H facility. The aim of this Benchmark is to provide validated CFD analysis tools providing a firm basis for quantifying the CHF Margin reliably for normal operation and operational transients conditions and allowing eventually the use of CMFD Codes for predicting DNB under accidental conditions [1]. Simulation of turbulent phenomena downstream of the spacer grid presents high complexity issues. A wide range of length scales are present increasing the difficulty of defining in detail the transient nature of turbulent flow. Calculations were performed with the commercial code ANSYS R CFX R and CFD modelling using Large Eddy Simulation (LES) turbulence models by comparison with measurements to determine their suitability in the prediction of the turbulence phenomena. One of the most important aspects to be taken into account in order to properly simulate the flow downstream of the spacer grids is the use of a suitable turbulence model. Time-averaged values for all three velocity components, timeaveraged RMS values of the fluctuating component of all three velocity components in several cross-planes downstream of the spacer grid and circulation data in a selected sub-channel are in good agreement with the measured data. These results could be of great value for future studies of spacer grid including heat transfer from the rods and as a basis of spacer grid simplifications.