Thermal-hydraulic study of a wire spacer fuel assembly

► Thermal-hydraulic investigation of wire-wrapped fuel rod bundle. ► Influence on the results of the turbulence model and geometry meshing. ► The heat transfer employs simplified boundary condition for the temperature. ► Flow features remain unchanged as the number of pin increases. ► Global parameters (i.e. friction factor) have a good agreement with experiments. The paper presents refined three-dimensional simulations of the flow and heat transfer in fuel assemblies as found or suggested for liquid metal coolant fast reactors. The wire spacers, helically wound along the pin axis, generate a strong secondary flow pattern in opposition to smooth pins. The eddy viscosity and second moment turbulence models yield to very similar predictions of global friction and heat transfer coefficients and within the range of available experimental correlations. The four configurations simulated range from a small test rig to the full scale reactor bundle (7, 19, 61 and 271 pins) in order to separate (a) global swirl boundary effects, where helical wires leaning against the casing deflect the flow in unison, from (b) homogeneous flow patterns in the core, where wire helices counteract each other. The 61 and 271 pins simulations show a clear decoupling of (a) and (b). The effect of the variation of the helix pass has also been investigated for the 7 pin geometry. Finally, the paper considers the variation of the results as function of the meshing and in particular with the level of detail used for the pin-wire connection.