Local flow characterization depending on a wire angular position of the wire-wrapped rod bundle*

Sodium-cooled fast reactors (SFRs) represent a class of fourth-generation nuclear reactor technologies that utilize sodium as a coolant. The wire-wrapped fuel rod bundle constitutes a significant innovation in SFR design, facilitating enhanced thermal dissipation from the reactor core to its periphery, thereby markedly improving cooling efficiency. Detailed comprehension of local flow dynamics within these bundles is essential for optimizing heat transfer and ensuring the reactor's operational safety. In this study, we focused on the local flow characteristics within a 37-pin wire-wrapped rod bundle, employing three-dimensional, three-component (3D 3C) velocity fields derived from magnetic resonance velocimetry (MRV) measurements. We revealed a cyclic vortex evolution pattern in the wake region, characterized by vortices forming, growing, weakening, and dissipating at 60° intervals relative to the wire's angular position. Furthermore, we identified the periodic nature of flow split factor variations, elucidating their dependency on wire orientation within interior subchannels and the dominant influence of edge swirling in edge subchannels. Our quantitatively derived experimental findings offer critical insights into the flow patterns within the rod bundle, providing essential information for the optimization of reactor design and performance analysis.