Flow Visualization on Wire-Wrapped 37-pin Rod Bundle for Sodium-cooled Fast Reactor by Magnetic Resonance Velocimeter*

Sodium-cooled Fast Reactor (SFR) is paid attention to as one of the 4th generation reactor concepts. It uses liquid sodium as a coolant and has the advantage of being able to operate under atmospheric pressure and using spent nuclear fuel. The fuel assembly uses helically wire-wrapped rods for enhanced mixing. However, the effects of such wire and rod bundle on the flow characteristics are hardly uncovered by flow visualization technique due to complicated flow geometry. Using Magnetic Resonance Velocimetry (MRV), we performed quantitative measurements of 3 dimensional (3D), 3 component (3C) mean velocities in a 3D-printed flow model of wire-wrapped 37-pin rod bundle with an aqueous solution as a working fluid. The MRV results were validated by comparing the flow distribution by subchannels against other independent experimental data. The flow was successfully resolved by MRV to show secondary flows behind the wires. We will discuss the effects of the wire-wrapped structure on the global flow, non-unform flow distributions caused by helical wires, etc. For example, we found that the subchannel region of a locally low flowrate is rotated with a 90-degree phase lag of wire rotation. In addition, we were able to observe a global swirling motion due to the helical wire structure.