A comparative PIV study on flow characteristics in 19-pin and 7-pin wire-wrapped bundle assemblies

The China initial Accelerator Driven System (CiADS) holds significant potential in the transmutation of long-lived nuclear waste and enhancing nuclear fuel utilization efficiency. Investigating the coolant flow characteristics in reactor fuel assemblies is crucial for safety and economic analysis. In this study, a Visual Hydraulic ExperimentaL Platform (VHELP) was established to investigate the velocity distribution in the CiADS fuel assembly. Polymethyl methacrylate and a 62.932 % weight percent sodium iodide solution were used as the bundle materials and the refractive index matching fluid. Particle Image Velocimetry was employed to systematically compare the axial and transverse velocity distributions in the 7-pin and 19-pin wire-wrapped fuel bundles. The results show that the normalized root mean square (RMS) transverse velocity in the internal flow channel of the two bundles reaches a maximum of 24 %. This value is lowered down to 19 % for the side-wall flow channel. In contrast, the normalized RMS axial velocity for the two bundles reaches a 60 % for the internal flow channel while this value increases to 70 % for the side-wall flow channel. Additionally, the maximum normalized transverse velocities at the internal-to-internal and internal-to-sidewall subchannel interfaces are approximately 40 % and 20 %, respectively, for the 7-pin and 19-pin bundles. These results contribute to a deeper understanding of the velocity distributions in wire-wrapped fuel bundles and advance research on the "side-wall effect" in the fuel assembly of liquid metal-cooled fast reactors.