A sodium cooled fast reactor is designed to attain a high burn-up core in a feasibility study on commercialized fast reactor cycle systems. In high burn-up fuel subassemblies, deformation of fuel pin due to the swelling and thermal bowing may decrease flow rate via change of flow area in the sub-assembly and influence the heat removal capability. A 2.5 times enlarged 7-pin bundle water model was applied to investigate the influence of pin bowing and wrapping wire. The test section consisted of a hexagonal acrylic duct tube and fluorinated resin pins which had a refractive index (1.336) nearly the same with water (1.333) and a high light transmission rate (93%). This enabled to visualize around the central pin through the outer pins. In this experiment, the bowing deformation of a fuel pin was simulated by compressing the central pin in the axial direction. Velocity distribution was measured by using Particle Image Velocimetry (PIV) in sub-channels around the central pin in reference and deformation conditions. The Laser Doppler Velocimetry (LDV) was used for the cross-check, the velocity distributions were consistent between PIV and LDV results. The measured velocity distribution around the wrapping wire revealed that the wire influenced to the velocity and RMS in the wide region above the pin surface.

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