A sodium cooled fast reactor is designed to attain a high burn-up core in commercialized fast reactor cycle systems. In high burn-up fuel subassemblies, deformation of fuel pin due to the swelling and thermal bowing may decrease local flow velocity via change of flow area in the subassembly and influence the heat removal capability. Therefore, it is important to obtain the flow velocity distribution in a wire wrapped pin bundle. In this study, water experiments were carried out to investigate the detailed velocity distribution in inner subchannel of the pin bundle geometry. These basic data are not only useful for understanding of pin bundle thermal hydraulics but also code validation.

A wire-wrapped 3-pin bundle water model was applied to investigate the detailed velocity distribution in an inner subchannel surrounded by 3 pins with the wrapping wire. The test section consists of an irregular hexagonal acrylic duct tube and fluorinated resin pins which have nearly the same refractive index with that of water and a high light transmission rate. This refractive index matching enables to visualize the inner subchannel through the outer pins. The velocity distribution in the inner subchannel with the wrapping wire was measured by PIV (Particle Image Velocimetry) through two sides of the duct tube. Typical flow velocity conditions in the pin bundle were 1.6m/s (Re = 13,500) and 0.36m/s (Re = 2,700).

Feature of stream regime in the subchannel existing wrapping wire was visualized in vertical and horizontal plane. The time averaged velocity field in the horizontal plane was reconstructed from the two vertical plane data in different directions. A detailed simulation code based on FEM was applied to the experimental analysis. The calculated velocity distributions were consistent with the experimental data.

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