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, the deformation of fuel pin due to the swelling and thermal bowing may decrease local flow velocity 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, the detailed flow velocity distribution in the subchannel has been obtained by PIV (Particle Image Velocimetry) measurement using a wire-wrapped 3-pin bundle water model.

The test section consisted of an irregular hexagonal acrylic duct tube and fluorinated resin pins which had nearly the same refractive index with that of water and a high light transmission rate. This enables to visualize the inner subchannel through the outer pins. Flow velocity conditions in the pin bundle were set from 0.036 m/s (Re = 270) to 1.6m/s (Re = 13,500).

From the PIV results, it was confirmed that the normalized flow velocity near the wrapping wire in low Re number condition was decreased relatively compared to that in high Re number condition. In the region away from the wrapping wire, the maximum flow velocity was increased by decreasing the Re number. In particular, the maximum flow velocity was more than twice the cross section average velocity in the laminar flow (Re = 270). Moreover, the PIV measurements by using the 3-pin bundle geometry without the wrapping wire were conducted. From the results, the effect of the wrapping wire on the flow field in the subchannel was understood. There experimental results useful not only for understanding of pin bundle thermal hydraulics but also code validation.

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