Experimental study on performance of U-pattern wire wrap spacer of fast reactor fuel assembly using PIV

To improve heat transfer performance between sodium coolant and fuel pin, U-pattern wire wrap spacer was proposed to have enhanced cross flow in gap and less pressure drop. From designed pattern of wire wrap spacer, it is expected to have stronger mixing flow between subchannel than that of conventional one. To evaluate the thermal-hydraulic performance of U-pattern wire wrap, two experimental approaches was considered. First, flow visualization and thermal mixing experiment were conducted with 19-pin rod bundle. Non-intrusive flow field measurement could be achieved by matching refractive index of structural material and that of working fluid. In this study, flow visualization experiment was performed with fluorinated ethylene propylene (FEP) and water. Velocity field was analyzed in various flow rates in isothermal condition by particle image velocimetry (PIV) technique, which can find velocity field by analyzing a pair of image for moving particles on designated plane. In addition, thermal mixing experiment was conducted to measure axial temperature distribution inside duct. From experimental results, swirl flow was generated at near wire wrap spacer region by analyzing lateral velocity distribution in subchannel. In center subchannel, locally-zero cross flow region was observed for conventional wire wrap spacer due to flow interactions induced by each pin from its regular pattern. Turbulent kinetic energy representing mixing effect of each subchannel was compared for conventional and U-pattern wire wrap. As a result, U-pattern wire wrap spacer had stronger magnitude of cross flow in center subchannel and minimized zero-cross flow region in same condition, which can enhance local heat transfer capability reducing radial temperature gradient inside duct. Therefore, it is expected to have better thermal-hydraulic performance of SFR fuel assembly with U-pattern wire wrap spacer suggested based on understanding of flow characteristics from wire wrap spacer. In future work, pattern effect of wire wrap spacer will be modeled in terms of correlations of mixing effect and pressure drop characteristics to implement subchannel analysis code. (author)