The rod bundle fuel assembly is characterized by compact structure and narrow flow passage, and it bears complex mechanical load. Due to the long-time exposure to radiation, the fuel element is possible to swell, bend or deform, and generates fragment pieces. The fragments and corrosion products, flowing with the coolant, can cause local blockage, resulting in local coolant evaporation and threaten the integrity of the fuel cladding. Therefore, it is necessary to study the characteristics of the flow field in the rod bundle channel under local blocking condition.In the current study, Particle Image Velocimetry (PIV) technology is used to visualize and measure the flow fields downstream of the blockages. Since the location and area of the local blockage can happen randomly, experiments are performed under difference blockage conditions, i.e., partial blockage in the side, corner, and interior sub-channels with different sizes of pieces (different blockage ratios).The experiment obtained the flow field data and pressure data downstream of the blockage. The results show that partial blockages will cause flow reversal. Thus, the fluid field can be separated into the mainstream zone and the backflow zone. In the mainstream zone, the average velocity is faster. In the backflow zone, flow reversal happens and vortices are generated downstream of the blockage, causing increase in the resistance. It is not conducive to the transfer of heat between coolant and fuel rods. The influence of the vortices increase with increasing blocking ratio, and decrease with increasing turbulence level downstream of blockage. Therefore, the coolant flow should be increased when a local blocking accident occurs. The degree of turbulence downstream of the blockage is enhanced, reducing the influence of the vortex. To ensure the integrity of the fuel cladding, which will mitigate the consequences of the accident. For the blockage of the interior sub-channels, in the backflow zone, two rows of asymmetrically distributed vortices, and the vortices interfere with each other and cause squeeze deformation. For the blockage of the side and corner sub-channels, the formed vortices have irregular shape and nonuniformed distribution, and the flow field is more complex and changeable. This is believed to be caused by the high intensity turbulence and the influence of the wall. Based on the pressure data, the variation of coefficient of local resistance with the Reynolds number under different blockage conditions is analyzed. It is found that the variation of coefficient of local resistance has a nonlinear increasing trend with the increase of the blocking ratio.Key words : PIV, blockage, rod bundle channel, flow field characteristics

Experimental Study of Characteristics of Flow Field in Rod Bundle Channel Under Blocking Conditions

Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE28/volume/85246

2021 28th International Conference on Nuclear Engineering (ICONE28)

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