However, the fuel assembly is located in the carrier in the transfer tube, which is a semi-closed rectangular cavity. The core of the cooling problem of the fuel assembly in the transfer channel is the natural convection heat transfer of the horizontal rod bundle. The fuel assembly usually employs an arrangement of 17 × 17. It is a key factor affecting the cladding temperature of the fuel assembly. The natural convection in the cross section is more complicated compared with the longitudinal flow along the transfer tube. The flow of cooling water in the cross section of the fuel assembly in the transfer tube is shown in Figure 1B. In the previous analysis, the flow field in the transfer tube can be decomposed into two directions: longitudinal flow along the transfer tube and crosswise flow in the cross section of the transfer tube, respectively ( Xi-dao et al., 2018). The water flows back to the pool through the upper part of the tube. The cooling water is heated in the fuel assembly and then flows out of the assembly through the top holes of the carrier due to the buoyancy. The water flows into the fuel assembly through the circular holes on both sides of the carrier. The cold water from the pool flows from the lower part of the tube to the fuel assembly. The driving force of natural circulation in the transfer tube comes from the density difference of hot and cold fluids.
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