Yuanzineng kexue jishu (Aug 2023)
Flow and Heat Transfer Analysis in Lead-bismuth Reactor Core under Rod Bending Condition
Abstract
Liquid metal-cooled fast reactors (LMFRs) are being investigated in many countries as the next-generation nuclear reactors. Fuel rods in LMFRs are commonly arranged in a triangular pitch and assembled in a hexagonal fuel wrapper. The fuel pin will be subjected to various forces during the irradiation process, leading to the phenomena of bending and deformation. As a result, the performance of the fuel assembly is directly related to reactor safety, reliability, and economy. To investigate the effect of rod bending on the flow and heat transfer characteristics, a computational fluid dynamics (CFD) model of the Karlsruhe Institute of Technology (KIT) 19 rods fuel assembly has been built. The model contains 19 duel rods using lead-bismuth eutectic as working fluid. The mesh of the assembly with and without wire, correlation method of turbulence model and fluid-solid coupling heat transfer were considered in the CFD model. The established model was validated, and the simulation results were found to be in good agreement with experimental data. The simulation results indicated that the SST k-ω turbulent model and Cheng-Tak turbulent Prandtl correlation can be combined to precisely obtain the flow and heat transfer behavior of the coolant in sub-channels. The results show that in the case of neglecting the wire spacer, the rod bending narrows the coolant channel and forms a local high-temperature region. In the deviating direction of bending, the coolant channel widens, promoting mixing and causing coolant temperature to decrease. The maximum circumferential temperature difference of the center rod bending, edge rod bending, and corner rod bending is 46 K, 17 K and 16 K, respectively. In the case of considering the wire spacer, the geometry of the bending rod has been simplified. The influence of the bending wire on the coolant flow resistance is reduced, which results in the failure to reflect the deterioration of coolant heat transfer due to bending. Therefore, the high-temperature region generated by rod bending is less significant due to the wire mixing effect, the high-temperature area is mainly concentrated in the narrow area where the rod contacts the wire. This study reveals the effect of fuel rod performance on the safety and reliability of lead-bismuth reactors, which provides a reference for the reactor safety design. Further study is necessary to reveal the complex heat transfer characteristics and flow mechanism in the case of rod bending with wire spacer. The work presented in this paper represents an important step toward the research on fuel rod bending scenarios in the LMFR fuel assembly.
