Sodium side inlet flow distribution characteristics of the steam generator in China demonstration fast reactor

<bold>Background</bold>The sodium-cooled fast reactor China Demonstration Fast Reactor (CFR600) represents one of the most promising directions for the future of nuclear energy in China, which providing advantages such as efficient fuel utilization, higher breeding potential, and enhanced safety features. As a critical component of CFR600, the steam generator (SG) plays a pivotal role in ensuring efficient heat transfer and maintaining the overall stability of the reactor system. However, one of the major challenges encountered in sodium-cooled fast reactors is the non-uniform distribution of coolant flow. This issue can significantly affect the thermal-hydraulic performance of SG and may compromise the operational safety of the reactor system. The uneven flow distribution is influenced by multiple design and operational factors, including the configuration of the flow equalizing grids, the structure and positioning of the inlet windows, the width of annular channels, the height of inlet nozzles, and the coolant flow rate. Therefore, optimizing these design parameters is crucial for improving the efficiency and reliability of the steam generator.<bold>Purpose</bold>This study aims to investigate the sodium-side flow distribution in the CFR600 steam generator and optimize the flow equalizing grid and inlet window design to improve flow uniformity and thermal-hydraulic performance.<bold>Methods</bold>Firstly, a computational fluid dynamics (CFD) approach was employed to perform steady-state simulations of the sodium-side inlet section of SG by using ANSYS CFX, and a grid independence study was first conducted to ensure accurate and efficient simulations, which determined that a mesh size of 7.25 million elements was sufficient to achieve reliable results while maintaining computational efficiency. Then, the effects of three different flow equalizing grid heights and three distinct inlet window structures on sodium-side flow distribution were systematically examined. Finally, a parameter sensitivity analysis was conducted to evaluate the influence of these design factors on flow uniformity. Additionally, a dimensionless non-uniformity index was introduced as a quantitative metric to assess the degree of flow maldistribution within the system.<bold>Results</bold>The simulation results demonstrate that the optimal flow equalizing grid height is 1 200 mm. Furthermore, the most effective inlet window design incorporates a combination of varying hole sizes and arrangements, which significantly improves flow uniformity. The implementation of flow equalizing devices markedly enhances both circumferential and axial flow uniformity in the sodium-side inlet region. Although these devices introduce a slight increase in pressure losses, they substantially improve the overall flow characteristics of the steam generator. Specifically, the maximum relative radial velocity is controlled within the range of 120%⁓150%, and the maximum non-uniformity index is reduced to below 0.35.<bold>Conclusions</bold>Results of this study confirm that optimizing the flow equalizing grid and inlet window structure can significantly enhance the uniformity of sodium coolant flow. These improvements contribute to better thermal-hydraulic performance, leading to higher efficiency and safer operation of the steam generator in CFR600. The findings provide valuable insights into the design optimization of sodium-cooled fast reactor steam generators, offering a theoretical foundation for future reactor developments and technological advancements in fast reactor design.