Multi-scale thermal-hydraulics methodology and its application to marine small nuclear reactor

<bold>Background</bold>Marine-based small nuclear reactors (SMRs) face unique thermal-hydraulic challenges due to periodic fluid oscillations and complex maritime movements, posing difficulties in design and safety assessments.<bold>Purpose</bold>This study aims to develop a multi-scale analysis method for systematically evaluating thermal-hydraulic behavior and safety of marine SMRs under dynamic marine conditions.<bold>Methods</bold>Firstly, STAR-CCM+ was selected as analysis tool to modify system, subchannel, and Computational Fluid Dynamics (CFD) codes for marine-specific factors impacting system behavior, component heat transfer, and fuel bundle analysis. The RELAP5 source code was modified to simplify the modeling of marine small reactors, and simulation calculations and analysis were considered for typical accident transients such as a small break with a diameter of 5 cm and main pump power loss. Then, targeted methods for system-level, component-level, and fuel bundle scales were established to simplify modeling of marine small reactor with a cross-scale data transmission mechanism enabling multi-scale simulation. Finally, a 5×5 rod bundle was selected as the calculation object, and the influence of different swing angles and swing periods on the average heat transfer coefficient of the rod bundle was calculated.<bold>Results</bold>Analysis reveals significant thermal-hydraulic changes under marine conditions, especially periodic oscillations in thermodynamic parameters and enhanced heat transfer coefficients under oscillatory conditions. The cross-scale accuracy and data coherence are enhanced by coupling CFD with system and subchannel codes.<bold>Conclusions</bold>The proposed multi-scale analysis method effectively evaluates the safety of marine nuclear power reactors and provides theoretical support for design optimization.