Fuel assemblies in Pressurized Water Reactor may encounter complex problem of fluid-structure interaction (FSI) under dynamic excitations, such as seismic events. The study focuses on modeling leakage flow in porous media, which is essential for understanding how fuel assemblies behave during such conditions. The research introduces a new approach to strongly coupled FSI in densely packed environments like PWR fuel assemblies, where traditional models such as Darcy’s law are inadequate. The methodology involves filtering local fluid equations, developing a dedicated structural model, coupling fluid and structural variables, and solving the resulting system numerically. Extending a previously validated framework for PWR fuel assemblies under axial flow and external loading, the authors present a leakage flow model to address variable gaps between assemblies. The porous medium approach establishes an equivalent fluid model across the entire assembly space, providing equations for velocity, pressure, and the forces exerted by the fluid on the structure. Additionally, a leakage flow model is developed, treating the inter-assembly fluid domain as a flat region of variable thickness and integrating the Navier-Stokes equations over this thickness. The coupled problem is discretized in time and space using finite element methods, with iterative procedures to resolve the fluid-structure interaction. The model’s accuracy is confirmed through experimental comparisons, including full-scale tests, demonstrating its ability to capture key physical phenomena. Notably, the wall by-pass model yields reliable results without calibration, based solely on geometric parameters. Future work may expand the model to include corner by-passes and further refine grid effects.