CFD validations of ballooning effects during LOCA against MASCARA experiments

• Magnetic Resonance Velocimetry (MRV) data post-processing for blocked subchannels in MASCARA. • Code_Saturne validated for ballooning-induced flow redistribution. • Hotspot localization by recirculation-driven heat transfer downstream of different blockages. This study investigates the thermal–hydraulic behavior through deformed fuel assemblies during LOCAs using CFD simulations. The MASCARA experiments uses a dedicated mock-up and Magnetic Resonance Velocimetry (MRV) to study how ballooned nuclear fuel rods affect coolant flow. We consider CFD validation against MASCARA experimental data. Employing the Code_Saturne solver, we reproduce flow redistribution and extrapolate thermal behavior in a 7x7 fuel rod bundle with different blockage configurations of 4x4 balloons. The chosen turbulence models with carefully designed meshes capture critical flow phenomena, including intense transverse velocities upstream and recirculation zones downstream of blocked regions. Validation against MRV data confirms the reliability of Code_Saturne in replicating experimental observations across various blockage conditions. Thermal simulations reveal the recirculation-driven heat accumulation and evacuation based on different geometry of deformed subchannels. The study is dedicated to improving sub-channel code predictions in ballooning effects and demonstrating the potential of high-fidelity CFD for multiscale nuclear safety assessments.