CFD analysis of the ALFRED reactor within the framework of the NEA international benchmark

This thesis work is framed within the context of NEA Lead-cooled Fast Reactor Benchmark dedicated to the Verification and Validation of Computational Fluid Dynamics codes for the design and safety analysis of Generation IV Lead-cooled Fast Reactors. The primary objective is the detailed thermal-hydraulic characterization of a representative fuel assembly of the ALFRED core, investigating the complex interactions between geometry, power distribution, and turbulence modeling in heavy liquid metal flows. The study investigates two primary operating regimes: High Flow (nominal conditions) and Low Flow (representative of Decay Heat Removal scenarios). In the nominal regime, results highlight a marked mass flow redistribution governed by the hexagonal wrapper geometry, resulting in a distinct axial velocity penalty of approximately 35% in the corner sub-channels. The thermal analysis reveals a competition between hydraulic efficiency and power distribution: under uniform power conditions, the Hot Spot is localized in the corner regions; however, the imposition of a realistic radial power profile shifts critical thermal peaks toward the central sub-channels. Conversely, in the Low Flow regime, the system exhibits characteristics of intrinsic safety; the predominance of molecular conduction over turbulent transport leads to a flattening of thermal gradients, significantly reducing inter-channel temperature discrepancies. Finally, the assessment of the Spacer Grids yielded critical insights for fuel design. Contrary to the anticipated enhancement in heat transfer; the presence of grids in the active region induced local cladding temperature overshoots. These thermal spikes are attributed to flow blockage and the formation of stagnation zones behind the structural elements – phenomena that locally outweigh the benefits of turbulent mixing.