Numerical evaluation of hybrid spacer grid integrating mixing channel with mixing vane for small modular reactors

Spacer grids in light-water reactor fuel assemblies must balance heat-transfer augmentation via subchannel mixing against the pressure-loss penalty. This trade-off is especially consequential in natural-circulation small modular reactors (SMRs), where mixing-vane grids deliver strong near-grid enhancement but can impose prohibitive form losses, whereas channel-type concepts reduce pressure drop but provide weaker mixing. To address this gap, this study presents the first CFD-based proposal and evaluation of hybrid spacer grids integrating a channel-type framework with strategically positioned mixing vanes. Four configurations—a mixing vane grid (MVG), a mixing channel grid (MCG), and hybrid grids with large (LVHG) and small vanes (SVHG)—were assessed using three-dimensional CFD simulations under representative SMR conditions. Crossflow factor, secondary flow intensity, and Nusselt-number distributions quantified mixing and heat transfer together with overall pressure drop. The SVHG showed the best balance, sustaining downstream heat-transfer enhancement within 85% of the MVG while reducing pressure drop by 34% relative to the MVG. Building on this screening, surrogate-based multi-objective optimization using the response surface method (RSM) coupled with a multi-objective genetic algorithm (MOGA) refined the SVHG vane geometry, yielding a Pareto-optimal design with improved thermal–hydraulic balance. These findings support small-vane hybrid grids for SMR fuel assemblies requiring effective passive cooling.