Lead-cooled fast reactors (LFRs) are part of the so-called Generation IV reactor technologies and use liquid lead as the primary coolant. While lead has several favorable attributes promoting economics, safety, and sustainability, its relatively high freezing point (327°C) requires coolant solidification to be included among the phenomena of interest in the development of the safety case and operational aspects for the nuclear plant. The assessment of current numerical modeling capabilities to predict coolant solidification and tracking of the solidification front propagation is thus essential to the development of LFRs.

Motivated by the current development of the Westinghouse LFR, this study presents transient computational fluid dynamics (CFD) simulations of lead solidification within a pool-type geometry cooled externally by forced convection of air using the Siemens STAR-CCM+ code. The geometry is representative of the main vessel of the LEFREEZ test facility built by Westinghouse and its partners as part of Phase 2 of the U.K. Department for Business, Energy and Industrial Strategy Advanced Modular Reactor (AMR) program in the United Kingdom and contains a partially submerged mock-up of a 19-pin fuel assembly. A simplified two-dimensional axisymmetric approach was first applied, and results using four different viscous modeling approaches (laminar, , SST, and ) predicted the same overall development of the solidification front through the domain, whose shape was influenced by the positioning of the external air inlet beneath the vessel. Simulations of the full three-dimensional model reproduced the solidification front as it moved past the included mock-up fuel bundle and illustrated the relatively complex and nonuniform nature of the external heat transfer. However, the CFD simulations significantly underpredicted the time taken for lead to solidify at each probe location, and consequently, the progression of the solidification front through the bundle was significantly slower. Overall, these results demonstrate that the current melting-solidification modeling capabilities can produce results that are physically consistent with expected behavior, but further work is required to address discrepancies with experimental data.