Boron Carbide–Stainless Steel Eutectic Interactions During Severe Accidents in Sodium-Cooled Fast Reactors: From Reaction Mechanisms to Melt Relocation and Safety Implications

Eutectic reactions between core materials strongly influence the progression of severe accidents in sodium-cooled fast reactors (SFRs). Contacting constituents, namely metallic fuel with steel cladding and boron carbide (B4C) absorber with stainless steel (SS), form liquid phases several hundred kelvin below the melting points of the individual materials. The resulting low-temperature liquefaction leads to early failure of cladding and control-rod structures, relocation of fuel and absorber, and changes in core reactivity, and therefore affects both the recriticality evaluation and the in-vessel retention (IVR) strategy. This focused narrative review mainly covers studies published in the last decade and earlier foundational work. It concentrates on B4C–SS reaction mechanisms, kinetics, thermodynamics, properties, experiments and models, with metallic fuel–cladding interaction as context. A transferability analysis distinguishes reactor-independent material-couple evidence from geometry-dependent and oxidation-specific results obtained mainly in post-Fukushima boiling water reactor studies. The principal limitations are the predominance of unirradiated small-scale tests in controlled atmospheres, incomplete multicomponent thermodynamic and transport-property data, and limited validation from single-rod to bundle and reactor scales. These uncertainties propagate through melt composition and boron redistribution to reactivity and IVR, defining priority needs for future SFR experiments and coupled modeling.