Whole-transient simulation of a total instantaneous blockage accident in a sodium-cooled fast reactor using ISAA-Na with a two-layer molten-pool model

Total instantaneous blockage (TIB) accidents in sodium-cooled fast reactors (SFRs) represent one of the most challenging beyond-design-basis scenarios, involving rapid progression from sodium boiling to molten pool formation. This study develops a computationally efficient molten pool model to enable seamless whole-transient analysis from early thermal-hydraulic degradation to late-phase pool behavior within ISAA-Na, the sodium-cooled fast reactor version of the Integrated Severe Accident Analysis code. The model employs a coarse-node framework with transient-adaptive Rayleigh convection treatment, integral moving-boundary heat conduction, and dynamic material relocation that includes progressive integration between the molten pool and the substrate layer as well as axial and radial relocation into the pools, achieving system-level accident-sequence coverage while preserving computational tractability. Benchmark assessment against the SCARABEE-BE+3 experiment demonstrates that the model captures the complete accident progression—sodium boiling, cladding failure, fuel melting, material ejection, and stratified pool formation—with quantitative agreement for principal measured quantities: steady-state temperatures within 0.7% error and boiling-onset timing within 4.4% error. Discrepancies in radial thermal coupling and two-phase interface prediction reflect the inherent trade-offs of the coarse-node approach. The model provides a practical tool for preliminary TIB analysis and parametric safety studies, though additional validation across diverse geometries and accident scenarios is required before licensing applications.