Fuel relocation dynamics in sodium-cooled reactor transients: numerical modeling and experimental benchmarking with CABRI-LTX data

Sodium-cooled fast reactors (SFRs) hold strategic significance in China's nuclear energy roadmap. To address the growing demand for advanced safety analysis tools, the integrated SFR simulation code ISAA-Na is under development. Fuel relocation dynamics, particularly cladding failure and fuel ejection during transient overpower (TOP) events, constitute critical phenomena requiring high-fidelity modeling. A numerical model was implemented in ISAA-Na to simulate multiphysics-coupled fuel relocation processes under accident conditions. The model employs a 1D compressible flow formulation with variable flow cross-sections, incorporating simplified yet physically consistent assumptions for interfacial heat/mass transfer and phase transitions. The model was benchmarked against the internationally recognized CABRI-LTX transient experiment. Key parameters including coolant flowrate, temperature transients, void front progression, and axial fuel distribution showed close agreement with experimental data. The ISAA-Na fuel relocation model demonstrates robust predictive capability for SFR severe accident scenarios, providing a validated tool for safety analysis of Chinese SFR designs.