Numerical study on hydrodynamic-thermodynamic coupled breakup of a melt jet in a sodium-cooled fast reactor under non-boiling conditions

Melt jet breakup in highly subcooled liquid sodium is a key multiphysics process in sodium-cooled fast reactor severe accidents, where interfacial solidification can modify hydrodynamic breakup. To characterize hydrodynamic-thermodynamic coupled breakup under non-boiling conditions, an improved least-squares moving particle semi-implicit framework is developed by incorporating a latent-heat-corrected enthalpy treatment and a liquid-fraction-dependent viscosity model. The phase-change module is verified using the one-dimensional Stefan problem, and hydrodynamic breakup is validated against a jet-injection benchmark, with an average jet-front velocity error of 0.9% and a maximum error of 8.4%. Systematic simulations show that surface crust formation suppresses Rayleigh-Taylor and Kelvin-Helmholtz instabilities, thereby prolonging breakup and increasing breakup length and time. The dimensionless breakup length is weakly sensitive to jet velocity because solidification dominates at low velocities whereas hydrodynamic instabilities regain dominance at higher velocities. A relative instantaneous contact interface temperature is introduced into an Epstein-type breakup-length framework, yielding a correlation that predicts simulations within ±30% and independent molten-aluminum/sodium experimental data within ±20%. Finally, within the direct-contact, zero vapor-film thermal-resistance limit, a non-boiling baseline global breakup model is proposed for bounding continuous melt-jet penetration and breakup under highly subcooled liquid-sodium conditions.