<p>The vapor diffusion and transport resulting from steam generator tube rupture (SGTR) accidents are a major concern threatening lead-based reactor core safety. In this study, a high-parameter SGTR experimental platform and the multi-phase multi-physics processes numerical simulation were developed to investigate the phase behavior and interaction mechanisms. This study revealed the interaction mechanisms of lead–bismuth liquid metal and water driven by flash vaporization, jet impingement boiling, and moderate boiling. The migration and evolution of the discrete phases (vapor–water mixture) were inferred from the temperature transient laws and a numerical simulation. The results revealed that the evolution of the discrete phases consists of three stages: cavity formation, flanking diffusion, and stable up-floating. The jet pressure significantly extended the disturbance period. Variations in the water temperature mainly affected the depressurization boiling process, altering the diffusion region of the discrete phases. The temperature of the liquid metal and the duration of the jet had a minimal impact on the behavior of the discrete phases. This study provides a crucial reference for constructing a complete picture of accident evolution.</p>