What you’re seeing is the moment heat transfer fails. In high-power systems like nuclear reactors, fuel rods rely on efficient heat removal to stay within safe operating limits. Under normal conditions, heat is carried away through nucleate boiling—tiny vapor bubbles form, detach, and allow cooler liquid to continuously contact the surface. But as heat flux increases, the system approaches a physical limit known as critical heat flux (CHF). At CHF, boiling behavior undergoes a phase transition: • Vapor bubbles stop detaching efficiently • They coalesce into a continuous insulating layer • Liquid can no longer reach the surface • Heat transfer collapses dramatically This regime is called film boiling. Instead of cooling the rod, the vapor layer traps heat, causing a rapid spike in surface temperature—often within milliseconds. That’s what this experiment captures. Facilities like Idaho National Laboratory deliberately push materials to this boundary under controlled conditions...