Characteristics of Heat Transfer During Boiling of Liquid Metal in the Fuel Assemblies of Fast Reactors in Emergency Regimes

The results of experimental studies of heat transfer during boiling of sodium-potassium alloy in the presence of natural convection in model FA of a fast reactor with seven fuel-rod simulators are reported. The method of investigation is described. Data are obtained for three regimes of boiling of the liquid metal in FA: stable (nucleate), unstable (slug), and secondary stable (disperse-annular). The unstable boiling regime developed in two directions depending on the state of the surface of the simulators: in the presence of manufacturing roughness – in the direction of damping of pulsations, stabilization of boiling, and subsequent stable heat removal in the presence of a fi lm of liquid on the surface of the simulators (disperse-annular regime of boiling) or in the presence of small roughness – in the direction of increasing amplitude of flow and temperature pulsations. Data on an emergency regime with pressure reduction in the circulation loop, initial superheating upon boiling-up of the liquid metal, heat transfer, and a cartogram of the two-phase flow regimes of liquid metals are presented. The boundaries of the regimes are described by analytical expressions.