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Cooling of the core in fast neutron reactors in accident conditions (ULOF, UTOP) leads to the study of regimes with reduced coolant flow rates or during natural convection in fuel assemblies. The results of experiments on heat exchange and the stability of circulation during boiling of a sodium and sodium-potassium alloy on the models of single and in a system of parallel fuel assemblies with natural coolant circulation are presented. It is shown that the boiling process of liquid metals in fuel assemblies has a complex structure, is characterized by both stable and pulsating regimes with significant fluctuations in flow rate, pressure, temperature, and the occurrence of a heat exchange crisis. The hydrodynamic interaction of the circulation contours can lead to a significant increase in the amplitude of fluctuations in the coolant flow rate in them ("resonance" of flow rate pulsations) and a possible "blocking" or inversion of the coolant flow rate, an increase in the temperature of the coolant, and the cladding of the fuel elements (interchannel instability effect) and crisis. The data of calculated and experimental studies of the liquid metal boiling in FA models are compared. The effect of the surface roughness of the fuel rods on the heat exchange and flow regimes during a liquid metal boiling in bundles is demonstrated. It has been shown experimentally that there is no steaming of the sodium cavity and no change in the boiling regime in the fuel assembly when a sodium cavity is located in a fuel assembly designed to compensate for the positive sodium void reactivity effect during sodium boiling. There is the possibility of continued cooling of the fuel elements in the fuel assembly. The data on heat exchange during boiling of liquid metals in bunches are generalized, and a cartogram of the flow regimes of a two-phase flow of liquid metals in bundles is constructed that differs significantly from the cartogram for water. The boundaries between flow regimes are approximated by simple dependencies. [ABSTRACT FROM AUTHOR]