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The results of experimental investigation of choking in vertical tubes with diameter 20, 30, and 40 mm and 19-rod assembly with pressure 0.6-4.1 MPa in the absence of directed circulation of the coolant and with water level above the section under study are presented. Previously determined correlations for determining the counterflow of the phases during choking for a wide range of pressure and diameter of the sections are refined. The results of calculations, performed with the KORSAR and RELAP5/MOD3.2, of choking in experimental sections are presented. Comparative analysis has revealed discrepancies between the computations and the experimental data. It is concluded on the basis of the results obtained that the models of choking for the tested codes need additional work with a correction of relations for the flow rate of the phases. It is recommended that this be done using the correlations obtained in the course of the present experimental study.The modern concepts of the development of enhanced-safety VVER dictate the need to validate the heat-engineering reliability fuel assemblies not only for normal operation but also during regimes of an accident. Analysis has shown that at the final stage of an accident with loss-of-coolant in the first loop and with the active and passive safety systems operational the core below the water level can be saved and the fuel assemblies can be cooled in the natural circulation regime [1] (Fig. 1). A characteristic feature of the thermohydraulic processes is low rate of circulation of the coolant. In addition, as a rule, low pressure 0.1-1 MPa in core is characteristic for such processes.When the fuel assemblies are cooled by coolant entering the core from below as well as from above, for a certain flow rate of the steam the hydrodynamic phenomenon of "choking" becomes possible [2,3]. Choking limits the flow of liquid from above to the fuel assemblies, i.e., the balance of the ingoing coolant and outgoing steam flows disrupted:where G 2 is the flow rate of the steam generated in the fuel assemblies, and G 1 and G 0 are the flow rates of the coolant entering the fuel assemblies from above and below, respectively.The limiting case of such a situation is the total cessation of directed motion of the coolant through the bottom section of the core followed by cooling of the fuel assemblies by water flowing only through the top cross section in a regime of counter motion of the phases (G 0 = 0). The onset of choking results in gradual drying of the core and overheating of the heat-transfer surfaces.The critical hydrodynamic phenomenon of choking is a limiting state of a counter flow of gas and liquid and determines the maximum possible flow rate of one gas with a prescribed flow rate of the other phase. Choking has been studied quite well experimentally in channels with different geometry [2][3][4][5][6]. Processes which occur in steam-generating channel with

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