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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
The results of a computational analysis of the behavior of melt in the facility used to localize melt for VVER-1200 (AES-2006, which were obtained using the GEFEST-ULR code, are presented. The main parameters of the melt are determined: the time variation of the component composition, the temperature and density of the oxide and metallic components and their relative arrangement. Using the KORSAR/GP thermohydraulic code, the minimum margin to crisis of heat transfer at the outer surface of the wall of the melt localization facility is analyzed. It is confirmed that heat transfer from the outer surface of the wall with normal operation of the cooling loop is reliable.One measure to increase the safety of nuclear power plants with VVER is to incorporate additional protection in the plant -a melt localization facility. The safety of the nuclear power plant increases because the melt is localized in the core and the in-core structures after the reactor vessel is destroyed in a serious accident within the sub-reactor room of the concrete shaft as well as because the effect of the emerged melt on the construction of the protective shell decreases.The main functions of this facility are: 1) receiving and arranging within its volume melt, solid fragments of the core and structural materials of the reactor; 2) preventing melt from flowing outside the localization zone and ensuring subcriticality; 3) stable heat removal to cooling water; 4) minimizing the emission of radioactive substances and hydrogen into the space of the hermetic shell; and 5) ensuring the integrity of the structures located in the sub-reactor room of the concrete shaft under various static and mechanical loads.The melt-localization facility performs its functions with minimum control by the operators. The melt localization facility for AES-2006 with VVER-1200 was developed using an evolutionary approach taking account of the tested scientific-technical solutions for the design of such a facility at the Tian Wan nuclear power plant (China) and the Kudankulam nuclear power plant (India) [1-3]. Specifically, the scheme of a dry water-cooled crucible in the subreactor space, sacrificial materials from a mixture of low-melting iron and aluminum oxides, a bi-layer vessel for preventing destruction by thermal stresses, gadolinium oxide as a sacrificial material to ensure melt subcriticality was chosen as the basic design solution.The melt-localization facility for AES-2006 was designed taking account of the concrete conditions for the arrangement in the concrete shaft relative to the reactor vessel a pool of cooling water and a tank from which the water can be fed onto the surface of the melt as well as the characteristics of the melt which enters from the reactor vessel.The present article is devoted to the numerical study of the dynamics of the formation of a melt pool and determining the relative reserve to crisis of heat transfer on the outer surface of the wall of the water-cooled vessel of the facility. Construction of the Melt Localization Facility.T...
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