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The results of experimental studies of the local hydrodynamics and mass transfer of coolant in the characteristic zone of VVER fuel assemblies using mixing lattices of structurally different types are presented. These studies were performed on an aerodynamic stand by the method of impurity diffusion for several experimental scaled models. The studies were performed on a section of self-similar flow, so that the results can be used to convert to natural conditions of coolant flow in the core in regular TVSA. These studies refine the local and mass-transfer characteristics of coolant flow in order to reduce the conservatism in the evaluation of the heat-engineering reliability of the VVER core with TVSA fuel assemblies. The results also comprise a database for verifying CFD codes and programs for cellular calculation of a VVER core.A promising direction for the development of nuclear power is to develop reactors capable of operating at higher power levels and more safely. Any increases of the unit reactor power and at the same time reactor safety depend on efficient operation of the fuel assemblies. Afrikantov Experimental Design Bureau of Mechanical Engineering (OKBM Afrikantov) has developed fuel assemblies (TVSA) with a new design for the core of a VVER-type reactor. These designs are in many ways competitive with the analogs in regard to reliability, safety, cost-effectiveness and technological adaptability [1]. A distinguishing feature of the modernized TVSA is a mixing lattice, which makes it possible to intensify the mass transfer of the coolant, create flow turbulence within individual cells and increase the margin to crisis of heat transfer. To obtain high intensity of intercellular mass transfer in TVSA, it has been proposed that two structurally different types of mixing lattices be used: swirling the flow around a fuel element and passing the coolant along the rows in the space between fuel elements.To validate the heat-engineering reliability of the improved VVER core with TVSA with different mixing lattices, it is necessary to determine how their design affects on the hydrodynamics and mass transfer of the coolant flow. The main method of studying mass transfer and hydrodynamics is experimental study of scaled and full-size models of fuel assemblies and the core on aero-and hydrodynamic stands. An experimental stand comprising an aerodynamic open loop through which air is pumped has been developed and built for such studies [2]. The studies were performed on 19-and 61-rod models of a TVSA fragment and a 57-rod model of a VVER core fragment, which includes segments of three TVSA and the inter-cassette space (Fig. 1). All experimental models, incorporating dummy fuel elements and zones of spacing and mixing lattices, are implemented in conformance with full geometric similarity (Fig. 2).The local mass transfer of the coolant flow in the TVSA models was studied by the impurity diffusion method, based on recording the transverse mass flow along the transported substance (paints, salt, gas and others) [3]....

The article presents the results of experimental studies of the local hydrodynamics of the coolant flow in the mixed core of the VVER reactor, consisting of the TVSA-T and TVSA-T mod.2 fuel assemblies. Modeling of the flow of the coolant flow in the fuel rod bundle was carried out on an aerodynamic test stand. The research was carried out on a model of a fragment of a mixed core of a VVER reactor consisting of one TVSA-T segment and two segments of the TVSA-T.mod2. The flow pressure fields were measured with a five-channel pneumometric probe. The flow pressure field was converted to the direction and value of the coolant velocity vector according to the dependencies obtained during calibration. To obtain a detailed data of the flow, a characteristic cross-section area of the model was selected, including the space cross flow between fuel assemblies and four rows of fuel rods of each of the TVSA fuel assemblies. In the framework of this study the analysis of the spatial distribution of the projections of the velocity of the coolant flow was fulfilled that has made it possible to pinpoint regularities that are intrinsic to the coolant flowing around spacing, mixing and combined spacing grates of the TVSA. Also, the values of the transverse flow of the coolant caused by the flow along hydraulically nonidentical grates were determined and their localization in the longitudinal and cross sections of the experimental model was revealed. Besides, the effect of accumulation of hydrodynamic flow disturbances in the longitudinal and cross sections of the model caused by the staggered arrangement of hydraulically non-identical grates was determined. The results of the study of the coolant cross flow between fuel assemblies interaction, i.e. between the adjacent TVSA-T and TVSA-T mod.2 fuel assemblies were adopted for practical use in the JSC of “Afrikantov OKB Mechanical Engineering” for assessing the heat engineering reliability of VVER reactor cores; also, they were included in the database for verification of computational hydrodynamics programs (CFD codes) and for detailed cell-based calculation of the reactor core.