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]....