The strategy for the development of nuclear power presupposes the construction of thermal water-cooled reactors and fast reactors [1][2][3]. At present, water moderated and cooled power reactors are the foundation of our country's nuclear power. The organizations of the State Corporation Rosatom have developed designs for new-generation reactor setups -AES-2006 and VVER-TOI, which are the result of evolutionary development and improvement of operating water moderated and cooled power reactors, which proved their reliability in thousands of reactor-years of accident-free operation. VVER are characterized by elevated power, the best economic and operational performance as well as enhanced safety compared with currently operating reactors. The thermal power is increased by, for example, using fuel assemblies with a new design incorporating heat-exchange intensifiers. The safety of the new designs of NPP with VVER is enhanced by means of the principle of technological diversity, which consists in combining active and passive safety systems. The passive systems ensure shutdown and prolonged removal of residual heat in the presence of a sealed loop or a depressurized loop and do not require operator intervention or an external long-time power source.To secure the possibility of increasing the power and validating the serviceability of new passive safety systems for VVER, a large-scale program of thermophysical and experimental studies has been completed.Increasing the Power Density and Efficiency of the VVER Core. The new NPP designs with VVER presuppose an improved design of the core as a whole and the fuel assemblies in particular. One promising avenue for increasing the capacity of power-generating units and the efficiency of the fuel cycle is to use fuel assemblies with improved thermohydraulic characteristics, which is accomplished by using mixing or intensifying lattices to increase heat transfer. Acting on the coolant flow these setups decrease the nonuniformity of the heating of the coolant (enthalpy) in the transverse section of a fuel assembly. They also make it possible to increase the turbulence properties of the flow, which causes moisture to settle on the walls of the fuel elements and promotes the removal of a large quantity of heat.In order to validate the heat-engineering reliability of a VVER core with fuel assemblies with mixing lattices, it is necessary to determine the effect of their design on the hydrodynamics and mass-transfer processes in the core. Important problems are the choice of the arrangement of the fuel assemblies and optimization of the design of the mixing lattices, which must possess the optimal combination of parameters such as the hydraulic resistance, the intensity of mixing of the coolant and a safety margin to crisis of boiling. The main method for studying the hydrodynamics and mass transfer in reactor fuel assemblies is the experimental investigation of fuel-assembly models on thermohydraulic stands: SVD-2, STF, and the TRASSER setup [4]. The main characteristics of the tripl...