The results of thermophysical experimental and computational-theoretical studies for validating the enhanced efficiency and safety of new-generation nuclear reactors are presented. Critical heat flows have been studied to validate the design of fuel assemblies with elevated parameters on models with different spacer and intermediate lattices. Passive safety systems with a system of second-stage water tanks have been investigated. Heat transfer during coolant boiling in the core has been studied. Systems for monitoring hydrogen concentration and removal in nuclear power plants with VVER and an in-tank sodium purification system for maintaining an admissible impurity concentration in fast reactors have been tested.