The paper deals with the technical and economic efficiency of combining NPP with the hydrogen energy complex (HEC) based on a closed hydrogen cycle. At the present stage of studying, the use of hydrogen produced through electrolysis (electrolysis hydrogen, EH) is a well-known approach to providing NPP with a base load during hours of minimum electrical load in the energy system (ES). Despite the existing concerns about the safety of using hydrogen fuel in the cycles of thermal power plants (TPP), this approach could improve NPP efficiency by accumulating the unused night-time electricity succeeded by generation of additional electricity during the hours of maximum electrical loads in ES. This provides further development of clean energy on the basis of atomic-hydrogen technologies. In the paper, the technical and economic efficiency of implementing the closed hydrogen cycle at NPP depending on the cost of the off-peak electricity required for HEC has been investigated. The assessment of the main technical and economic efficiency indicators of the basic equipment for HEC based on a closed hydrogen cycle preventing hydrogen from penetrating into the main steam cycle has allowed analyzing the efficiency of combining NPP with HEC. For this purpose, an increase of the steam-turbine unit (STU) capacity due to an increase in steam consumption along with the flowing section efficiency has been estimated; the required heat exchanging area for a closed system of hydrogen steam superheating (HSS) has been determined on the basis of the heat and mass transfer process calculations; and capital expenditures (CAPEX) as well as operating expenses (OPEX) have been calculated. As an example, a comparative assessment of two alternative methods for implementing a closed hydrogen cycle has been performed: at elevated and atmospheric pressure of combustion products. The calculation results have allowed determining key figures of comparative technical and economic efficiency of implementing the proposed schemes for combining NPP with HEC based on a closed hydrogen cycle. Zones of economic efficiency depending on the cost of off-peak electricity for the needs of the hydrogen energy complex have been determined as well. The calculations have shown that reducing the pressure of combustion products to the atmospheric one may result in a certain decrease in the efficiency of the closed hydrogen cycle. These results could be used in developing and optimizing the systems for increasing an economically sound maneuverability of NPP based on combining with HEC. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.