The article considers a novel approach to a challenging issue of modern power systems related with the future of nuclear power energy as having clear environmental advantages. To solve the problem, a number of alternative ways of using hydrogen fuel as a source of clean energy were proposed, which are aimed to increase the capacity and maneuverability of operating and designed two-circuit nuclear power plants (NPPs) with installed water coolants. In particular, it was suggested to use hydrogen energy to improve the performance of steam generators and capacity of NPP units by heating the feedwater. The revealed economic benefits of the given approach include the effect from replacing environmentally harmful stations based on gas turbine units. A research was made into the impact of pressure in the hydrogen-oxygen combustion chamber on the effectiveness of the proposed solutions. The necessary design parameters to the hydrogen heating system, the gross and net power ratings of the power unit, and the net present value were determined. It is shown that lowering the pressure in the hydrogen-oxygen combustion chamber leads to the increase in the net present value by 24 to 71 million dollars and by 43 to 90 million dollars for the operating and designed NPPs, respectively. Meanwhile, using the proposed approach to the designed NPPs can ensure a higher capacity for the power units, which equals 179 and 163 MW at atmospheric and elevated pressures, respectively. The efficiency of off-peak energy may equal 26.25 to 27.21 and 26.53% to 27.55% for the operating and designed NPPs, respectively. It should be noted that the proposed schematics prove to be economically competitive across the marginal cost of off-peak energy. The accrued net present value equals 106 to 404 million dollars and 170 to 468 million dollars at the elevated pressure, and 130 to 475 million dollars and 213 to 558 million dollars at the atmospheric pressure for the operating and designed NPPs, respectively.