It is shown the potential role of nuclear energy in sustainable energy supply ensuring and mitigating of the negative environmental impacts of electricity production from fossil fuels. Given is the capability of fast neutron reactors with sodium coolant (FNR) for solving the problems of fuel resources optimal use and waste minimization by closing the nuclear fuel cycle. The analysis of the impact of nuclear power plants with FNR on the environment and personnel is made. The architecture of a promising two-component nuclear energy of XXI century is described.