The paper reviews two alternative options of nuclear energy system development: with advanced fast reactors that in future should replace thermal reactors currently under construction, and fusion-fission reactors capable of providing thermal reactors with fuel and minimize the risk of environmental radioactive pollution. The comparison of these options indicates that with the existing conceptions of nuclear energy development prospects based on fast reactors it will require natural uranium resources approximately twice as much as the modern estimates of its economically feasible reserves, and the fuel cycle of these reactors will pose higher environmental risk. The nuclear industry infrastructure will have to be changed completely to produce fast reactors. In case of nuclear energy system development with fusion-fission reactors, natural uranium demands will correspond to the modern estimates of economically attractive reserves, the risk of radioactive pollution will be reduced by approximately 10 times, and the current nuclear industry infrastructure will be preserved.

Key words: nuclear energy system, closed fuel cycle, fusion-fission reactor, suppressed fission, molten-salt blanket, fast reactors, thermal reactors.