The Monte Carlo method is used to simulate electronuclear systems consisting of two-cascade subcritical zones: a liquid-metal fast reactor, which is used as a booster, and a thermal reactor, where most of the energy is released. Reactors of the type VVÉR-1000, MSBR-1000, and CANDU-6 are considered. The systems considered, functioning in the safe regime (keff = 0.94–0.98), possess much higher maximum power in the entire range of keff than similar systems without an intermediate fast booster reactor. At the same time, for high thermal neutron flux and with both fast and thermal zones present nuclear wastes can be efficiently transmuted in them, decreasing the required proton current in the beam by approximately a factor of 10. This is especially important when the liquid-salt thermal breeding reactors are considered as the basic electricity producing zone.