A compact nuclear power generation system comprising: a nuclear reactor; the nuclear reactor including a reactor core provided with a plurality of fuel assemblies each formed by a plurality of fuel rods bundled together to form a unified structure; each fuel rod being formed by metal fuel sealed in a sheathing tube; the metal fuel containing at least one of uranium-235/238 and plutonium-239, or both; a reactor vessel that houses therein the reactor core; a primary coolant charged into the reactor vessel and formed of metal sodium that is varied in temperature and in density depending on reactivity of the metal fuel to automatically control reactivity of the metal fuel; a neutron reflector provided around the outer periphery of the reactor core and operating to reflect neutrons radiated from the metal fuel in the reactor core back towards the metal fuel so as to maintain the effective multiplication factor for neutrons radiated from the metal fuel at approximately unity or higher to maintain the critical state of the metal fuel; the neutron reflector having a height not larger than one-half of the height of the fuel rods and being carried for movement in the height-wise direction of the fuel rods; the neutron reflector being moved by a movement mechanism from a lower part towards an upper part of the reactor core, as the neutron reflector fronts to the fuel rods of the reactor core, so as to set the metal fuel progressively to the critical state from the lower part towards the upper part; the system being characterized by further comprising a main heat exchanger installed outside the nuclear reactor and supplied via a conduit with the primary coolant heated by the nuclear reactor; a secondary coolant formed of supercritical carbon dioxide being circulated within the main heat exchanger so as to be heated by heat exchange with the primary coolant, a turbine driven by the secondary coolant heated by the main heat exchanger; and a generator run in operation by the driving of the turbine. a nuclear reactor; the nuclear reactor including a reactor core provided with a plurality of fuel assemblies each formed by a plurality of fuel rods bundled together to form a unified structure; each fuel rod being formed by metal fuel sealed in a sheathing tube; the metal fuel containing at least one of uranium-235/238 and plutonium-239, or both; a reactor vessel that houses therein the reactor core; a primary coolant charged into the reactor vessel and formed of metal sodium that is varied in temperature and in density depending on reactivity of the metal fuel to automatically control reactivity of the metal fuel; a neutron reflector provided around the outer periphery of the reactor core and operating to reflect neutrons radiated from the metal fuel in the reactor core back towards the metal fuel so as to maintain the effective multiplication factor for neutrons radiated from the metal fuel at approximately unity or higher to maintain the critical state of the metal fuel; the neutron reflector having a height not larger than one-half of the height of the fuel rods and being carried for movement in the height-wise direction of the fuel rods; the neutron reflector being moved by a movement mechanism from a lower part towards an upper part of the reactor core, as the neutron reflector fronts to the fuel rods of the reactor core, so as to set the metal fuel progressively to the critical state from the lower part towards the upper part; the system being characterized by further comprising a main heat exchanger installed outside the nuclear reactor and supplied via a conduit with the primary coolant heated by the nuclear reactor; a secondary coolant formed of supercritical carbon dioxide being circulated within the main heat exchanger so as to be heated by heat exchange with the primary coolant, a turbine driven by the secondary coolant heated by the main heat exchanger; and a generator run in operation by the driving of the turbine.