A compact nuclear power generation system includes a reactor (3) comprising a core (2) which uses metal fuel containing either or both of uranium-235/238 and plutonium-239. A reactor vessel (1) houses the core (2). Metal sodium primary coolant (8) is heated by the core (2). A neutron reflector (9) maintains the effective multiplication factor of neutrons emitted from the core (2) at approximately one or more to bring the core into a critical state. The neutron reflector is movable from a lower part towards an upper part of the core. The heated metal sodium is supplied to a main heat exchanger (15) which is located outside the reactor. A secondary coolant of supercritical carbon dioxide, which circulates through the main heat exchanger, is in heat exchange with the heated metal sodium. The heated secondary coolant drives a turbine (20). A power generator (21) can be operated by the driven turbine.