Provided is a nuclear reactor system and method therefor, for increasing the speed of conversion of a radionuclide to a stable nuclide to reduce radionuclide concentration using thermal neutrons produced by reducing the velocity of fast neutrons, while simultaneously subjecting fast-neutron-induced thermal energy of a primary cooling material to heat exchange with a secondary cooling material in a heat exchanger (7), and feeding the energy to a turbine system to generate power, the system having a nuclear reactor container (1) comprising a first container (11), and a second container (12), a plurality of metal fuel assemblies (22) and a liquid metal, which is the primary cooling material, being disposed in the first container, and the second cooling material capable of dual use as a neutron moderator and a MA radioactivity-extinguishing assembly or FP-extinguishing assembly (24) being loaded in the second container.