Abstract: A liquid metal cooled nuclear reactor for a nuclear power plant on the modular principle is disposed in a reactor cavern having cooling surfaces. A reactor tank contains a reactor core and one or more heat exchangers and primary pumps and is surrounded by a double tank of nodular graphite cast iron serving as a heat accumulator and having a plurality of detachably interconnected and superposed rings and a base. With this configuration, not only is the structure simplified but advantages are also achieved for operation, inspection and repair or exchange. Producing the double tank from cast iron has appreciable advantages for the construction of the entire plant with respect to external influences. The function as a heat accumulator has appreciable advantages for the construction of the heat removal systems of the cooling surfaces.