A cooling circuit that removes heat from the primary reactor circuit in a heat exchanger and conducts it for further use. In most cases, the secondary circuit contains water at normal pressure, which is converted to steam by the heat transferred and propels the turbine. In the case of a nuclear reactor cooled by molten sodium, which is highly reactive when in contact with water, the secondary circuit also contains sodium and it is the tertiary circuit that contains water and generates steam to drive the turbine.