A system for evacuating the residual heat from a nuclear reactor comprises: a first heat exchanger, which transfers heat from a primary fluid of the reactor to a secondary fluid; a second heat exchanger, where the secondary fluid is cooled by an auxiliary fluid which crosses a cooling duct; and a control portion, subject to thermal expansion by effect of the heating, induced by an increase in the temperature of the primary fluid beyond a preset threshold, of the secondary fluid in the control portion; the control portion being connected to a mechanical actuator device moved by the thermal expansion of the control portion to open the cooling duct and allow the passage of auxiliary fluid into the cooling duct and through the second heat exchanger.