A passive nuclear reactor shutdown cooling system comprising: - a reactor vessel housing, a nuclear fuel core, the reactor vessel housing containing a primary coolant heated by the nuclear fuel core; - a steam generator fluidly coupled to the reactor vessel and containing a liquid secondary coolant; the primary coolant circulating in a first closed flow loop between the reactor vessel and steam generator, the primary coolant transferring heat to the secondary coolant in the steam generator and producing secondary coolant steam; - a heat exchanger including an inventory of a liquid third coolant and a tube bundle, the tube bundle submerged in the third coolant; the secondary coolant circulating via gravity flow in a second closed flow loop between the submerged tube bundle and the steam generator; wherein the secondary coolant steam is extracted from the steam generator and flows in the second closed flow loop to the tube bundle, condenses forming condensate, and the condensate flows back to the steam generator; wherein the reactor vessel, steam generator, and heat exchanger are commonly disposed inside a metal reactor containment vessel; wherein the condensing secondary coolant steam transfers heat to the inventory of liquid third coolant in the heat exchanger and produces third coolant steam and the third coolant steam is extracted from the heat exchanger and flows in a third closed flow loop to an array of heat dissipater ducts, condenses forming liquid third coolant, and the liquid third coolant flows back to the heat exchanger, thereby replenishing the inventory of liquid third coolant. - a reactor vessel housing, a nuclear fuel core, the reactor vessel housing containing a primary coolant heated by the nuclear fuel core; - a steam generator fluidly coupled to the reactor vessel and containing a liquid secondary coolant; the primary coolant circulating in a first closed flow loop between the reactor vessel and steam generator, the primary coolant transferring heat to the secondary coolant in the steam generator and producing secondary coolant steam; - a heat exchanger including an inventory of a liquid third coolant and a tube bundle, the tube bundle submerged in the third coolant; the secondary coolant circulating via gravity flow in a second closed flow loop between the submerged tube bundle and the steam generator; wherein the secondary coolant steam is extracted from the steam generator and flows in the second closed flow loop to the tube bundle, condenses forming condensate, and the condensate flows back to the steam generator; wherein the reactor vessel, steam generator, and heat exchanger are commonly disposed inside a metal reactor containment vessel; wherein the condensing secondary coolant steam transfers heat to the inventory of liquid third coolant in the heat exchanger and produces third coolant steam and the third coolant steam is extracted from the heat exchanger and flows in a third closed flow loop to an array of heat dissipater ducts, condenses forming liquid third coolant, and the liquid third coolant flows back to the heat exchanger, thereby replenishing the inventory of liquid third coolant. wherein the secondary coolant steam is extracted from the steam generator and flows in the second closed flow loop to the tube bundle, condenses forming condensate, and the condensate flows back to the steam generator; wherein the reactor vessel, steam generator, and heat exchanger are commonly disposed inside a metal reactor containment vessel; wherein the condensing secondary coolant steam transfers heat to the inventory of liquid third coolant in the heat exchanger and produces third coolant steam and the third coolant steam is extracted from the heat exchanger and flows in a third closed flow loop to an array of heat dissipater ducts, condenses forming liquid third coolant, and the liquid third coolant flows back to the heat exchanger, thereby replenishing the inventory of liquid third coolant.