Patent Number: 
Section: claims

1. A passive nuclear reactor shutdown cooling system comprising:a reactor vessel housing a nuclear fuel core, the reactor vessel containing a primary coolant heated by the 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 reactor vessel, steam generator, and heat exchanger being commonly disposed inside a metal reactor containment vessel having a shell;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 condensing secondary coolant steam transfers heat to the inventory of liquid third coolant in the heat exchanger and produces third coolant steam;wherein 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; andwherein the heat dissipater ducts are integrally attached to the shell of the metal reactor containment vessel, the third coolant steam transferring heat via the heat dissipater ducts to the shell of the metal reactor containment vessel and condenses into liquid third coolant. 2. The system according to claim 1, wherein steam is extracted at a higher location from the steam generator than where the condensate returns to the steam generator. 3. The system according to claim 1, wherein the reactor vessel, steam generator, and heat exchanger are all vertically elongated in shape. 4. The system according to claim 1, further comprising a reservoir containing water surrounding the reactor containment enclosure. 5. The system according to claim 4, wherein the water in the reservoir has a temperature lower than the temperature of the third coolant steam to form a heat sink for condensing the third coolant steam. 6. The system according to claim 4, wherein the reservoir has an annular shape and is formed between the reactor containment vessel and an outer containment enclosure structure. 7. A passive nuclear reactor shutdown cooling system comprising:a cylindrical inner reactor containment vessel having a first metal shell and a cylindrical outer containment enclosure structure concentrically arranged with the reactor containment vessel and having a second metal shell;a water-filled annular cooling reservoir disposed between the reactor containment vessel and the containment enclosure structure providing a cooling reservoir, water in the annular cooling reservoir in wetted contact with the first and second metal shells of the containment vessel and the containment enclosure structure respectively;a reactor vessel disposed inside the reactor containment vessel and housing a nuclear fuel core, the reactor vessel containing a primary coolant heated by the 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 comprising a hollow cylindrical shell including an inventory of a liquid third coolant and a tube bundle, the tube bundle submerged in the third coolant, the reactor vessel, steam generator, and heat exchanger being commonly disposed inside the reactor containment vessel;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 condensing secondary coolant steam transfers heat to the inventory of liquid third coolant in the heat exchanger and produces third coolant steam;wherein the third coolant steam is extracted from the heat exchanger and flows in a third closed flow loop to an array comprising a plurality of longitudinally elongated 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; andwherein the heat dissipater ducts are integrally attached directly to an interior surface of the first metal shell of the reactor containment vessel, the third coolant steam transferring heat via the heat dissipater ducts to the shell of the reactor containment vessel and condenses into liquid third coolant;wherein the heat dissipater ducts are circumferentially spaced around the interior surface of the reactor containment vessel. 8. The system according to claim 7, wherein the water in the annular cooling reservoir has a temperature lower than a temperature of the third coolant steam to form a heat sink for condensing the third coolant steam when the third coolant steam flows through the heat dissipater ducts. 9. The system according to claim 8, wherein a heat transfer path between the third coolant steam in the heat dissipater ducts and the annular cooling reservoir via the first metal shell of the containment vessel is in a direction transverse to the flow of the third coolant steam through the heat dissipater ducts.