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 heat exchanger including an inventory of liquid cooling water and a tube bundle, the tube bundle submerged in the liquid cooling water;the primary coolant circulating via gravity flow in a first closed flow loop between the submerged tube bundle and the reactor vessel;wherein the primary coolant transfers heat to the inventory of liquid cooling water in the heat exchanger and is cooled before flowing back to the reactor vessel in the first closed flow loop;wherein the reactor vessel and heat exchanger are commonly disposed inside a metal reactor containment vessel having a shell;wherein the heat transferred from primary coolant to the inventory of cooling water in the heat exchanger produces coolant steam;wherein the coolant steam is extracted from the heat exchanger and flows in a second closed flow loop to an array of heat dissipater ducts Integrally attached to an interior surface of the shell of the metal reactor containment vessel in a thermally conductive relationship, condenses, and flows back to the heat exchanger, thereby replenishing the inventory of liquid cooling water. 2. The system according to claim 1, wherein the heat dissipater ducts are circumferentially spaced around the interior surface of the shell of the reactor containment vessel. 3. The system according to claim 1, further comprising a reservoir containing water surrounding the reactor containment vessel, wherein the water in the reservoir is in contact with an exterior surface of the shell of the reactor containment vessel and has a temperature lower than the temperature of the coolant steam forming a heat sink which condenses the steam inside the heat dissipater ducts. 4. The system according to claim 3, wherein a heat transfer path between the 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 coolant steam through the heat dissipater ducts. 5. The system according to claim 3, wherein the reservoir has an annular shape and is formed between the reactor containment vessel and an outer containment enclosure structure. 6. The system according to claim 1, wherein hot primary coolant is extracted from a riser column inside the reactor pressure vessel and is routed via piping to a tubeside of the tube bundle in the heat exchanger. 7. The system according to claim 6, wherein cooled primary coolant is re-introduced back into an annular downcomer region of the reactor vessel surrounding the riser column. 8. The system according to claim 1, wherein the primary coolant flows on a tubeside of the tube bundle of the heat exchanger and the liquid coolant is on a shellside of the heat exchanger. 9. The system according to claim 1, further comprising a steam generator fluidly coupled to the reactor vessel and containing a secondary coolant, the primary coolant during normal operation of the reactor when the passive nuclear reactor shutdown cooling system is not in operation circulating 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. 10. The system according to claim 1, wherein the heat exchanger comprises a hollow cylindrical shell holding the inventory of liquid cooling water and the submerged tube bundle therein. 11. A passive nuclear reactor shutdown cooling system comprising:an inner reactor containment vessel haying a first metal shell and an outer containment enclosure structure having a second metal shell;an annular cooling reservoir filled with cooling water disposed between the reactor containment vessel and the containment enclosure structure, the cooling water in the annular cooling reservoir forming a heat sink in which the cooling water is in wetted contact with the first metal shell of the containment vessel and the second metal shell of the containment enclosure structure;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 heat exchanger disposed inside the reactor containment vessel, the heat exchanger comprising a hollow shell including an inventory of a first coolant liquid and a tube bundle, the tube bundle submerged in the first coolant liquid;the primary coolant circulating via gravity flow in a first closed flow loop between the submerged tube bundle and the reactor vessel;wherein the heated primary coolant transfers heat to the inventory of first coolant liquid in the heat exchanger and is cooled before flowing back to the reactor vessel in the first closed flow loop;wherein the heat transferred from primary coolant to the inventory of first coolant liquid in the heat exchanger produces first coolant steam in the heat exchanger;wherein the first coolant steam is extracted from the heat exchanger and flows in a second closed flow loop to an array of heat dissipater ducts integrally attached to an interior surface of the first metal shell of the reactor containment vessel in a thermally conductive relationship;wherein the first coolant steam in the heat dissipater ducts transfers heat to the cooling water in the annular cooling reservoir through the first metal shell of the reactor containment vessel, condenses into first coolant liquid, and flows back to the heat exchanger, thereby replenishing the inventory of first coolant liquid. 12. The system according to claim 11, wherein a heat transfer path between the coolant steam in the heat dissipater ducts and the annular cooling reservoir via the first metal shell of the reactor containment vessel is in a direction transverse to the flow of the first coolant steam through the heat dissipater ducts. 13. The system according to claim 1, wherein the heat dissipater ducts are welded to the interior surface of the shell of the metal reactor containment vessel. 14. The system according to claim 11, wherein the heat dissipater ducts are welded to the interior surface of the first metal shell of the reactor containment vessel.