Patent Number: 
Section: claims

1. A passive reactor cooling system usable after a loss-of-coolant accident, the system comprising:a containment vessel in direct thermal contact with an external heat sink;a reactor well disposed inside the containment vessel;a reactor vessel disposed at least partially, in the reactor well, the reactor vessel containing primary coolant and a nuclear fuel core beating the primary coolant which is circulated between the reactor vessel and a steam generator in a closed primary coolant flow loop;a cooling water tank disposed inside the containment vessel and containing an inventory of emergency cooling water in selective fluid communication with the reactor well via at least one flow control apparatus, the flow control apparatus having a closed position preventing flow of cooling water to the reactor well and an open position providing flow of cooling water to the reactor well; anda heat exchanger comprising a plurality of longitudinally-extending heat dissipater ducts integrally attached directly to an inside surface of the containment vessel in a thermally conductive relationship, the heat exchanger in fluid communication with the reactor well and water tank via a closed cooling water flow loop in which flow is driven via gravity;wherein following a loss of primary coolant, the water tank is configured and operable to flood the reactor well with cooling water which is converted into steam by heat from the fuel core and flows through the closed cooling water flow loop to the heat exchanger;the steam flowing through the heat dissipater ducts of the heat exchanger and transferring heat to the external heat sink directly through the containment vessel which condenses the steam. 2. The system according to claim 1, wherein the steam condenses in the heat exchanger forming condensate, and the condensate flows via gravity back to the water tank via the closed cooling water flow loop. 3. The system according to claim 2, wherein the condensate flows from the water tank back to the reactor well via the flow control apparatus. 4. The system according to claim 1, wherein the flow control apparatus comprises at least one flow conduit and a dump valve movable between the open and closed positions, the dump valve controlling the flow of cooling water to the reactor well from the cooling water tank through the at least one flow conduit. 5. The system according to claim 2, wherein a top of the reactor well is sealed and enclosed by a closure structure, the closure structure capturing the steam produced in the reactor well which is directed to the heat exchanger via the closed cooling water flow loop by steam inlet piping penetrating the closure structure. 6. The system according to claim 5, wherein the closure structure is formed at least in part by a ring-shaped reactor support, flange attached to and extending circumferentially around a perimeter of the reactor vessel. 7. The system according to claim 5, wherein a top of the cooling water tank is sealed and enclosed, the condensate flowing back to the water tank from the heat exchanger via the closed cooling water flow loop through outlet condensate piping penetrating the enclosed closed top of the cooling water tank. 8. The system according to claim 1, wherein the cooling water tank has a volumetric capacity at least as large as the volumetric capacity of the reactor well to optimize cooling the reactor core during a loss of primary coolant event. 9. The system according to claim 1, wherein the containment vessel comprises a cylindrical metal shell in thermal communication with the external heat sink. 10. The system according to claim 9, wherein the heat dissipater ducts are parallel to each other and circumferentially spaced apart around a circumference of the inner surface of the containment vessel. 11. The system according to claim 1, wherein the heat dissipater ducts are vertically oriented, each heat dissipater duct having upper and lower ends fluidly coupled to a common upper inlet ring header and a common lower outlet ring header attached to the inner surface of the containment vessel. 12. The system according to claim 1, wherein each heat dissipater duct is formed of a half-section of pipe or tube defining parallel longitudinal legs which are seam welded to the interior surface of the containment vessel such that the steam and condensate flowing in each heat dissipater duct is in immediate contact with the interior surface of the containment vessel. 13. The system according to claim 1, wherein the external heat sink comprises an annular reservoir holding water that surrounds and contacts an exterior surface of the containment vessel. 14. The system according to claim 13, wherein the water in the annular reservoir has a temperature lower than the temperature of the steam for condensing the steam. 15. The system according to claim 13, wherein the annular reservoir is formed between the containment vessel and an outer containment enclosure structure. 16. The system according to claim 2, wherein the closed flow loop includes:inlet steam piping fluidly coupling the heat exchanger to an enclosed top portion of the reactor well which prevents escape of the steam to an environment inside the containment vessel; andoutlet condensate piping fluidly coupling the heat exchanger to an enclosed top portion of the cooling water tank;wherein the enclosed reactor well and cooling water tank form an integral part of the closed cooling water flow loop. 17. The system according to claim 1, wherein the reactor well and cooling water tank are formed in a concrete monolith disposed in the containment vessel, the reactor well and cooling water tank sharing a common wall therebetween. 18. A passive reactor cooling system usable after a loss-of-coolant accident, the system comprising:a metal containment vessel comprising a shell in direct thermal contact with an external annular water-filled reservoir which defines an external heat sink;a monolithic concrete structure disposed inside the containment vessel and defining a reactor well;a vertically elongated reactor vessel having a lower portion disposed in the reactor well and an upper portion, the reactor vessel containing primary coolant and a nuclear fuel core heating the primary coolant which is circulated between the reactor vessel and a steam generator in a closed primary coolant flow loop;a cooling water tank disposed inside the containment vessel and containing an inventory of emergency cooling water in selective fluid communication with the reactor well via at least one flow conduit controlled by a dump valve, the dump valve having a closed position preventing flow of cooling water to the reactor well and an open position providing flow of cooling water to the reactor well; anda heat exchanger comprising a plurality of longitudinal heat dissipater ducts integrally attached directly to an inside surface of the containment vessel shell in a thermally conductive relationship, the heat exchanger in fluid communication with the reactor well and cooling water tank via a closed cooling water flow loop in which flow is driven via gravity;wherein following a loss of primary coolant, the cooling water tank is configured and operable to flood the reactor well with cooling water which is converted into steam by heat from the fuel core and flows through the closed cooling, water flow loop to the heat dissipater ducts; andwherein the steam condenses in the heat dissipater ducts via rejection of heat to the external heat sink directly through the containment vessel shell forming condensate which flows via gravity back to the cooling water tank via the closed cooling water flow loop. 19. The system according to claim 18, wherein the cooling water tank is formed in the monolithic concrete structure adjacent the reactor well, the reactor well and cooling water tank sharing a common wall therebetween and the at least flow conduit formed through the common wall. 20. A passive reactor cooling system usable after a loss-of-coolant accident, the system comprising:a containment vessel in thermal communication with an external heat sink;a reactor well disposed inside the containment vessel;a reactor vessel disposed at least partially in the reactor well, the reactor vessel containing primary coolant and a nuclear fuel core heating the primary coolant which is circulated between the reactor vessel and a steam generator in a closed primary coolant flow loop;a cooling water tank disposed inside the containment vessel and containing an inventory of emergency cooling water in selective fluid communication with the reactor well via at least one flow control apparatus, the flow control apparatus having a closed position preventing flow of cooling water to the reactor well and an open position providing flow of cooling, water to the reactor well; anda heat exchanger attached to an inside surface of the containment vessel, the heat exchanger in fluid communication with the reactor well and water tank via a closed cooling water flow loop in which flow is driven via gravity;wherein following a loss of primary coolant, the water tank is configured and operable to flood the reactor well with cooling water which is converted into steam by heat from the fuel core and flows through the closed cooling water flow loop to the heat exchanger;wherein the heat exchanger comprises a plurality of longitudinally-extending heat dissipater ducts integrally attached to the containment vessel and in thermal communication with the external heat sink via the containment vessel;wherein each heat dissipater duct is formed of a half-section of pipe or tube defining parallel longitudinal legs which are seam welded to the interior surface of the containment vessel such that the steam and condensate flowing in each heat dissipater duct is in immediate contact with the interior surface of the containment vessel.