Patent Number: 
Section: claims

1. A passively-cooled spent nuclear fuel pool system comprising:a spent nuclear fuel pool comprising:a body of liquid water having a surface level, at least one spent nuclear fuel rod submerged in the body of liquid water that heats the body of liquid water;a lid covering the spent nuclear fuel pool to form a hermetically sealed vapor space between the surface level of the body of liquid water and the lid, the vapor space including a first vapor space section and a second vapor space section, the lid comprising a first lid section and a second lid section; anda passive heat exchange sub-system comprising:a riser conduit comprising a first riser inlet section having a first inlet positioned within the first vapor space section, a second riser inlet section having a second inlet positioned within the second vapor space section and a primary riser section, wherein the primary riser section receives water vapor from the first and second vapor space sections;at least one downcomer fluidly coupled to the primary riser section for receiving the water vapor from the primary riser section, the water vapor condensing within the at least one downcomer to form a condensed water vapor; andat least one return conduit fluidly coupled to the at least one downcomer, the at least one return conduit having an outlet located within the body of liquid water for returning the condensed water vapor to the body of liquid water. 2. The system according to claim 1 further comprising a containment vessel comprising a cylindrical shell having an inner surface defining an interior cavity, the spent nuclear fuel pool housed within the interior cavity of the containment vessel. 3. The system according to claim 2 further comprising a containment enclosure surrounding the containment vessel, a heat sink space formed between the containment vessel and the containment enclosure. 4. The system according to claim 3 wherein the heat sink space contains water, and wherein the at least one downcomer is coupled to the inner surface of the containment vessel such that thermal energy from the water vapor is transferred to the water in the heat sink space through the at least one downcomer conduit and the containment vessel. 5. The system according to claim 1 further comprising a plurality of the downcomer conduits arranged in a circumferentially spaced apart manner about the inner surface of the containment vessel, and wherein each of the plurality of downcomer conduits is in intimate surface contact with the inner surface of the containment vessel. 6. The system according to claim 1 wherein the riser conduit comprises a thermal insulating layer. 7. The system according to claim 1 wherein the condensed water vapor mixes with the body of liquid water within the spent nuclear fuel pool. 8. The system according to claim 1 further comprising a first divider extending from the lid a partial distance into the body of liquid water to divide the vapor space into the first vapor space section located between the first lid section and the body of liquid water and the second vapor space section located between the second lid section and the body of liquid water. 9. The system according to claim 8 wherein the first and second vapor space sections are hermetically isolated from one another by the first divider and the first and second lid sections so that the water vapor in the first vapor space section cannot flow into the second vapor space section and the water vapor in the second vapor space section cannot flow into the first vapor space section. 10. The system according to claim 1 wherein each of the first and second vapor space sections is a hermetically sealed space. 11. The system according to claim 1 further comprising a gasket coupled to each of the first and second lid sections to create the hermetically sealed vapor space. 12. The system according to claim 8 wherein the spent nuclear fuel pool further comprises:a third lid section; anda second divider extending from the lid a partial distance into the body of liquid water, the first and second dividers dividing the vapor space into the first vapor space section, the second vapor space section and a third vapor space section, the third vapor space section located between the third lid section and the body of liquid water. 13. The system according to claim 12 wherein the riser conduit further comprises a third riser inlet section having a third inlet positioned within the third vapor space section, the third riser inlet section extending from the primary riser section to the third vapor space section. 14. The system according to claim 1 wherein a peripheral wall of the spent nuclear fuel pool is formed of concrete, and wherein the first and second riser inlet sections extend through the concrete. 15. The system according to claim 1 wherein each of the first and second vapor space sections is at a sub-atmospheric pressure. 16. The system according to claim 1 wherein the passive heat exchange sub-system further comprises an inlet manifold fluidly coupling the riser conduit to the at least one downcomer conduit and an outlet manifold fluidly coupling the at least one downcomer conduit to the at least one return conduit. 17. The system according to claim 1 wherein the passive heat exchange sub-system comprises a closed-loop fluid circuit. 18. The system according to claim 1 further comprising:a nuclear reactor; andthe nuclear reactor, the spent nuclear fuel pool and the passive heat exchange sub-system housed within a thermally conductive containment vessel, the at least one downcomer conduit coupled to the thermally conductive containment vessel such that thermal energy from the water vapor is transferred through the at least one downcomer conduit and the thermally conductive containment vessel to a heat sink. 19. A passively-cooled spent nuclear fuel pool system comprising:a spent nuclear fuel pool comprising a body of liquid water having a surface level, at least one spent nuclear fuel rod submerged in the body of liquid water that heats the body of liquid water;a lid covering the spent nuclear fuel pool to create a hermetically sealed vapor space between the surface level of the body of liquid water and the lid; anda passive heat exchange sub-system fluidly coupled to the vapor space, the passive heat exchange sub-system configured to: (1) receive water vapor from the vapor space; (2) remove thermal energy from the received water vapor, thereby condensing the water vapor to form a condensed water vapor; and (3) return the condensed water vapor to the body of liquid water;a heat sink;wherein the passive heat exchange sub-system comprises at least one riser conduit receiving the water vapor from the vapor space and at least one downcomer conduit receiving the water vapor from the at least one riser conduit, the at least one downcomer conduit being in thermal cooperation with the heat sink to transfer thermal energy from the water vapor to the heat sink, thereby condensing the water vapor in the at least one downcomer conduit and facilitating thermosiphon flow of the water vapor through the passive heat exchange sub-system;a thermally conductive containment vessel enclosing the spent nuclear fuel pool, the heat sink located outside of the thermally conductive containment vessel; andthe at least one downcomer conduit coupled to the thermally conductive containment vessel such that the thermal energy from the water vapor is transferred to the heat sink through the at least one downcomer conduit and the thermally conductive containment vessel. 20. The system according to claim 19 wherein the passive heat exchange sub-system comprises at least one inlet located in the vapor space and at least one outlet located in the body of liquid water. 21. The system according to claim 19 further comprising:a containment enclosure at least partially surrounding the thermally conductive containment vessel to form a heat sink space therebetween; andthe heat sink being a liquid reservoir located within the heat sink space. 22. The system according to claim 21 wherein the heat sink space is an annular space circumferentially surrounding the thermally conductive containment vessel. 23. The system according to claim 21 wherein the containment enclosure has an open top end. 24. The system according to claim 21 further comprising a plurality of heat exchange fins extending from an outer surface of the thermally conductive containment vessel into the liquid reservoir.