Patent Number: 
Section: claims

1. A nuclear steam supply system with natural gravity-driven coolant circulation, the system comprising:a vertically-oriented reactor vessel comprising an elongated cylindrical shell forming an internal cavity configured for containing primary coolant and a nuclear reactor fuel core;a vertically-oriented steam generating vessel comprising an elongated cylindrical shell defining an internal cavity configured for containing secondary coolant, a top tubesheet, and a bottom tubesheet;a plurality of heat transfer tubes extending vertically between the top and bottom tubesheets, the tubes including a lower preheater section, an intermediate steam generator section, and an upper superheater section, wherein secondary coolant in a liquid state enters a shell side of the preheater section at a bottom of the steam generating vessel and flows upward to the steam generator section where a portion of the secondary coolant boils to produce steam which in turn flows upward into the superheater section at a top of the steam generating vessel;a vertical riser pipe extending vertically between the top and bottom tubesheets, the riser pipe fluidly connected to the reactor vessel;a fluid coupling forming a flow conduit for exchanging primary coolant between the steam generating vessel and reactor vessel; anda tubular recirculation shroud surrounding the tubes in the steam generator section, the shroud configured to recirculate a portion of the liquid secondary coolant in the steam generator section back to the preheater section;wherein primary coolant flows upward through the riser pipe and downward through the tubes on the tube side of the steam generating vessel to heat the secondary coolant. 2. The system according to claim 1, wherein the primary coolant flows in a parallel counter flow arrangement to the secondary coolant through the steam generating vessel. 3. The system according to claim 1, wherein a water level of secondary coolant is formed in the steam generating vessel above a top of the recirculation shroud, the water level defining a liquid-steam interface in which the recirculation shroud is submerged in the liquid secondary coolant. 4. The system according to claim 3, wherein the recirculation shroud forms an annular downcomer between the shroud and the shell of the steam generating vessel for recirculating the liquid secondary coolant, the liquid secondary coolant flowing downwards in the annular downcomer to the preheater section, reverses direction around a bottom of the recirculation shroud, and flows upwards inside the recirculation shroud back to the top of recirculation shroud to complete a recirculation flow loop. 5. The system according to claim 1, wherein the preheater section includes a plurality of horizontally oriented flow baffles configured to form a combination perpendicular cross-flow and parallel counter-flow pattern of secondary coolant flowing through the preheater. 6. The system according to claim 5, wherein the flows baffles include a first configuration of circular baffles attached to the shell of the steam generating vessel and a second configuration of circular baffles attached to the riser pipe. 7. The system according to claim 6, wherein the first and second configurations of baffles are arranged in an alternating pattern in a vertical direction to create a staggered shell-side fluid flow path through the preheater section. 8. The system according to claim 4, further comprising an interface plate forming a horizontally oriented demarcation between the preheater section from the steam generator section, the interface plate comprising a plurality of holes which receives the tubes which pass through interface plate. 9. The system according to claim 8, wherein the interface plate has a convex top surface to sweep debris to an outer periphery of the steam generating vessel. 10. The system according to claim 8, wherein the recirculation shroud terminates at a point spaced above and proximate to the interface plate. 11. A steam generator for a nuclear steam supply system, the steam generator comprising:a vertically-oriented steam generating vessel comprising an elongated cylindrical shell defining an internal cavity configured for containing secondary coolant, a top tubesheet, a secondary coolant outlet nozzle proximate to the top tubesheet, a bottom tubesheet, and a secondary coolant inlet nozzle proximate to the bottom tubesheet;a plurality of heat transfer tubes extending vertically between the top and bottom tubesheets, the tubes including a preheater section, a steam generator section, and a superheater section, wherein secondary coolant in a liquid state enters a shell side of the preheater section via the inlet nozzle and flows upward to the steam generator section where a portion of the secondary coolant boils to produce steam which in turn flows upward into the superheater section and exits the steam generating vessel through the outlet nozzle; anda vertical riser pipe extending vertically between the top and bottom tubesheets, the riser pipe in fluid communication with the tubes and configured for fluid coupling to a reactor vessel containing primary coolant;a double-walled fluid coupling forming a flow conduit for exchanging primary coolant between the steam generating vessel and reactor vessel, the fluid coupling configured so that primary coolant from the reactor vessel flows through the fluid coupling into the steam generator riser pipe and returns from the steam generating vessel to the reactor vessel through the fluid coupling;a bottom collection plenum formed below the bottom tubesheet by the fluid coupling and configured for fluid coupling to the reactor vessel, the collection plenum in fluid communication with the tubes;a top distribution plenum formed above the top tubesheet, the distribution plenum in fluid communication with the riser pipe and tubes; anda tubular recirculation shroud surrounding the tubes in the steam generator section, the shroud configured to recirculate a portion of the liquid secondary coolant in the steam generator section back to the preheater section. 12. The steam generator according to claim 11, wherein the steam generating vessel is configured so that primary coolant flows upward through the riser pipe and downward through the tubes on a tube side of the steam generating vessel to heat the secondary coolant which flows upward on the shell side inside the recirculation shroud. 13. The steam generator according to claim 11, wherein a water level of secondary coolant is formed in the steam generating vessel above a top of the recirculation shroud, the water level defining a liquid-steam interface. 14. The system according to claim 12, wherein the recirculation shroud forms an annular downcomer between the shroud and the shell of the steam generating vessel for recirculating the liquid secondary coolant, the liquid secondary coolant flowing downwards in the annular downcomer to the preheater section, reverses direction around a bottom of the recirculation shroud, combines with the liquid secondary coolant exiting the preheater section, and the combined liquid secondary coolant flow flows upwards inside the recirculation shroud back to the top of recirculation shroud to complete a recirculation flow loop. 15. The system according to claim 14, further comprising a horizontally oriented interface plate forming a demarcation between the preheater section from the steam generator section, wherein the recirculation shroud terminates at a point spaced above and proximate to the interface plate which precludes the liquid secondary coolant flow from the annular downcomer from re-entering the preheater section. 16. The steam generator according to claim 11, wherein the preheater section includes a plurality of horizontally oriented flow baffles configured to form a combination perpendicular cross-flow and parallel counter-flow pattern of secondary coolant flowing through the preheater. 17. The steam generator according to claim 16, wherein the flows baffles include a first configuration of circular baffles attached to the shell of the steam generating vessel and a second configuration of circular baffles attached to the riser pipe. 18. The steam generator according to claim 17, wherein the first and second configurations of baffles are arranged in an alternating pattern in a vertical direction to create a staggered flow of liquid secondary coolant through the preheater section. 19. The steam generator according to claim 16, wherein the steam generator section does not include horizontal oriented flow baffles. 20. The steam generator according to claim 11, wherein the bottom tubesheet has a convexly rounded top surface.