Patent Number: 
Section: claims

1. A nuclear steam supply system with natural gravity-driven coolant circulation, the system comprising:a vertically elongated reactor vessel comprising a shell forming an internal cavity;a reactor core comprising nuclear fuel disposed within the internal cavity;a steam generating vessel comprising a shell and a plurality of heat exchanger sections disposed in a vertically stacked relationship, the steam generating vessel shell being formed separately from the reactor vessel shell and fluidly connected together via a fluid coupling;a closed-loop primary coolant system formed between the reactor vessel and the steam generating vessel, the primary coolant system having a primary coolant flowing through the reactor pressure vessel to cool the reactor core and through the steam generating vessel to transfer heat to a secondary coolant flowing through the steam generating vessel,wherein the primary coolant system is configured to induce gravity driven circulation of the primary coolant through the reactor vessel and steam generating vessel;wherein the secondary coolant flows vertically through each of the heat exchanger sections of the steam generating vessel and changes phase from liquid to steam;wherein the fluid coupling comprises the reactor vessel further including an inner first primary coolant outlet nozzle and an outer first primary coolant inlet nozzle concentrically aligned with the first primary coolant outlet nozzle, and the steam generating vessel further including an outer second primary coolant outlet nozzle and an inner second primary coolant inlet nozzle concentrically aligned with the second primary coolant outlet nozzle; andwherein the first primary coolant outlet nozzle is fluidly connected directly to the second primary coolant inlet nozzle, and the second primary coolant outlet nozzle is fluidly connected directly to the first primary coolant inlet nozzle, the primary coolant exiting the reactor vessel through the first primary coolant outlet nozzle and entering the steam generating vessel through the second primary coolant inlet nozzle, and the primary coolant exiting the steam generating vessel through the second primary coolant outlet nozzle and entering the reactor vessel through the first primary coolant inlet nozzle;wherein a total length of the fluid coupling between the reactor vessel and steam generating vessel is less than or equal to a diameter of the reactor vessel. 2. A nuclear steam supply system with natural gravity-driven coolant circulation, the system comprising:a vertically elongated reactor vessel comprising a shell forming an internal cavity;a reactor core comprising nuclear fuel disposed within the internal cavity;a steam generating vessel comprising a shell and a plurality of heat exchanger sections disposed in a vertically stacked relationship, the steam generating vessel shell being formed separately from the reactor vessel shell and fluidly connected together via a fluid coupling;a closed-loop primary coolant system formed between the reactor vessel and the steam generating vessel, the primary coolant system having a primary coolant flowing through the reactor pressure vessel to cool the reactor core and through the steam generating vessel to transfer heat to a secondary coolant flowing through the steam generating vessel,wherein the primary coolant system is configured to induce gravity driven circulation of the primary coolant through the reactor vessel and steam generating vessel;wherein the secondary coolant flows vertically through each of the heat exchanger sections of the steam generating vessel and changes phase from liquid to steam;wherein primary coolant flows in the primary coolant system from the reactor vessel to the steam generating vessel without assistance from a pump;wherein operation of the reactor core heats the primary coolant to a degree sufficient to cause natural circulation of the primary coolant through the closed-loop primary coolant system between the reactor vessel and the steam generating vessel;wherein the reactor vessel and the steam generating vessel are independently supported;wherein the heat exchanger sections of the steam generating vessel includes a lower steam generating section and an upper superheater section disposed above the lower steam generating section, the superheater section being operable to heat secondary coolant to superheated steam conditions;wherein the steam generating vessel further includes a preheater section disposed below the lower steam generating section;wherein the preheater section is formed by a lower portion of the lower steam generating section;wherein the secondary coolant flows upwards through the steam generating vessel and the primary coolant flows downwards through the steam generating vessel;wherein the secondary coolant enters a bottom portion of the steam generating vessel as a saturated liquid and leaves an upper portion of the steam generating vessel as superheated steam;wherein the steam generating vessel includes a central vertically-extending riser pipe which is fluidly coupled to the reactor vessel, the primary coolant flowing from the reactor vessel upwards through the riser pipe;wherein the secondary coolant flows vertically upwards the steam generating vessel between the shell of the steam generating vessel and the riser pipe;wherein the primary coolant further flows vertically downwards through the stacked heat exchanger sections in the steam generating vessel;a centrally-located vertical riser disposed in the internal cavity of the reactor vessel, the vertical riser being fluidly connected to a vertical riser pipe of the steam generating vessel via a laterally extending inlet/outlet coupling between the vessels;wherein the heat exchanger sections each include a pair of vertically spaced apart tubesheets and a tube bundle comprising a plurality of vertically-oriented tubes extending between the tubesheets;wherein the primary coolant flows downwards through the tubes of each heat exchanger section on a tube side;wherein the secondary coolant flows upwards between the tubes on a shell side of each heat exchanger section in a parallel cross flow type arrangement;wherein the tubes in each heat exchanger section are arranged in a circumferentially spaced pattern around the riser pipe between the shell of the steam generating vessel and riser pipe;a pressurizer mounted on a top end of the steam generating vessel, the pressurizer including a head which closes the top end;wherein the pressurizer forms a liquid/gas interface of the primary coolant at the top end of the steam generating vessel;wherein the steam generating vessel includes a intermediate flow plenum disposed between a top tubesheet of the lower steam generating section and a bottom tubesheet of the upper superheater section, the primary coolant flowing through intermediate plenum from the upper superheater section to the lower steam generating section;an external secondary coolant bypass piping loop fluidly connecting the shell side of the upper superheater section and the shell side of the lower steam generating section, the bypass piping loop arranged to bridge over the intermediate flow plenum;a bottom flow plenum disposed at a bottom end of the steam generating vessel below the stacked heat exchanger sections, the bottom flow plenum being operable to collect primary coolant flowing downwards through the steam generating vessel for return to the reactor vessel;wherein the reactor vessel further includes a first outlet nozzle and a first inlet nozzle concentrically aligned with the first outlet nozzle, and wherein the steam generating vessel further includes a second outlet nozzle and a second inlet nozzle concentrically aligned with the second outlet nozzle;wherein the first outlet nozzle is fluidly connected to the second inlet nozzle, and the second outlet nozzle is fluidly connected to the first inlet nozzle;wherein the first and second inlet nozzles and first and second outlet nozzles collectively define a closely coupled primary coolant fluid coupling having a lateral length that is less than a diameter of the steam generating vessel. 3. A nuclear steam supply system with natural gravity-driven coolant circulation, the system comprising:a vertically elongated reactor vessel having a first vertical axis and comprising a shell forming an internal cavity;a vertical riser and a downcomer disposed in the cavity of the reactor vessel for conducting a flowing fluid, the downcomer being in fluid communication with the vertical riser;a reactor core comprising nuclear fuel disposed within the internal cavity of the reactor vessel;a vertically elongated steam generating vessel having a second vertical axis and a cylindrical shell, the steam generating vessel including a plurality of heat exchanger sections fluidly connected in a vertically stacked relationship, the steam generating vessel shell being formed separately from the reactor vessel shell and fluidly connected together via a fluid coupling;a reactor primary coolant system having a gravity-driven closed flow loop with a primary coolant flowing through the downcomer and vertical riser of the reactor vessel for heating by the reactor core, the primary coolant further flowing through the heat exchanger sections of the steam generating vessel to transfer heat to a secondary coolant flowing through the steam generating vessel;the heat exchanger sections of the steam generating vessel including a lower steam generating section and an upper superheater section disposed above the lower steam generating section, the superheater section being operable to heat secondary coolant to superheated steam conditions;the steam generating vessel further including a tubeless intermediate flow plenum disposed between a top tubesheet of the lower steam generating section and a bottom tubesheet of the upper superheater section, the primary coolant flowing through the intermediate plenum from the upper superheater section to the lower steam generating section;an external secondary coolant bypass piping loop fluidly connecting a shell side of the upper superheater section and a shell side of the lower steam generating section, the bypass piping loop arranged to bridge over the intermediate flow plenum;wherein the secondary coolant flows vertically through the heat exchanger sections of the steam generating vessel and changes phase from liquid to steam;wherein the first vertical axis of the reactor vessel is laterally offset from the second vertical axis of the steam generating vessel. 4. The system of claim 3, wherein the downcomer is formed by an annular space between the shell of the reactor vessel and the vertical riser which is axially aligned with the first vertical axis concentrically aligned with the vertical riser. 5. The system of claim 3, further comprising a central riser pipe in steam generating vessel and tube bundles of the heat exchanger sections being arranged radially around the riser pipe. 6. The system of claim 3, wherein primary coolant flows in the primary coolant system from the reactor vessel to the steam generating vessel without assistance from a pump. 7. The system of claim 3, wherein operation of the reactor core heats the primary coolant to a degree sufficient to cause natural circulation of the primary coolant through the closed-loop primary coolant system between the reactor vessel and the steam generating vessel. 8. The system of claim 3, wherein the reactor vessel and the steam generating vessel are independently supported. 9. The system of claim 3, wherein the steam generating vessel further includes a preheater section disposed below the lower steam generating section; and wherein the preheater section is formed by a lower portion of the lower steam generating section. 10. The system of claim 3, wherein the secondary coolant flows upwards through the steam generating vessel and the primary coolant flows downwards through the steam generating vessel; and wherein the secondary coolant enters a bottom portion of the steam generating vessel as a saturated liquid and leaves an upper portion of the steam generating vessel as superheated steam. 11. The system of claim 3, wherein the steam generating vessel includes a central vertically-extending riser pipe which is fluidly coupled to the reactor vessel, the primary coolant flowing from the reactor vessel upwards through the riser pipe; wherein the secondary coolant flows vertically upwards the steam generating vessel between the shell of the steam generating vessel and the riser pipe; and wherein the primary coolant further flows vertically downwards through the stacked heat exchanger sections in the steam generating vessel. 12. The system of claim 3, wherein:the stacked heat exchanger sections each include a tube bundle comprising a plurality of vertically-oriented tubes extending between the tubesheets;the primary coolant flows downwards through the tubes of each heat exchanger section on a tube side;the secondary coolant flows upwards between the tubes on a shell side of each heat exchanger section in a parallel cross flow type arrangement;wherein the steam generating vessel includes a central vertically-extending riser pipe which is fluidly coupled to the reactor vessel, the primary coolant flowing from the reactor vessel upwards through the riser pipe; andwherein the tubes in each heat exchanger section are arranged in a circumferentially spaced pattern around the riser pipe between the shell of the steam generating vessel and riser pipe. 13. The system of claim 3, further comprising a pressurizer mounted on a top end of the steam generating vessel, the pressurizer including a head which closes the top end; and wherein the pressurizer forms a liquid/gas interface of the primary coolant at the top end of the steam generating vessel. 14. The system of claim 3, wherein the reactor vessel further includes a first outlet nozzle and a first inlet nozzle concentrically aligned with the first outlet nozzle, and wherein the steam generating vessel further includes a second outlet nozzle and a second inlet nozzle concentrically aligned with the second outlet nozzle,wherein the first outlet nozzle is fluidly connected directly to the second inlet nozzle, and the second outlet nozzle is fluidly connected directly to the first inlet nozzle; andwherein the first and second inlet nozzles and first and second outlet nozzles collectively define a closely coupled primary coolant fluid coupling having a lateral length that is less than a diameter of the steam generating vessel.