Patent Number: 
Section: claims

1. A nuclear power generation system including:a steam generator including a vertically elongated steam generating vessel fluidly coupled to a reactor vessel having an internal cavity;a reactor core comprising nuclear fuel disposed within the internal cavity and operable to heat a primary coolant;a primary coolant flow loop formed between the reactor vessel and the steam generating vessel, the primary coolant flow loop being configured and operable to circulate primary coolant through the reactor vessel and steam generating vessel via thermally induced gravity flow; anda secondary coolant flow loop formed between the steam generating vessel and a low pressure turbine, the secondary coolant flow loop being configured and operable to circulate secondary coolant through the steam generating vessel in which the primary coolant heats and converts the secondary coolant from liquid to steam, the steam flowing through the secondary coolant flow loop to the low pressure steam turbine;wherein a temperature differential between the primary coolant leaving the steam generating vessel and secondary coolant entering the steam generating vessel is at least 175 degrees F. sufficient to induce natural thermally driven gravity circulation of the primary coolant through the primary coolant flow loop;wherein the pressure of steam entering the low pressure turbine is less than 400 psia; andwherein the secondary coolant flow loop does not include a high pressure turbine. 2. The nuclear power generation system of claim 1, wherein the temperature of steam entering the low pressure turbine has a temperature of at least 575 degrees F. and is superheated steam. 3. The nuclear power generation system of claim 1, further comprising a single feedwater heater disposed in the secondary coolant flow loop between an outlet from the low pressure turbine and the steam generating vessel, the feedwater heater configured to heat the secondary coolant using secondary coolant extracted from the low pressure turbine;wherein the secondary coolant flow loop does not include any additional feedwater heaters between the low pressure turbine outlet and the steam generating vessel. 4. The nuclear power generation system of claim 3, further comprising:a condenser disposed in the secondary coolant flow loop between an outlet from the low pressure turbine and the steam generating vessel; anda single feedwater heater disposed in the secondary coolant flow loop between the condenser and the steam generating vessel, wherein the secondary coolant flow loop does include any additional feedwater heaters between the condenser and the steam generating vessel. 5. The nuclear power generation system of claim 4, wherein the steam generating vessel and reactor vessel are disposed inside a containment vessel, and the turbine, condenser, and feedwater heater are disposed outside the containment vessel. 6. The nuclear power generation system of claim 4, wherein secondary coolant is heated in the feedwater heater using secondary coolant extracted from the low pressure feedwater heater at two different extraction points having different secondary coolant temperatures and pressures. 7. The nuclear power generation system of claim 1, wherein the primary coolant flows 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. 8. The nuclear power generation system of claim 1, wherein the steam generating vessel includes a vertically stacked preheat section, main steam generating section, and a superheater section. 9. A nuclear power generation system including:a steam generator including a vertically elongated steam generating vessel fluidly coupled to a reactor vessel having an internal cavity;a reactor core comprising nuclear fuel disposed within the internal cavity and operable to heat a primary coolant;a primary coolant flow loop formed between the reactor vessel and the steam generating vessel, the primary coolant flow loop being configured and operable to circulate primary coolant through the reactor vessel and steam generating vessel via thermally induced gravity flow;a secondary coolant flow loop formed between the steam generating vessel and a single steam turbine, the secondary coolant flow loop being configured and operable to circulate secondary coolant through the steam generating vessel in which the primary coolant heats and converts the secondary coolant from liquid to steam, the steam flowing through the secondary coolant flow loop to an inlet on the single steam turbine;a condenser disposed in the secondary coolant flow loop between an outlet from the single turbine and the steam generating vessel, the condenser configured to cool and condense steam exiting the single turbine thereby converting the secondary coolant from steam to liquid; anda single feedwater heater disposed in the secondary coolant flow loop between the condenser and the steam generating vessel, the feedwater heater configured to receive and heat the liquid secondary coolant from the condenser,wherein the secondary coolant flows directly from the feedwater heater into the steam generating vessel without any intervening feedwater heaters between the condenser and the steam generating vessel;wherein a temperature differential between the primary coolant leaving the steam generating vessel and secondary coolant entering the steam generating vessel is sufficient to induce natural thermally driven gravity circulation of the primary coolant through the primary coolant flow loop; andwherein the single steam turbine is a low pressure turbine characterized by steam entering the turbine at a pressure less than 400 psia. 10. The nuclear power generation system of claim 9, wherein the secondary coolant flow loop does not include any additional turbines. 11. The nuclear power generation system of claim 9, wherein the steam is superheated. 12. The nuclear power generation system of claim 9, wherein the temperature of steam entering the steam turbine has a temperature of at least 575 degrees F. and is superheated steam. 13. The nuclear power generation system of claim 9, wherein a temperature differential between primary coolant leaving the steam generating vessel and secondary coolant entering the steam generating vessel is at least 175 degrees F. 14. The nuclear power generation system of claim 13, wherein the temperature differential is between and including 175-215 degrees F. 15. The nuclear power generation system of claim 9, wherein the secondary coolant is heated in the feedwater heater using secondary coolant extracted from the turbine at two different extraction points, at least one of the extraction points being at sub-atmospheric pressure. 16. The nuclear power generation system of claim 15, wherein each of the extraction points is at sub-atmospheric pressure. 17. The nuclear power generation system of claim 9, wherein the feedwater heater is a shell and tube heat exchanger, colder secondary coolant from the condenser flowing on the tube side and hotter secondary coolant at least partially in steam form extracted from turbine flowing on the shell side for heating the tube side secondary coolant. 18. The nuclear power generation system of claim 17, wherein the secondary coolant extracted from the turbine is at sub-atmospheric pressure. 19. The nuclear power generation system of claim 9, wherein the steam generating vessel includes vertically stacked heat exchangers comprising a preheat section, a steam generator section, and a superheater section, the secondary coolant being converted from liquid to superheated steam flowing upwards through the steam generating vessel. 20. A method for inducing thermally driven gravity flow of primary coolant through a nuclear reactor, the method comprising:providing a vertically elongated steam generating vessel fluidly coupled to a reactor vessel housing a nuclear fuel core which heats a primary coolant;circulating the primary coolant through a primary coolant flow loop formed between the steam generating vessel and reactor vessel, the primary coolant entering the steam generating vessel from the reactor vessel at a first temperature and exiting the steam generating vessel at a second temperature lower than the first temperature;heating a secondary coolant in the steam generating vessel using the primary coolant which converts the secondary coolant from a liquid entering the steam generating vessel to steam exiting the steam generating vessel, the secondary coolant entering the steam generating vessel at a third temperature and exiting the steam generating vessel at a fourth temperature higher than the third temperature;circulating the secondary coolant through a secondary coolant flow loop having an external portion outside to the steam generating vessel;expanding the steam in a single steam turbine for producing electric power;condensing the steam in a surface condenser to convert the secondary coolant from steam back into liquid form;heating the liquid secondary coolant received from condenser in a single feedwater heater to the third temperature using fluid extracted from the turbine; andflowing the heated liquid secondary coolant from the feedwater heater directly to the steam generating vessel without any intervening feedwater heaters between the condenser and the steam generating vessel;wherein the temperature differential between the second temperature of the primary coolant and the third temperature of the secondary coolant is at least 175 degrees F. selected to induce natural thermally driven gravity circulation of the primary coolant through the primary coolant flow loop; andwherein the single steam turbine is a low pressure turbine characterized by steam entering the turbine at a pressure less than 400 psia.