Patent Number: 
Section: claims

1. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel divided into an upper plenum containing primary coolant, a lower plenum containing primary coolant, and a steam generator plenum including a first section defining a first volume, a second section defining a second volume, a steam outlet and a feedwater inlet, the steam generator plenum being interposed between the upper plenum and the lower plenum and containing secondary coolant,wherein secondary coolant in the first section is kept separated from secondary coolant in the second section;a nuclear reactor core comprising fissile material disposed in the lower plenum;one or more risers arranged to convey primary coolant upward from the nuclear reactor core to the upper plenum;a plurality of tubes passing through the steam generator plenum and arranged to convey primary coolant downward from the upper plenum to the lower plenum; anda steam separator operatively connected with the steam outlet and the feedwater inlet of the steam generator plenum to separate secondary coolant in a steam phase from secondary coolant in a water phase and return the secondary coolant in the water phase to the steam generator plenum by way of the feedwater inlet,wherein the steam separator is outside of the pressure vessel and elevated above the steam generator plenum so that natural circulation of the secondary coolant occurs in the steam generator plenum. 2. The apparatus of claim 1, wherein the one or more risers comprise:a hollow cylindrical central riser disposed within the pressure vessel;wherein the steam generator plenum comprises a steam generator annulus encircling the hollow cylindrical central riser. 3. The apparatus of claim 1, further comprising:an upper annular tube sheet connecting with upper ends of the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum and defining the boundary between the upper plenum and the steam generator plenum; anda lower annular tube sheet connecting with lower ends of the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum and defining the boundary between the lower plenum and the steam generator plenum. 4. The apparatus of claim 1, wherein the one or more risers comprise:a plurality of riser tubes passing through the steam generator plenum;wherein the riser tubes are inboard of and surrounded by the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum. 5. The apparatus of claim 4, wherein the steam generator plenum comprises a single connected volume through which passes both the riser tubes and the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum. 6. The apparatus of claim 4, wherein the steam generator plenum comprises a cylindrical volume through which passes both the riser tubes and the tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum. 7. The apparatus of claim 4, wherein the riser tubes have the same cross-section as the tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum. 8. The apparatus of claim 4, further comprising:an upper tube sheet connecting with upper ends of the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum and with upper ends of the riser tubes, wherein the upper tube sheet defines the boundary between the upper plenum and the steam generator plenum; anda lower tube sheet connecting with lower ends of the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum and with lower ends of the riser tubes, wherein the lower tube sheet defines the boundary between the lower plenum and the steam generator plenum. 9. The apparatus of claim 8 wherein at 25° C. the tension in the riser tubes is greater than the tension in the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum. 10. The apparatus of claim 1, wherein the steam generator plenum contains secondary coolant as a steam/water mixture. 11. The apparatus of claim 1, wherein there is no pump operatively disposed between the steam generator plenum and the steam separator, and the steam generator plenum and the steam separator are configured to circulate the secondary coolant by natural circulation. 12. The apparatus of claim 1, wherein an outer annular wall of the steam generator plenum comprises an inside wall of the pressure vessel. 13. The apparatus of claim 1, further comprising:a containment structure containing both the pressure vessel and the steam separator, the containment structure being configured to contain radioactive steam escaping from the pressure vessel. 14. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel divided into an upper plenum containing primary coolant, a lower plenum containing primary coolant, and a steam generator plenum including a first section defining a first volume, a second section defining a second volume, a steam outlet and a feedwater inlet, the steam generator plenum being interposed between the upper plenum and the lower plenum and containing secondary coolant,wherein secondary coolant in the first section is kept separated from secondary coolant in the second section;a nuclear reactor core comprising fissile material disposed in the lower plenum;one or more risers arranged to convey primary coolant upward from the nuclear reactor core to the upper plenum;a plurality of tubes passing through the steam generator plenum and arranged to convey primary coolant downward from the upper plenum to the lower plenum; anda steam separator disposed outside of the pressure vessel and operatively connected with the steam generator plenum to separate secondary coolant in a steam phase from secondary coolant in a water phase and return the secondary coolant in the water phase to the steam generator plenum by way of the feedwater inlet,wherein there is no pump arranged to pump secondary coolant between the steam generator plenum and the steam separator,wherein the steam separator is arranged to allow natural circulation of the secondary coolant in the steam generator plenum. 15. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel divided into an upper plenum containing primary coolant, a lower plenum containing primary coolant, and a steam generator plenum including a first section defining a first volume, a second section defining a second volume, a steam outlet and a feedwater inlet, wherein the steam generator plenum is interposed between the upper plenum and the lower plenum and containing secondary coolant,wherein secondary coolant in the first section is kept separated from secondary coolant in the second section;a nuclear reactor core comprising fissile material disposed in the lower plenum;one or more risers arranged to convey primary coolant upward from the nuclear reactor core to the upper plenum;a plurality of tubes passing through the steam generator plenum and arranged to convey primary coolant downward from the upper plenum to the lower plenum; anda steam separator disposed outside of the pressure vessel and operatively connected with the steam generator plenum to separate secondary coolant in the steam phase from secondary coolant in the water phase and return the secondary coolant in the water phase to the steam generator plenum by way of the feedwater inlet, wherein the steam separator is arranged to allow natural circulation of the secondary coolant in the steam generator plenum. 16. The apparatus of claim 15, wherein the steam generator plenum does not include or contain a shell-and-tube steam generator in which one of primary coolant and secondary coolant flows in one direction in tubes of the shell-and-tube steam generator and the other of primary coolant and secondary coolant flows in an opposite direction in the shell of the shell-and-tube steam generator. 17. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel divided into an upper plenum containing primary coolant, a lower plenum containing primary coolant, and a steam generator plenum including a first section defining a first volume, a second section defining a second volume, a steam outlet and a feedwater inlet, wherein the steam generator plenum is interposed between the upper plenum and the lower plenum and containing secondary coolant,wherein secondary coolant in the first section is kept separated from secondary coolant in the second section;a nuclear reactor core comprising fissile material disposed in the lower plenum;a plurality of riser tubes passing through an inboard cylindrical region of the steam generator plenum and arranged to convey primary coolant upward from the nuclear reactor core to the upper plenum;a plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum, wherein the plurality of tubes arranged to convey primary coolant downward from the upper plenum to the lower plenum pass through an outboard annular region of the steam generator plenum that surrounds the inboard cylindrical region of the steam generator plenum; anda steam separator disposed outside of the pressure vessel and operatively connected with the steam generator plenum to separate secondary coolant in a steam phase from secondary coolant in a water phase and return the secondary coolant in the water phase to the steam generator plenum by way of the feedwater inlet, andwherein the steam separator is elevated above the steam generator plenum so that natural circulation of the secondary coolant occurs in the steam generator plenum.