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 a top, a bottom, an elongated cylindrical shell extending between the top and bottom defining an internal cavity, a top tubesheet, and a bottom tubesheet;a vertical riser pipe extending vertically between the top and bottom tubesheets, the riser pipe fluidly connected to the reactor vessel;a plurality of heat transfer tubes extending vertically between the top and bottom tubesheets; anda flange-less 90 degree primary coolant fluid coupling forming a flow conduit for exchanging primary coolant between the steam generating vessel and reactor vessel;the fluid coupling comprising a reactor vessel outer inlet nozzle and a reactor vessel inner outlet nozzle inside the outer inlet nozzle and arranged concentrically thereto;the inner outlet nozzle having an inlet end directly coupled to the reactor vessel, and an outlet end directly coupled to the riser pipe in the steam generating vessel;the outer inlet nozzle external to the steam generating vessel and having an inlet end directly coupled to a bottom outlet nozzle of the steam generating vessel, and an outlet end directly coupled to the shell of the reactor vessel;the outer inlet nozzle comprising a vertically-extending upper eccentric cone section defining the inlet end of the outer inlet nozzle which is directly coupled to the bottom outlet nozzle of the steam generating vessel, and an adjoining horizontally-extending stub pipe section defining the outlet end of the outer inlet nozzle which is directly coupled to the shell of the reactor vessel;wherein the eccentric cone section comprises a vertically straight inner wall and a straight opposing inclined outer wall obliquely angled to the inner wall;wherein a closed primary coolant loop is formed in which primary coolant flows from the reactor vessel through the eccentric cone into the steam generator vessel and returns from the steam generating vessel to the reactor vessel through the eccentric cone. 2. The system according to claim 1, wherein the riser pipe extends through the eccentric cone and is fluidly coupled to the reactor vessel. 3. The system according to claim 1, wherein the primary coolant flows from the reactor vessel upwards through the riser pipe of the steam generator vessel at a first temperature, reverses direction, and flows downward through the tubes into the fluid coupling at a second temperature lower than the first temperature. 4. The system according to claim 1, further comprising an annular bottom collection plenum formed between the riser pipe and the eccentric cone section, the bottom collection plenum being in fluid communication with a tube side of the steam generating vessel to collect primary coolant flowing outwards from the heat transfer tubes through the bottom tubesheet. 5. The system according to claim 1, wherein the eccentric cone section is fluidly coupled to the bottom tubesheet and the heat transfer tubes. 6. The system according to claim 1, wherein the eccentric cone section is physically located externally to the shell of the steam generating vessel. 7. The system according to claim 1, wherein the stub pipe section is oriented 90 degrees to the eccentric cone section and joined to the eccentric cone section via an angled miter joint. 8. The system according to claim 7, wherein the eccentric cone section has a top end coupled to the bottom tubesheet and a narrower bottom end coupled to the stub pipe section. 9. The system according to claim 1, further comprising a reactor primary coolant outlet nozzle having a first end welded to a reactor shroud internal to the reactor vessel shell and a second end welded to an inlet nozzle of the steam generating vessel at the bottom of riser pipe, the reactor primary coolant outlet nozzle being disposed inside the eccentric cone section of the fluid coupling. 10. The system according to claim 1, wherein a total horizontal length of the fluid coupling between the reactor vessel and steam generating vessel is less than or equal to the diameter of the steam generating vessel. 11. The system according to claim 1, wherein no portion of a primary coolant pump resides inside the outer inlet nozzle. 12. The system according to claim 1, wherein the inner outlet nozzle inside the outer inlet nozzle of the fluid coupling includes a straight section of pipe oriented parallel to the inclined outer wall of the eccentric cone section.