Patent Number: 
Section: claims

1. A nuclear steam supply system comprising:a vertically elongated reactor vessel comprising an internal cavity with a central axis and a primary coolant flow nozzle extending outwards from the reactor vessel;a reactor core comprising nuclear fuel disposed within the internal cavity;a steam generating vessel comprising at least one heat exchanger section, the steam generating vessel being fluidly coupled to the reactor vessel;wherein the reactor vessel comprises a cylindrical shell having an annular upper flange portion, and a head having an annular head flange portion, the upper flange portion being removably coupled to the head flange portion, and wherein the upper flange portion extends inwards into the internal cavity, and the head flange portion extends outward from the internal cavity; andwherein an outer surface of the upper flange portion of the shell is substantially flush with a vertical outer surface of the cylindrical shell of the reactor vessel below the upper flange portion. 2. The system according to claim 1 wherein the shell includes a first shell portion below the upper flange portion having a first inner radius, and the head includes a first head portion having a second inner radius, which is smaller than the first inner radius. 3. The system according to claim 2, wherein the upper flange portion of the shell that extends into the central cavity defines a third inner radius which is about the same as the second inner radius of the first head portion and smaller than the first inner radius of the first shell portion. 4. The system according to claim 1, wherein the shell includes a first shell portion below the upper flange portion having an inner radius, and the head includes a first head portion above the head flange portion having an outer radius, which is smaller than or equal to the inner radius. 5. The system according to claim 1 wherein the shell includes a first shell portion below the upper flange portion having a first inner surface, and the head includes a first head portion above the head flange portion having a second inner surface, and wherein the second inner surface is closer to the central axis than the first inner surface along respective parallel radial lines extending from the central axis. 6. The system according to claim 1 wherein the shell includes a first shell portion below the upper flange portion of the shell, the first shell portion having a first inner surface and the upper flange portion of the shell having a second inner surface, and wherein the second inner surface is closer to the central axis than the first inner surface along respective parallel radial lines extending from the central axis. 7. The system according to claim 6, wherein the primary coolant flow nozzle is disposed on the first shell portion. 8. The system according to claim 7, wherein the primary coolant flow nozzle is formed as an integral forging with the shell of the reactor vessel. 9. The system according to claim 7, further comprising an annular support flange attached to the first shell portion of the reactor vessel, the support flange configured for mounting on a horizontal surface to support a weight of the reactor vessel. 10. The system according to claim 1 wherein the shell includes a first shell portion having an inner surface, and the head includes a first head portion above the head flange portion having an outer surface, and wherein the outer surface is at least as close to the central axis as the inner surface along respective parallel radial lines extending from the central axis. 11. The system according to claim 1 wherein the shell includes a first shell portion having a first outer surface, the head includes a first head portion having a second outer surface, and a plurality of fasteners secure the upper flange portion to the head flange portion, and wherein the fasteners are each disposed further from the central axis than the second outer surface and nearer the central axis than the first outer surface. 12. The system according to claim 11, further comprising a plurality of circumferentially arranged longitudinal bores formed in both the upper flange portion of the shell and the head flange portion of the head that receive the fasteners, wherein a first distance between each bore and the central axis of the internal cavity of the reactor vessel is less than a second distance between the central axis and an outer surface of the first shell portion of the reactor vessel. 13. The system according to claim 12, wherein the longitudinal bores in the upper flange portion of the shell extend partially into the upper flange portion and the longitudinal bores in the head flange portion of the head extend completely through the head flange portion. 14. The system according to claim 12, wherein the fasteners are threaded fasteners.