Patent Number: 
Section: claims

1. A nuclear reactor steam generator, comprising:three or four plenums proximate with a first plane;three or four plenums proximate with a second plane; anda plurality of steam-generating tubes that form a coolant flowpath from one of the three or four plenums located proximate with the first plane to at least one of the three or four plenums proximate with the second plane,wherein the steam generator is configured to be installed in a reactor vessel such that the first plane intersects a bottom portion of a riser column positioned in the reactor vessel and the second plane intersects a top portion of the riser column,wherein each of the three or four plenums proximate with the first plane and each of the three or four plenums proximate with the second plane includes an approximately flat tubesheet that faces in a direction of a middle portion of the riser column, andwherein the approximately flat tubesheet includes a plurality of perforations, the perforations being of lower density near an edge closer to the riser column than near an edge closer to a reactor vessel wall. 2. The nuclear reactor steam generator of claim 1, wherein each of the three or four plenums proximate with the second plane is directly above a corresponding one of the three or four plenums proximate with the first plane. 3. The nuclear reactor steam generator of claim 1, wherein at least some of the plurality of perforations include an orifice for reducing pressure at an inlet of a steam-generating tube. 4. The nuclear reactor steam generator of claim 3, wherein the orifice included with the at least some of the plurality of perforations introduces a pressure drop of at least 15.0% of an overall pressure drop brought about by a length of steam generator tubing extending between a first plenum located at the first plane and a second plenum located at the second plane. 5. The nuclear reactor steam generator of claim 1, wherein certain ones of the plurality of steam-generating tubes are interleaved with certain other ones of the plurality of steam-generating tubes. 6. A nuclear reactor steam generator, comprising:a top portion having three or four plenums disposed in a plane around a riser column, wherein:each of the three or four plenums includes an approximately flat tubesheet that faces a bottom portion of the steam generator, andwherein the approximately flat tubesheets of the plenums include a plurality of perforations, andwherein the plurality of perforations changes in density between an area near an inner edge of the plenums and an area near an outer edge of the plenums. 7. The nuclear reactor steam generator of claim 6, wherein the density of the perforations changes from a smaller number at the area near the inner edge of the at least one plenums to a larger number near the outer edge of the at least one plenums. 8. The nuclear reactor steam generator of claim 6, wherein the plurality of perforations is arranged into a plurality of concentric arcs. 9. The nuclear reactor steam generator of claim 6, wherein each perforation of the plurality of perforations is between 15.0 and 20.0 mm in diameter. 10. A nuclear reactor steam generator, comprising:three or four means for inletting a working fluid, wherein each of the means for inletting the working fluid are disposed around a riser column at a first height and are each approximately perpendicular to a longitudinal axis of the riser column;three or four means for outletting a working fluid, wherein each of the means for outletting the working fluid are disposed around the riser column at a second height and are each approximately perpendicular to the longitudinal axis of the rise column; andmeans for conducting heat to the working fluid from a reactor coolant and conveying the working fluid from at least one of the means for inletting to at least one of the means for outletting,wherein each of means for inletting comprise approximately flat tubesheets that face a bottom portion of the steam generator, andwherein the approximately flat tubesheets of the means for inletting each include a plurality of perforations, and a number of the plurality of perforations per unit of area of at least one of the approximately flat tubesheets changes between an area near an inner edge of each respective means for inletting and an area near an outer edge of each respective means for inletting. 11. The nuclear reactor steam generator of claim 10, wherein the means for inletting and the means for outletting comprise means for reducing a pressure of the working fluid. 12. The nuclear reactor steam generator of claim 10, wherein the approximately flat tubesheets couple the means for inletting to the means for conducting heat to and conveying the working fluid. 13. The nuclear reactor steam generator of claim 10, wherein each of the means for outletting comprise approximately flat tubesheets that face the top portion of the steam generator. 14. The nuclear reactor steam generator of claim 13, wherein the approximately flat tubesheets couple the means for outletting to the means for conducting heat to and conveying the working fluid. 15. The nuclear reactor steam generator of claim 13, wherein the approximately flat tubesheets of the means for outletting each include a plurality of perforations, and a number of the plurality of perforations per unit of area of at least one of the approximately flat tubesheets changes between an area near an inner edge of each respective means for outletting and an area near an outer edge of each respective means for outletting. 16. A nuclear reactor steam generating system, comprising:a nuclear reactor comprising a riser column; anda steam generator installed around the riser column, the steam generator comprising:first, second, and third plenums proximate with a first plane, the first plane intersecting a bottom portion of the rise column,first, second, and third plenums proximate with a second plane, the second plane being approximately parallel with the first plane and intersecting a top portion of the riser column; anda plurality of steam-generating tubes that form a coolant flowpath from one of the first, second, and third plenums located proximate with the first plane to at least one of the fourth, fifth, and sixth plenums proximate with the second plane,wherein the first, second, and third plenums proximate with the first plane and the first, second, and third plenums proximate with the second plane include approximately flat tubesheets that faces in a direction of a middle portion of the column, andwherein the approximately flat tubesheets include a plurality of perforations, the perforations being of lower density near an edge closer to the riser column than near an edge closer to a reactor vessel wall.