Patent Number: 
Section: claims

1. A method of operating a nuclear reactor steam generator, comprising:conveying a working fluid from a first group of plenums to a plurality of flowpaths, each of the plenums having an approximately flat tubesheet with a plurality of perforations, wherein a number of the plurality of perforations per unit of area of the approximately flat tubesheet changes between an area of the approximately flat tubesheet near an inner edge of the at least one plenum and an area near an outer edge of the at least one plenum, the conveying further comprising conveying the working fluid to the plurality of flowpaths through the plurality of perforations;vaporizing the working fluid in at least some of the plurality of flowpaths, the vaporizing resulting, at least in part, from transferring thermal energy from a reactor coolant to the at least some of the plurality of flowpaths; andtransferring the vaporized coolant to a second group of plenums. 2. The method of claim 1, wherein the conveying further comprises reducing pressure of the working fluid. 3. The method of claim 2, wherein the working fluid is reduced in pressure by an amount of at least 15.0% of an overall pressure drop resulting, at least in part, from a flowpath extending between a first plenum of the first group of plenums located at a first plane and a second plenum of the second group of plenums located at a second plane. 4. The method of claim 1, wherein transferring further comprises transferring the working fluid to the second group of plenums through an approximately flat tubesheet of each plenum of the second group of plenums. 5. The method of claim 1, wherein the flowpaths extend helically between the first group of plenums and the second group of plenums. 6. The method of claim 4, wherein the transferring further comprises transferring the working fluid to the second group of plenums through a plurality of perforations in the approximately flat tubesheet of at least one plenum of the second group of plenums. 7. A method of operating a nuclear reactor steam generator, comprising:conveying a working fluid from a first set of a plurality of chambers to a plurality of tubes, wherein the chambers of the first set of chambers are disposed around a riser column;vaporizing the working fluid in at least some of the plurality of tubes, the vaporizing resulting, at least in part, from transferring thermal energy from a reactor coolant to the at least some of the plurality of tubes; andtransferring the vaporized coolant from the plurality of tubes to a second set of a plurality of chambers, wherein chambers of the second set of chambers are disposed around the riser column;wherein the pressure of the working fluid is reduced by an amount of at least 15.0% of an overall pressure drop resulting, at least in part, from a tube extending between a first chamber of the first set of chambers located at a first height and a second plenum of the second set of chambers located at a second height. 8. The method of claim 7, wherein the conveying further comprises reducing a pressure of the working fluid. 9. The method of claim 7, wherein the conveying further comprises conveying the working fluid to the plurality of tubes through an approximately flat tubesheet of each chamber of the first set of chambers. 10. The method of claim 7, wherein transferring further comprises transferring the working fluid to the second set of chambers through an approximately flat tubesheet of each chamber of the second set of chambers. 11. The method of claim 7, wherein the tubes extend helically between the first set of chambers and the second set of chambers. 12. The method of claim 9, wherein the coupling further comprises conveying the working fluid to the plurality of tubes through a plurality of perforations in the approximately flat tubesheet of each chamber of the first set of chambers. 13. The method of claim 10, wherein the transferring further comprises transferring the working fluid to the second set of chambers through a plurality of perforations in the approximately flat tubesheet of each chamber of the second set of chambers. 14. The method of claim 12, wherein a number of the plurality of perforations per unit of area of the approximately flat tubesheet changes between an area of the approximately flat tubesheet near an inner edge of the at least one chamber and an area near an outer edge of the at least one chamber. 15. A method of operating a nuclear reactor steam generator, comprising:conveying a working fluid from a first group of plenums into an internal portion of a plurality of flowpaths, wherein the plenums of the first group of plenums are disposed around a riser column at a first height, each of the plenums having an approximately flat tubesheet with a plurality of perforations, wherein a number of the plurality of perforations per unit of area of the approximately flat tubesheet changes between an area of the approximately flat tubesheet near an inner edge of the at least one plenum and an area near an outer edge of the at least one plenum;conveying a reactor coolant from the riser column along an external portion of the plurality of flowpaths, wherein the reactor coolant is conveyed from a second height to the first height;vaporizing the working fluid in at least some of the plurality of flowpaths, the vaporizing resulting, at least in part, from transferring thermal energy from the reactor coolant to the at least some of the plurality of flowpaths; andtransferring the vaporized coolant from the plurality of flowpaths to a second group of plenums, wherein the plenums of the second group of plenums are disposed around the riser column at the second height.