Patent Number: 
Section: claims

1. A support grid for a nuclear fuel assembly, said nuclear fuel assembly including a generally cylindrical fuel rod with a diameter, said support grid comprising:a plurality of generally tubular frame members defining frame assembly including a plurality of generally cylindrical cells, each said cell including at least one sidewall;at least one generally cylindrical tubular member including a cell contact portion with a greater diameter and at least one helical fuel rod contact portion with a lesser diameter, said cell contact portion and said fuel rod contact portion joined by a transition portion, said greater diameter being generally equivalent to said cell radius, and said lesser diameter being generally equivalent to said fuel rod diameter such that a fuel rod disposed in said tubular member would engage said lesser diameter; andeach of said at least one tubular member disposed in one cell of said plurality of generally circular cells so that said cell contact portion engages said cell sidewall. 2. The support grid of claim 1 wherein each said at least one tubular member has a wall with a uniform thickness. 3. The support grid of claim 1 wherein each said at least one helical fuel rod contact portion extends 360 degrees about said tubular member. 4. The support grid of claim 1 wherein each said at least one helical fuel rod contact portion includes two helical fuel rod contact portions. 5. The support grid of claim 4 wherein each said helical fuel rod contact portion extends 180 degrees about said tubular member. 6. The support grid of claim 4 wherein each said helical fuel rod contact portion extends 90 degrees about said tubular member. 7. The support grid of claim 1 wherein each said transition portion is a generally smooth curve. 8. The support grid of claim 1 wherein:said cell contact portion includes a platform; andsaid transition portion is a sharp curve. 9. The support grid of claim 8 wherein each said fuel rod contact portion includes a concave platform. 10. The support grid of claim 1 wherein:said fuel rod contact portion includes a concave platform; andsaid transition portion is a sharp curve. 11. The support grid of claim 1 wherein:said cell contact portion includes a platform; andsaid transition portion is generally flat with angled ends. 12. The support grid of claim 11 wherein each said fuel rod contact portion includes a concave platform. 13. The support grid of claim 1 wherein:said fuel rod contact portion includes a concave platform; andsaid transition portion is generally flat with angled ends. 14. The support grid of claim 1 wherein:said frame assembly includes a plurality of generally tubular frame members defining said generally cylindrical cells; andeach said tubular frame members coupled to each other at 90 degree intervals about the perimeter of each tubular frame member. 15. The support grid of claim 1 wherein each said at least one helical fuel rod contact portion has a variable pitch. 16. The support grid of claim 1 wherein each said at least one helical fuel rod contact portion has a constant pitch. 17. The support grid of claim 1 wherein:said tubular member has a cylinder axis and at least a first axial portion and a second axial portion;said at least one helical fuel rod contact portion extending over both said first axial portion and said second axial portion; andwherein said at least one helical fuel rod contact portion on said first axial portion has a first pitch and said at least one helical fuel rod contact portion on said second axial portion has a second pitch. 18. The support grid of claim 1 wherein:each said each said at least one helical fuel rod contact portion may have a clockwise twist or a counterclockwise twist;at least one tubular member including a helical fuel rod contact portion with a clockwise twist; andat least one tubular member including a helical fuel rod contact portion with a counterclockwise twist. 19. The support grid of claim 1 wherein each said at least one helical fuel rod contact portion includes four helical fuel rod contact portions. 20. The support grid of claim 7 wherein each said helical fuel rod contact portion extends 90 degrees about said tubular member.