Patent Number: 
Section: claims

1. A support grid for a nuclear fuel assembly, said nuclear fuel assembly having a generally cylindrical fuel rod with a diameter, said support grid comprising:a frame assembly having a plurality of generally uniform cells, each said cell having at least one sidewall and a width;at least one generally cylindrical tubular member having 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 width, 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 uniform cells so that said cell contact portion engages said cell sidewall. 2. The support grid of claim 1 wherein said at least one tubular member has a wall with a uniform thickness. 3. The support grid of claim 1 wherein each of said at least one helical fuel rod contact portion extends 360 degrees about said tubular member. 4. The support grid of claim 1 wherein 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 said at least one helical fuel rod contact portion includes four helical fuel rod contact portions. 8. The support grid of claim 7 wherein each said helical fuel rod contact portion extends 90 degrees about said tubular member. 9. The support grid of claim 1 wherein said frame assembly includes a plurality of substantially flat, elongated strap members disposed in two interlocked sets, a vertical set and a horizontal set, said vertical and horizontal sets of strap members being generally perpendicular to each other and wherein the strap members in each set are generally evenly spaced so that said strap members form generally square cells. 10. The support grid of claim 9 wherein each said strap member is free from protrusions. 11. The support grid of claim 9 wherein said at least one helical fuel rod contact portion includes four helical fuel rod contact portions. 12. The support grid of claim 11 wherein each said helical fuel rod contact portion extends 90 degrees about said tubular member. 13. The support grid of claim 12 wherein:each strap member has a generally uniform height so that each cell is a generally square cell having a top and a bottom;each said generally square cell has two diagonal axes, said axes being generally perpendicular to each other; andeach said helical fuel rod contact portion is generally aligned with a diagonal axis at the top of said cell and aligned with a different diagonal axis at the bottom of said cell. 14. The support grid of claim 13 wherein said transition portion is a generally smooth curve. 15. The support grid of claim 1 wherein said transition portion is a generally smooth curve. 16. The support grid of claim 1 wherein:said cell contact portion includes a platform; andsaid transition portion is a sharp curve. 17. The support grid of claim 16 wherein said fuel rod contact portion includes a concave platform. 18. The support grid of claim 1 wherein:said fuel rod contact portion includes a concave platform; andsaid transition portion is a sharp curve. 19. The support grid of claim 1 wherein:said cell contact portion includes a platform; andsaid transition portion is generally flat with angled ends. 20. The support grid of claim 19 wherein said fuel rod contact portion includes a concave platform. 21. 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. 22. The support grid of claim 1 wherein said frame assembly includes a plurality of tubular frame members defining cylindrical cells. 23. The support grid of claim 22 wherein said tubular frame members are coupled to each other at 90 degree intervals about the perimeter of each tubular frame member. 24. The support grid of claim 23 wherein said at least one tubular member has a wall with a uniform thickness. 25. The support grid of claim 23 wherein said at least one helical fuel rod contact portion extends 360 degrees about said tubular member. 26. The support grid of claim 23 wherein said at least one helical fuel rod contact portion includes two helical fuel rod contact portions. 27. The support grid of claim 26 wherein each said helical fuel rod contact portion extends 180 degrees about said tubular member. 28. The support grid of claim 26 wherein each said helical fuel rod contact portion extends 90 degrees about said tubular member. 29. The support grid of claim 23 wherein said at least one helical fuel rod contact portion includes four helical fuel rod contact portions. 30. The support grid of claim 29 wherein each said helical fuel rod contact portion extends 90 degrees about said tubular member.