Patent Number: 047160179
Section: claims

1. In a nuclear reactor fuel assembly having a structural tube with a predetermined inside diameter, a generally cylindrical insert of an axial length substantially smaller than the axial length of said structural tube and having a generally cylindrical passageway of a predetermined diameter smaller than said predetermined inside diameter for providing an effectively reduced inside diameter for said structural tube, said insert comprising: means, having an outside diameter approximately equal to said predetermined inside diameter, for coaxially centering said insert within said structural tube;  forming lobes, operable when expanded to locally deform against said structural tube thereby locking said insert within said structural tube. 2. The insert of claim 1, further comprising projections extending radially into the passageway of said insert and operable to cooperate with an expansion tool to accurately position said expansion tool with respect to said forming lobes. 3. The insert of claim 3, wherein said projections are expandable with said forming lobes and wherein said projections are operable, in an unexpanded configuration, to pass an expansion tool moving in a first direction through said passageway and to register said expansion tool when moving in a direction opposite to said first direction, with said forming lobes. 4. The insert of claim 1, wherein said coaxial centering means comprises a pair of centering lobes, each having an outside diameter approximately equal to said predetermined inside diameter, disposed at ends of said insert. 5. The insert of claim 4, wherein said coaxial centering means is provided with a plurality of circumferentially spaced axial slots. 6. The insert of claim 1, wherein said coaxial centering means comprises an elongated central lobe formed on said insert and having an outside diameter approximately equal to said predetermined inside diameter. 7. The insert of claim 6, wherein said coaxial centering means is provided with a plurality of circumferentially spaced axial slots. 8. The insert of claim 1, wherein said coaxial centering means comprises a pair of lobes disposed at ends of said insert and a central lobe, all of said lobes having an outside diameter approximately equal to said predetermined inside diameter. 9. The insert of claim 8, wherein said coaxial centering means is provided with a plurality of circumferentially spaced axial slots. 10. The insert of claim 1, wherein said forming lobes comprise a pair of annular lobes having generally arcuate cross-sections formed at axially spaced apart locations along said insert, said annular lobes having a nominal thickness, said insert having reduced thickness portions axially disposed on either side of said annular lobes to facilitate expansion of said lobes. 11. The insert of claim 10, further comprising a plurality of axially extending expansion slots formed in said insert in a region of said forming lobes, said expansion slots being circumferentially spaced about said forming lobes.