Fuel assembly hold-down device

Abstract

The invention provides a hold-down device which maintains a net downward force on a fuel assembly in a nuclear reactor under all thermal and hydraulic conditions in the reactor core region. The hold-down device is particularly applicable in reactors having stainless steel support barrels and Zircaloy guide tubes. The hold-down device of the preferred embodiment of the invention is comprised of coil springs coaxially disposed about alignment posts which extend upwardly from the top end plate of each fuel assembly. A hold-down plate is slideably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the top end plate. In use, the coil springs bias the hold-down plate upwardly against a core alignment plate to provide a downward force on the fuel assembly. The hold-down plate may also serve as a lifting surface for the fuel assembly during fuel handling.

What we claim is: 
     
       1. In a nuclear fuel assembly including, .Iadd.a plurality of .Iaddend.longitudinally extending rigid support .[.means.]. .Iadd.tubes.Iaddend.; upper and lower end fittings supported by said support .[.means.]. .Iadd.tubes .Iaddend.at opposite ends thereof; a plurality of elongated fuel elements in parallel relationship and extending substantially between said upper and lower end fittings and supported by said support .[.means.]. .Iadd.tubes.Iaddend., hold-down apparatus for said fuel assembly in which said upper end fitting comprises: an end plate supported by said support .[.means.]. .Iadd.tubes .Iaddend.against downward motion relative thereto and extending transversely thereof;   .[.alignment means.]. .Iadd.a plurality of hollow cylindrical posts .Iaddend.projecting longitudinally upward from and supported against upward motion relative to said end plate, .Iadd.each of .Iaddend.said .[.alignment means.]. .Iadd.posts .Iaddend.having an outwardly enlarged shoulder affixed thereto and spaced upwardly from said end plate.[.;.]..Iadd., said posts and end plate including passageways extending vertically therethrough in registry with said tubes whereby a control element may be inserted therewithin into said fuel assembly; .Iaddend.   force transmitting means slidably mounted on said .[.alignment means.]. .Iadd.posts .Iaddend.and movable therealong between said shoulder and said end plate.Iadd., and including a hold-down plate having at least one hub portion, an aperature in and extending through said hub portion for relative slidable passage therethrough of said posts, .Iaddend.and .[.having a portion thereof.]..Iadd.leg portions .Iaddend.extending laterally beyond said .[.shoulder.]. .Iadd.shoulders .Iaddend.for receiving a downward force applied to said .Iadd.leg .Iaddend..[.portion.]. .Iadd.portions.Iaddend.; and   spring means .Iadd.coaxially disposed about a plurality of said posts and being .Iaddend.interposed .Iadd.in compression .Iaddend.between said force transmitting means and said end plate for biasing said force transmitting means upwardly from said plate whereby a said downward force on said force transmitting means is yieldably transmitted to said fuel assembly. .[.2. The apparatus of claim 1 wherein said force transmitting means comprise a hold-down plate having at least one hub portion; an aperture in and extending through said hub portion for relative slidable passage therethrough of said alignment means; and leg means extending substantially radially outward from said hub portion beyond said enlarged   
     
     
        shoulder on said alignment means..]. 3. The apparatus of claim .[.2.]. .Iadd.1 .Iaddend.wherein said .[.alignment means comprise a plurality of cylindrical posts extending upwardly from said end plate and.]. .Iadd.posts are .Iaddend.symmetrically disposed about the vertical axis passing through the center of gravity of said fuel assembly; and said hold-down plate includes said same plural number of apertured hub portions for relative insertion therethrough of said .[.alignment.]. posts, said hub portions being joined and positioned relative to one another .[.and.]. by said leg .[.means.]. .Iadd.portions.Iaddend.. .[.4. The apparatus of claim 1 wherein said spring means comprise a coil spring, said coil spring being in compression and coaxially disposed about said alignment means..]. 
     
     
        . The apparatus of claim 3 wherein said spring means comprise coil springs, said coil springs being in compression and coaxially disposed about a plurality of said .[.alignment.]. posts. .[.6. The apparatus of claim 1 wherein said fuel assembly includes a hollow tube open at its upper end and extending at least the full extent of said fuel elements parallel thereto and in vertical alignment with said alignment means; and said alignment means and said end plate include a passageway extending vertically therethrough in registry with said hollow tube whereby a control element may be inserted therewithin into said fuel assembly..]. .[.7. The apparatus of claim 3 wherein said fuel assembly includes hollow tubes open at their upper ends and extending at least the full extent of said fuel elements thereto and in vertical alignment with alignment posts; and said alignment posts and said end plate include passageways extending vertically thereto in registry with said hollow tubes whereby a control element may be inserted therewithin into said fuel assembly..]. .[.8. The apparatus of claim 7 wherein said hollow tubes comprise said support means extending between said fuel assembly end fittings..]..Iadd. 9. The apparatus of claim 5 wherein the posts are rigidly secured to the end plate. .Iaddend.