Patent Number: 048204757
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is shown an elevational view of a fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 20. The fuel assembly 20 is the type used in a pressurized water reactor (PWR) and basically includes a lower end structure or bottom nozzle 22 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 24 which project upwardly from the bottom nozzle 22. The assembly 20 further includes a plurality of transverse grids 26 axially spaced along the guide thimbles 24 and an organized array of elongated fuel rods 28 transversely spaced and supported by the grids 26. Also, the assembly 20 has an instrumentation tube 30 located in the center thereof and an upper end structure or top nozzle 32 attached to the upper ends of the guide thimbles 24. With such an arrangement of parts, the fuel assembly 20 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 28 in the array thereof in the assembly 20 are held in spaced relationship with one another by the grids 26 spaced along the fuel assembly length. Each fuel rod 28 includes nuclear fuel pellets (not shown) and the opposite ends of the rod are closed by upper and lower end plugs 34,36. The fuel pellets composed of fissile material are responsible for creating the reactive power of the PWR. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract some of the heat generated therein for the production of useful work. Burnable Absorber Rod Cluster Assembly In the operation of a PWR it is desirable to prolong the life of the reactor core as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, provide means to maintain the reactivity relatively constant over its lifetime. Such means commonly takes the form of a cluster of reduced length burnable absorber rods 38 supported by a holddown assembly 40. As seen in FIG. 1, the holddown assembly 40 stationarily supports the absorber rods 38 in the guide thimbles 24 of some of the fuel assemblies 20 to assist the movable control rods (not shown) in the guide thimbles of other assemblies (not shown) in maintaining a substantially constant level of neutron flux or reactivity in the core throughout its operating cycle. Referring now to FIGS. 2 and 3, in addition to FIG. 1, the holddown assembly 40 which supports the absorber rods 38 in spaced side-by-side relationship includes a lower flat perforated support plate 42 which fits within the fuel assembly top nozzle 32 and rests on the adapter plate 44 of the top nozzle. The holddown assembly 40 also includes a sleeve 46, being attached at its lower end within a central opening 48 in the support plate 42 and extending upwardly therefrom, and an upper holddown plate 50 which receives the sleeve 46 and is slidable vertically along it. Further, a holddown coil spring 52 is disposed about the sleeve 46 and extends between the lower support plate 42 and the upper holddown plate 50. Thus, the support plate 42 is held down against the top nozzle adapter plate 44 by the coil spring 52 which is compressed by the upper core plate (not shown) acting through the upper holddown plate 50 which abuts the upper core plate. This arrangement assures that the absorber rods 38 which are attached to the support plate 42 cannot be ejected from the reactor core by coolant flow forces while any thermal growth of the rods is accommodated. Absorber Rod Push Down Attachment Joint Heretofore, each of the burnable absorber rods had a threaded upper end, such as seen in the cross-referenced application, by which the rod was connected to the support plate 42. Such threaded connection presented complications when attempting to detach the rods 38 from the holddown assembly 40 in order to consolidate them for storage once they have reached a spent condition. The push out attachment joint of the present invention, as seen in FIG. 1, and better yet in FIGS. 4 and 5, and being generally identified by the numeral 54, was originated to overcome and eliminate such complications. Referring to FIGS. 4 to 9, there is shown the preferred embodiment of the push out attachment joint 54 for detachably connecting the upper end 56 of each absorber rod 38 to the support plate 42. Basically, the attachment joint 54 includes a plug insert 58 defined on the upper end 56 of each absorber rod 38, a hole 60 defined through the support plate 42 for receiving each plug insert 58, and attaching means, generally designated 62, for securing the plug insert 58 in an interengaging connection in the hole 60 and thereby connecting each absorber rod 38 to the support plate 42. In its preferred embodiment as seen in FIGS. 6 to 9, the attaching means 62 includes a thin hollow annular wall 64 extending axially outwardly from the plug insert 58 on the rod upper end 56 and a conical-shaped recess 66 defined in the support plate 42 and connected with the hole 60 therein. As shown in FIGS. 8 and 9, the recess flares or tapers outwardly from the hole 60 from the diameter of the latter to a maximum diameter greater than the diameter of the hole 60. Before the joint 54 is formed, the hollow annular wall 64 on the plug insert 58 is generally cylindrical in shape, as seen in FIGS. 6 and 7. After the joint 54 is completed as illustrated in FIGS. 4 and 5, the annular wall 64 has a deformed shape in which it conforms with the conical shape of the recess 66. Such interengaging connection formed by the annular wall 64 deformed into contact with the conical-shaped recess 66 together with the larger diameter flange 68 on the upper rod end 56 below the plug insert 58 which contacts the bottom side of the support plate 42 maintains the plug insert 58 within the support plate hole 60 so as to rigidly attach each absorber rod 38 to the support plate 42. However, the annular wall 64 is redeformable upon application of a predetermined downwardly and axially-directed force on the plug insert 58 so as to remove the plug insert 58 from the support plate hole 60 by forcibly pushing the plug insert 58 out of the hole. The center 70 of the deformed wall 64 provides a pilot hole for receipt of an appropriate punch tool (not shown) for driving the plug insert 58 downwardly and concurrently straightening the wall 64 enough to allow passage through the hole 60 and thereby detachment of the absorber rod 38 from the support plate. In order to prevent the plug insert 58, and thereby the absorber rod 38, from rotating in the support plate hole 60, the attaching means 62 of the attachment joint 54 also includes an eccentric portion which is located in offset relation to the axes of the absorber rod 38 and support plate hole 60. The eccentric portion of the attaching means 62, as seen in FIGS. 6, 8 and 9, is in the form of a groove or cavity 72 defined in the support plate 42 adjacent the outer periphery of the recess 66 and connected therewith in an offset relation thereto. When the annular wall 64 is deformed to its conical shape to complete the joint 54, a small portion 74 of the material of the peripheral rim or edge 76 of the annular wall 64 is pressed into the offset cavity 72. In such manner, the deformed annular wall 64 and plug insert 58 are anchored against rotation relative to the recess 66 and hole 60. However, the wall portion 74 readily slides out of the cavity 72 as the wall 64 is redeformed during detachment of the absorber rod 38 from the support plate 42. In one modified embodiment illustrated in Fig. 10, the push out attachment joint 54' has attaching means 62' in the form of a concave-shaped groove 78 in the support plate 42 within the hole 60 therein and the annular wall 64' on the plug insert 58' is deformed into conformity with the shape of the groove 78. Rotation can be prevented by the provision of eccentric means similar to that shown in relation to the preferred embodiment. Another modified embodiment of the attachment joint 54" is shown in FIG. 11. There, the attaching means 62" is in the form of a solid section 80 extending outwardly from the plug insert 58" and beyond the hole 60 in the support plate 42 and a collar 82 slid over and attached on the solid section 80 of the plug insert 58" and having an outside diameter greater than that of the hole 60. The solid section 80 has a concave groove 84 formed therein and the collar 82 has a portion 86 bulged into the groove 84 so as to attach the collar to the plug insert section. Rotation can be prevented in a way similar to that shown in the preferred embodiment. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.