Patent Number: 048881511
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayers et al, assigned U.S. Ser. No. 089,384 and filed Aug. 25, 1987, a division of U.S. Ser. No. 774,850, filed Sept. 12, 1985, now abandoned. PA0 2. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayers et al, assigned U.S. Ser. No. 089,203 and filed Aug. 25, 1987, a division of U.S. Ser. No. 774,850, filed Sept. 12, 1985, now abandoned. PA0 3. "Reconstitutable Control Rod Spider Assembly" by John M. Shallenberger et al, assigned U.S. Ser. No. 162,841 and filed Mar. 2, 1988. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with a reconstitutable control rod spider assembly having removable control rods with detachable split upper end plugs. 2. Description of the Prior Art In a typical nuclear reactor, such as a pressurized water type, the reactor core includes a multiplicity of fuel assemblies. Each fuel assembly is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and attached at opposite ends to the nozzles. Also, a plurality of transverse support grids are axially spaced along and attached to the guide thimbles. Further, a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles are supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles. One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill. This patent shows a control rod spider assembly which includes a plurality of control rods and a spider structure supporting the control rods at their upper ends. The spider structure, in turn, is connected to a control drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube. The spider structure typically includes a plurality of radially extending vanes supported on and circumferentially spaced about a central hub. The vanes are flat metal plates positioned on edge and being connected at their inner ends to the central hub. Cylindrical shaped control rod connecting fingers are mounted to and supported by the vanes, with some of the vanes having only a single connecting finger and other vanes having a spaced pair of connecting fingers associated therewith. Typically, the upper end plug of each control rod has a threaded outer end which is receivable into a bore in the lower portion of one finger of the spider structure and threadable into a tapped hole formed in the finger at the inner end of the bore. The end plug is then secured or locked therein by a key or pin inserted into the side of the finger and the end plug and then welded therein. Generally speaking, the above-described control rod spider assembly is not reconstitutable, that is, the assembly cannot readily be taken apart and worn or damaged components replaced. Instead, the whole assembly must be discarded. In many older nuclear reactor plants, the control rods are approaching the end of their design life. Furthermore, in a number of newer plants with modified reactor internals, the control rods are experiencing accelerated wear. In most cases wear occurs over local areas on the cladding of some, but not all, control rods of a given control rod spider assembly. Wear is believed to be caused by coolant flow-induced vibration. The severity of the clad wear is dependent upon the type of nuclear plant and internals design and typically occurs only on a limited number of control rods of a given assembly. Consequently, there is a growing interest in and need for reconstitutable control rod spider assemblies wherein selected individual control rods that are prematurely worn or damaged can be removed and replaced. A spider assembly which allows removal of a control rod from a spider structure by severing or cutting off the threadably fastened top end of the control rod and the fastener is disclosed in French patent application No. 86/08381. A spider assembly which seems to allow removal of a control rod from a spider by removing a set screw and unthreading the upper end plug of the control rod from a support rod axially movably mounted to a spider vane finger is disclosed in Nakazato U.S. Pat. No. 4,711,756. Spider assemblies which allow removal of a control rod from a spider structure by deforming a locking element on the top end of the control rod are disclosed in U.S. patents to Edwards et al U.S. Pat. No. 4,314,885 and Walton U.S. Pat. No. 4,381,283. However, none of these disclosures would appear to be the optimum approach to providing a reconstitutable control rod spider assembly. SUMMARY OF THE INVENTION The present invention provides a reconstitutable control rod spider assembly designed to satisfy the aforementioned needs. The reconstitutable control assembly of the present invention incorporates a removable control rod with a split upper end plug for detachably attaching the control rod to a vane finger of the spider. Specifically, the split upper end plug is composed of two separate upper and lower portions. The upper and lower portions of each upper end plug are attached together by screw threading and are torqued to incorporate joint preload. Also, a locking feature is defined by the portions in order to prevent their loosening during reactor operation. Locking is accomplished by swaging/crimping a thin-walled upper segment of the lower portion onto a flats-bearing middle section of the lower portion of the upper end plug. The flats are thus provided on the upper portion to prevent subsequent joint loosening. Removal of a control rod is accomplished by unscrewing the rod which will rotate the lower portion of the upper end plug relative to the upper portion thereof and overcome the crimp retainer locking feature. A replacement control rod can then be reinstalled and crimped for rod retention. The thin-walled upper section to be crimped is virgin material on the lower portion of the upper end plug on the replacement control rod. The detachable attachment features incorporated by the separate upper and lower portions of the upper end plug do not adversely impact an existing desired feature such as the flexure joint which is retained by the control rod upper end plug. As an alternative to the above-described locking feature, a radially flexible and expandable thread-defining coil can be employed between the threaded lower section and middle segment of the respective upper and lower end plug portions to provide frictional resistance to rotation once attachment of the portions has been completed. The coil makes it possible to rotate the control rod against the frictional resistance posed by the coil without the need for overriding or overcoming a crimp retainer-type locking feature. Where the crimp locking feature was previously employed, the middle section and upper segment of the respective upper and lower end plug portions are of cylindrical configuration and form a slip fit type joint instead. Accordingly, the present invention is directed to an end plug for use in facilitating replacement of a neutron absorber control rod on a control assembly spider structure. The end plug comprises a pair of separate upper and lower plug portions with the upper plug portion having integral upper, middle and lower sections and the lower plug portion having integral upper, middle and lower segments. The upper section of the upper plug portion is configured for rigid attachment to the control assembly spider structure for supporting the control rod. The lower segment of the lower plug portion is configured for rigid connection to the control rod. Also, the lower section of the upper plug portion and the middle segment of the lower plug portion are complementarily configured for rigid threaded attachment to one another. Further, the middle section of the upper plug portion and the upper segment of the lower lug portion are complementarily configured for interlocking attachment to one another so as to resist unthreaded detachment of the upper and lower plug portions from one another. Also, the present invention sets forth in a reconstitutable control assembly including a spider structure and at least one control rod, an attachment joint for detachably fastening the control rod to the spider structure. The attachment joint comprises: (a) a hollow connecting finger on the spider structure; and (b) an elongated detachable split upper end plug on the control rod having a pair of separate upper and lower plug portions. The upper plug portion has integrally-connected tandemly-arranged upper, middle and lower sections. The lower plug portion has integrally-connected tandemly-arranged upper, middle and lower segments. More particularly, the upper section of the upper plug portion is rigidly attached to the connecting finger on the spider structure, whereas the lower segment of the lower plug portion is rigidly connected to the control rod. Also, the lower section of the upper plug portion and the middle segment of the lower plug portion have complementarily threaded means defined thereon for rigidly threadably attaching the upper and lower plug portions to one another. Further, the middle section of the upper plug portion and the upper segment of the lower plug portion have complementary interlocking means defined thereon for locking the upper and lower plug portions together so as to resist unthreading of the upper and lower plug portions from one another. Further, the present invention is directed to a modified end plug for use in facilitating replacement of a neutron absorber control rod on a control assembly spider structure. The modified end plug comprises a pair of separate upper and lower plug portions with the upper plug portion having integral upper, middle and lower sections and the lower plug portion having integral upper, middle and lower segments. Also, the end plug includes means in the form of a radially flexible and expandable thread-defining coil. Further, the upper section of the upper plug portion is configured for rigid attachment to a control assembly spider structure for supporting a control rod, whereas the lower segment of the lower plug portion is configured for rigid connection to the control rod. Still further, the lower section of the upper plug portion and the middle segment of the lower plug portion are threaded complementarily to receive therebetween the radially flexible and expandable thread-defining coil for threaded engagement therewith in a relationship in which the thread-defining coil frictionally resists rotation of the upper and lower plug portions relative to one another. Finally, the middle section of the upper plug portion and the upper segment of the lower plug portion are configured complementarily for defining a slip fit joint between the upper and lower plug portions. These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.