Patent Number: 048204757
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending U.S. patent application dealing with related subject matter and assigned to the assignee of the present invention: "Standardized Reduced Length Burnable Absorber Rods For A Nuclear Reactor" by Barry R. Cooney et al, assigned U.S. Ser. No. 718,902 and filed Apr. 1, 1985. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors and, more particularly, is concerned with a unique attachment joint for burnable absorber rods which allows easy detachment and consolidation of the spent absorber rods. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and 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. Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable poison rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core. The overall advantages to be gained in using burnable poison rods at stationary positions in a nuclear reactor core are described in U.S. Pat. Nos. 3,361,857 to Rose and 3,510,398 to Wood. With the increased use of an 18-month fuel cycle in operation of nuclear reactors in contrast to the more usual 12-month cycle, the need to consolidate spent burnable absorber assemblies, in an efficient manner, has become an important factor in the overall reactor fuel servicing. Previously with the use of the 12-month fuel cycle, burnable absorber assemblies were only required in the first cycle and so the spent absorber assemblies could be stored with the spent fuel assemblies. Now, in the 18-month fuel cycle, burnable absorber assemblies are required in the second and third cycles of the fuel. Thus, there are more spent absorber assemblies generated than there are spent fuel assemblies so all of the spent absorber assemblies cannot be stored in spent fuel assemblies. This means that the absorber rods must be consolidated to save space in the spent fuel pool. The present design of the burnable absorber cluster assembly, for example as illustrated and described in the patent application cross-referenced above, does not lend itself to consolidation since the upper ends of the rods are fastened to a base or support plate of the assembly with threaded connections which are locked in place. To remove the rods from the support plate it is necessary to shear the rods therefrom which is not an easy task to perform remotely while the irradiated assembly is submerged in water. Consequently, a need exists for a different approach to attaching the absorber rods to the support plate which will still ensure that the rods are held securely in their desired positions within the guide thimbles during use in the fuel assembly in the reactor core, but after being used up will allow easy detachment of the spent rods from the support plate in order to facilitate their efficient consolidation in the spent fuel pool. SUMMARY OF THE INVENTION The present invention provides a burnable absorber rod push out attachment joint designed to satisfy the aforementioned needs. The modifications to the burnable absorber assembly necessary to implement the push out attachment joint of the present invention is probably slightly less expensive than the present threaded connection and will not adversely affect reliability of the absorber assembly. However, most of the benefits to be gained from the attachment joint of the present invention are realized when it comes time to consolidate the spent absorber rods. Most importantly, the attachment joint allows the spent rod to be detached from the support plate without the complication of having to shear or cut it remotely. Instead, detachment of the spent rod is achieved from above by simply pushing down on its upper end using a suitable tool which causes the joined interlocking portion of the upper end to deform slightly and thereby release from an opening in the assembly support plate through which it was lodged and attached to the support plate. Accordingly, the present invention sets forth in a cluster assembly including a support plate and at least one burnable absorber rod, an attachment joint for detachably connecting an end of the absorber rod to the support plate, comprising: (a) a plug insert defined on the end of the rod; (b) means defining at least one hole through the support plate for receiving the plug insert; and (c) attaching means connected with at least one of the support plate hole-defining means and the plug insert and being deformable for securing the plug insert in an interengaging connection to the support plate in which the plug insert is disposed within the support plate hole, the attaching means also being redeformable upon application of a predetermined axially-directed force on the plug insert so as to remove the plug insert from the support plate hole and thereby detach the rod from the support plate. Also, the attaching means includes an eccentric portion which prevents the plug insert on the rod end from rotating in the hole. More particularly, in the preferred embodiment, the attaching means of the attachment joint includes a hollow annular wall extending outwardly from the plug insert on the rod end and means defining a conical-shaped recess in the support plate connected with the hole therein. The recess is flared outwardly from the hole to a maximum diameter greater than the diameter of the hole. Also, the annular wall on the plug insert is deformed into conformity with the shape of the recess. Further, the eccentric portion which prevents the plug insert from rotating in the hole includes means defining a cavity in the support plate being connected in offset relation to the recess, and a portion of the annular wall deformed into the offset cavity. In one modified embodiment, the attaching means of the attachment joint includes means defining a concave-shaped groove in the support plate within the hole therein, and the annular wall on the plug insert is deformed into conformity with the shape of the groove. In another modified embodiment, the attaching means includes a solid section extending outwardly from the plug insert on the rod end and beyond the hole in the support plate. A collar is attached on the solid section of the plug insert and has an outside diameter greater than that of the hole. Also, the solid section has a concave groove formed therein and the collar has a portion bulged into the groove so as to attach the collar to the section. These and other advantages and attainments 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.