Patent Number: 046844996
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 asssignee of the present invention: 1. "Standardized Reduced Length Burnab1e Absorber Rods For A Nuclear Reactor" by Barry R. Cooney et al, assigned U.S. Ser. No. 718,902 and filed Apr. 1, 1985 (W.E. 52,386). 2. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayers et al, assigned U.S. Ser. No. 774,850 and filed Sept. 12, 1985 (W.E. 52,875). 3. "Nuclear Reactor Fuel Assembly With A Removable Top Nozzles" by John M. Shallenberger et al, assigned U.S, Ser. No. 644,758 and filed Aug. 27, 1984 (W.E. 51,311I), now U.S. Pat. No. 4,631,168. 4. "Improved Guide Thimble Captured Locking Tube In A Reconstitutable Fuel Assembly" by Robert K. Gjertsen et al, assigned U.S. Ser. No. 775,208 and filed Sept. 12, 1985 (W.E. 52,881), BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a unique releasable latching structure for burnable absorber rods which readily permits attachment and detachment of such rods directly to and from the adapter plate of the fuel assembly top nozzle and facilitates 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 and attached to 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 abosrber 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 (3,510,388) to Wood. Also, the availability of assemblies of burnable absorber rods on a rapid response basis is required at reactor fuel reload time. The present design of the burnable absorber assemblies, being similar to those illustrated and described in the first two patent applications cross-referenced above, includes a plurality of precisely spaced apart absorber rods and thimble plugs fastened at their upper ends to a support Plate which also mounts a central hold-down device. In view of the multiplicity of components which make up the absorber assemblies and the precise spacing required between them when they are assembled together, it has been found necessary to assemble the absorber assemblies at a manufacturing facility located remote from the reactor site. The final absorber assemblies are then shipped with the fuel assemblies to the reactor site. This means that the particular absorber assembly design must be specified well in advance of the time of actual reload. However, it is desirable to have the flexibility of specifying the burnable absorber assembly configurations at the latest possible time so that the nuclear reload design can be fine tuned based on the latest reactor operations input. The ultimate absorber assembly configurations specified may advantageously include, for example, twelve burnable absorber rods and twelve thimble plugs per assembly or other combinations of absorber rods and thimble plugs. To accommodate these variations in configurations with the present design, within the time frame mentioned above, would require final assembly of the burnable absorber assemblies at the reactor site. This is not a practical alternative in view of the present design of the absorber assembly. Consequently, a need exists for a different approach to absorber assembly design which will provide greater flexibility in arriving at what the final configuration should be, based on substantially current reactor operating information. SUMMARY OF THE INVENTION The present invention provides a releasable latching structure for members, such as burnable absorber rods and thimble plugs, which is designed to satisfy the aforementioned needs. The releasable latching structure of the present invention permits handling of absorber rods and thimble plugs in a manner which allows their final arrangement to be specified just before reload time so that the latest reactor operating information can be taken into consideration. At the same time, there is no onsite manufacturing required. Basically, absorber rods and thimble plugs individually are easily attached directly to the top nozzle adapter plate which eliminates the need for the support plate and hold-down device of the previous design. Now, an inventory of individual absorber rods and thimble plugs can be sent to the reactor site prior to refueling. Then, when the final burnable absorber design for each fuel assembly is specified, the required configuration of absorber rods and thimble plugs can be inserted into the fuel assembly. The approach avoids the need for onsite manufacturing of absorber assemblies. An additional benefit of the present invention is that, later on, the spent absorber rods can be easily released from the adapter plate and readily consolidated in a consolidation cannister. Accordingly, the present invention is directed to a latching structure for releasably attaching an elongated member at an end thereof to a top nozzle adapter plate of a nuclear fuel assembly. The latching structure includes: (a) a mounting body attached to the end of the elongated member and extending axially outward therefrom; and (b) a spring latch disposed about the mounting body and having at least one latch finger movable toward and away from the body between an outer latching position in which the finger is adapted to engage the adapter plate and retain the elongated member in a stationary relationship with respect to the adapter plate and an inner unlatching position in which the finger is adapted to disengage from the adapter plate and allow removal of the member from the adapter plate. More particularly, the mounting body includes an inner plug portion attached to the end of the elongated member, an outer end portion having a groove defined therein, and a middle body portion having a configuration which generally defines a recessed region surrounding the mounting body at the middle portion thereof. Further, the spring latch includes an outer ring portion disposed about the outer end portion of the mounting body and having a bulge formed therein which extends into the groove in the outer end portion so as to connect the spring latch to the mounting body. Also, the spring latch includes a plurality of circumferentially spaced apart latch fingers connected at their outer ends to the outer ring portion in cantilever fashion and extending along the middle body portion of the mounting body. The fingers are radially deflectible toward and away from the middle body portion between the outer latching and inner unlatching positions. ln their outer latching positions, the fingers are adapted to engage the adapter plate and are generally disposed outside of the recessed region surrounding the mounting body middle portion. In their inner unlatching positions, the fingers are adapted to disengage from the adapter plate and are generally disposed within the recessed region surrounding the mounting body middle portion. Each of the latch fingers includes a latching key defined on an inner end thereof being configured to engage with and disengage from the adapter plate when the finger is deflected between its respective latching and unlatching positions. The present invention is also set forth in a fuel assembly including a top nozzle having an adapter plate with at least one passageway defined therethrough, at least one guide thimble connected to the adapter plate, and a burnable absorber rod disposed within the guide thimble. The invention relates to a releasable latching structure for releasably interconnecting an end of the absorber rod to the adapter plate which includes: (a) engagable means defined in the adapter plate; (b) a mounting body attached to the end of the absorber rod and extending axially upward therefrom through the passageway and above the adapter plate; and (c) a spring latch disposed about the mounting body above the adapter plate and having at least one latch finger extending downwardly toward the adapter plate. The latch finger is deflectible toward and away from the mounting body between an outer latching position in which the finger engages the engagable means on the adapter plate and retains the absorber rod disposed in the guide thimble and an inner unlatching position in which the finger disengages from the engagable means on the adapter plate and allows removal of the absorber rod from the guide thimble. Also, means is defined on the mounting body and spring latch for attaching the spring latch to the body. More particularly, the engagable means is in the form of a recess formed in the adapter plate within the passageway therein. The latch finger extends into the adapter plate passageway for engaging with and disengaging from the passageway recess when the finger is deflected between its respective latching and unlatching positions. Further, the latch finger has a latching key defined on its lower end being configured to engage the adapter plate within the recess formed in the passageway of the plate. Also, the mounting body has a portion generally coextensive with the latch finger and defining a recessed region along the mounting body. The latch finger is disposed generally outside of the recessed region when in its latching position and generally within the recessed region when in its unlatching position. 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,