Patent Number: 047724463
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. "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. 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. 3. "Nuclear Reactor Fuel Assembly With a Removable Top Nozzle" by John M. Shallenberger et al., assigned U.S. Ser. No. 644,758 and filed Aug. 27, 1984, 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, now U.S. Pat. No. 4,684,500. 5. "Burnable Absorber Rod Releasable Latching Structure" by Robert K. Gjertsen, assigned U.S. Ser. No. 807,142 and filed Dec. 10, 1985, now U.S. Pat. No. 4,684,499. BACKGROUND OF THE INVENTION The present invention relates to an apparatus for insertion and removal of releasable burnable absorber rods from the adapter plate of the top nozzle of a nuclear reactor fuel assembly. 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 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. to Rose (3,361,857) and to Wood (3,510,398). 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 illustated 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. A burnable absorber assembly in which the burnable absorber rods have a releasable latching structure is illustrated and described in the fifth patent application cross-referenced above. The advantage of the releasable latching structure is that the configuration of the burnable absorber rods can be specified at the latest possible time because the assembly does not have to include the burnable absorber rods until it is installed. Thus, the nuclear reload design can be fine tuned based on the latest reactor operations input. The ultimate absorber assembly 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. Consequently, a need exists for a device that can be used to insert and remove the burnable absorber rods and thimble plugs from the burnable absorber assembly. SUMMARY OF THE INVENTION The present invention provides apparatus for releasably engaging an elongated member, such as a burnable absorber rod or a thimble plug, that is releasably connected to the top nozzle of a nuclear reactor fuel assembly. The top nozzle has an adapter plate disposed at its lower end having at least one passageway therethrough through which the elongated member is disposed. The elongated member has a releasable latching structure at one end that is able to be engaged by the apparatus having at least one latching member movable between a latched position in which the latching member is able to engage said adapter plate and secure the absorber rod in a stationary relationship with respect to the adapter plate and an unlatched position in which the latching member is able to disengage from said adapter plate so that the elongated member be removed from the fuel assembly. The apparatus comprises a hollow releasing member for moving the latching member of the latching structure between its latched position and its unlatched position, an engaging member connected to the releasing member and extending downwardly through the hollow portion of the releasing member, and an actuating member extending downwardly through the hollow portion of the releasing member and coacting with the engaging member to releasably engage the latching structure. More particularly, the present invention enables a plurality of such elongated members to be inserted or removed from a nuclear reactor fuel assembly and comprises a frame and first and second plates disposed within the frame, the second plate disposed below and spaced apart from the first plate. At least one of the first and second plates is capable of vertical movement relative to the other. The apparatus includes means for moving the frame toward and away from the fuel assembly and means for moving the first and second plates within the frame toward and away from the adapter plate of the top nozzle of the fuel assembly. The apparatus thus includes means for varying the vertical distance between the first and second plates between a first distance and a decreased second distance. Means associated with the first and second plates maintains the first and second plates in a position whereby the vertical distance between said plates is the second distance. Further, the apparatus includes a hollow releasing member extending downwardly from the second plate for moving the latching member of the latching structure between its latched position and its unlatched position, an engaging member connected to the releasing member and extending downwardly from the second plate through the hollow portion of the releasing member, and an actuating member extending downwardly from the first plate through the hollow portion of the releasing member and coacting with the engaging member to releasably engage the latching structure when the vertical distance between the first and second plates is the second distance. The present invention coacts with the releasable latching structure that secures the burnable absorber rods and thimble plugs to the top nozzle adapter plate of a nuclear reactor fuel assembly so that the final arrangement of the absorber rods and thimble plugs can be specified at the reactor site. Thus, the latest reactor operating information can be considered when determining the arrangement of the absorber rods and thimble plugs in the design of the fuel assembly. The approach avoids the need for off site manufacturing of the final absorber rod and thimble plug assemblies. Instead, an inventory of individual absorber rods and thimble plugs can be sent to the reactor site prior to fueling the reactor. Once the design of the fuel assembly is specified, the configuration of the absorber rods and thimble plugs can be inserted in the fuel assembly. Later, the spent absorber rods can easily be released from the top nozzle adapter plate and replaced. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon 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.