Patent Number: 047028823
Section: summary

BACKGROUND OF THE INVENTION Cross Reference to Related Applications Reference is hereby made to the following copending applications directed to related subject matter and commonly assigned with the present application: 1. Nuclear Reactor Fuel Assembly With A Removable Top Nozzle, Ser. No. 537,775, filed Sept. 30, 1983; PA0 2. Removable Top Nozzle And Tool For A Nuclear Reactor Fuel Assembly, Ser. No. 564,058, filed Dec. 21, 1983; PA0 3. Improved Method And Apparatus For Removably Mounting A Top Nozzle On A Nuclear Reactor Fuel Assembly, Ser. No. 644,756, filed Aug. 27, 1984; and PA0 4. Nuclear Reactor Fuel Assembly with a Removable Top Nozzle, Ser. No. 644,758, filed Aug. 27, 1984. FIELD OF THE INVENTION The present invention relates generally to a nuclear reactor fuel assembly top nozzle design, and more specifically to a nuclear reactor fuel assembly top nozzle design which simplifies access to the fuel assembly rod bundle and which transmits tensile loads from the top nozzle via grid skirt extensions of a top spacer grid. DESCRIPTION OF THE PRIOR ART In most nuclear reactors the core portion comprises a large number of fuel assemblies which include elongated fuel elements or rods grouped together in bundles and supported by a skeletal framework. The fuel assemblies are generally elongated structures which are supported by and aligned with upper and lower transversely extending core support plates. As a part of the skeletal structure, conventional fuel assemblies generally include a plurality of control rod guide tubes or thimbles held in an organized array by spacer grids axially spaced along and attached to the control rod guide thimbles. The top and bottom nozzles are secured to the control rod guide thimbles on opposite ends thereof, thereby forming an integral fuel assembly. The top and bottom nozzles respectively extend slightly above and below the ends of the fuel rods, capturing the rods therebetween. An example of a reactor core having a fuel assembly of this convention design is described in U.S. Pat. No. 3,235,463 to Sankovich. If, during operation of a fuel assembly in a nuclear reactor, one or more fuel rods develop cracks or other flaws, they must be removed from the reactor. Since, in conventional fuel assemblies, the fuel rods are part of an integral assembly where the guide thimbles are semi-permanently attached to the top and bottom nozzles, it is difficult to detect and remove any failed or defective rods. To gain access to such rods, it is typically necessary to remove the affected fuel assembly from the reactor core and then break the attachments which secure the nozzles to the control rod guide thimbles. In doing so, the fuel assembly is often rendered unfit for further use in a reactor because of the damage done to both the guide thimbles and to the nozzles. In view of the high costs associated with replacing fuel assemblies, efforts have been made to develop reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. Several different types of reconstitutable fuel assemblies have been devised, the majority of which employ threaded arrangements for attaching the top nozzle to the control rod guide thimbles whereby the top nozzle can be removed to gain access to the fuel rods contained in the skeletal assembly. A type of reconstitutable fuel assembly is described by Klaumb et al. in U.S. Pat. Nos. 3,770,583 and 3,814,667 which illustrate a top nozzle of the type having a hold-down device which essentially comprises coil springs coaxially disposed about upwardly extending alignment posts that are threaded through an end plate with fastener nuts located on the underside of the plate. An upper hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the end plate. A radially enlarged shoulder on the upper end of the alignment posts retain the hold-down plate on the posts. In an attempt to improve upon the Klaumb et al. device, Anthony et al. set forth a different threaded joint arrangement as illustrated in U.S. Pat. No. 3,992,259. Another reconstitutable fuel assembly contruction is illustrated and described in U.S. Pat. No. 3,828,868 to Jabsen which employs a threaded arrangement for attaching the top nozzle to the control rod guide thimbles so that the top nozzle can be removed to gain access to the fuel rods. The top nozzle removal method described essentially comprises unscrewing a multitude of threaded connections to release the top nozzle from the guide thimbles and then pulling the top nozzle over the external threads during removal. The sequence is reversed during top nozzle replacement. The reconstitutable fuel assemblies which employ such threaded arrangements not only suffer from high manufacturing costs, complicated designs, difficulty in removing and reattaching the top nozzle, but, in addition, it has been found that after irradiation, the threaded connection may become corroded and inoperable, thus requiring destructive operations to the nozzle and/or the guide thimbles in order to remove the top nozzle which results in costly damage and generally renders the fuel assembly unfit for further use. Further, repeated removal and replacement of the top nozzle from and to the threaded portions of the guide thimbles as well as retightening the threaded connections increases the risk of damaging the threads. Any thread seizure or damage incurred during removal and replacement can ruin the fuel assembly skeleton and require that a new fuel assembly be substituted for it. Further, conventional reconstitutable fuel assembly designs utilize guide thimbles to support both tensive and compressive loads on the fuel assembly as well as for providing guidance for control elements. It is these multiple functions which give rise to arrangements which require complex and intricate components including numerous loose parts designed to permit the top nozzle to be disconnected from and reconnected to the fuel assembly. An additional nonwelded top nozzle attachment arrangement is illustrated and described in U.K. Pat. No. 1,228,610. However, this fuel assembly is equally difficult to disassemble since the upper ends of the guide thimbles are fixed in holes formed in the top nozzle by being expanded after insertion in order to conform to the shape of the holes and annular grooves therein. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a nuclear fuel assembly having an easily detachable and replaceable top nozzle. It is a further object of the present invention to provide a top nozzle design which permits quick and simple access to fuel rods of a fuel assembly without introducing loose parts to the fuel assembly. It is a further object of the present invention to provide a fuel assembly design wherein tensive loads on the top nozzle are transmitted to the fuel assembly via a top spacer grid. It is a still further object of the present invention to provide a fuel assembly design wherein no fastening of guide thimbles to the top nozzle is required. In order to accomplish these as well as other objects, the present invention comprises a fuel assembly having a longitudinally extending control rod guide thimble and a transversely extending top spacer grid. The fuel assembly further comprises a top nozzle and an attaching structure for removably mounting the top nozzle on the fuel assembly. The attaching structure comprises a first means for transferring compressive loads on the fuel assembly through the top nozzle and directly to the control rod guide thimble; and a second means for transferring tensive loads on the fuel assembly through the top nozzle and the transversely extending top spacer grid to the guide thimble. Preferably, the top nozzle includes a transversely extending adapter plate having an aperture therein. An upper end of the guide thimble is sized to clearance fit in the adapter plate aperture. A load collar is affixed to the the guide thimble adjacent the upper end thereof for transferring compressive loads through the adapter plate to the guide thimble. Further, the top spacer grid is preferably provided with an upstanding skirt extension and a latch for securing the skirt extension to the top nozzle. The invention is further directed to a method for assembling and disassembling a top nozzle of a fuel assembly having a guide thimble with an upper end, a top spacer grid, and a plurality of upstanding fuel rods. According to the invention, the method comprises the steps of providing means on the guide thimble for supporting compressive loads on the fuel assembly and providing means for resilently latching the top spacer grid to the top nozzle. In order to assemble the top nozzle on the fuel assembly, it is lowered onto the means for supporting compressive loads and then latched to the top spacer grid with the resilent latching means. Preferably, a load collar attached to the upper end of the guide thimble is used to support the compressive load and the top nozzle includes an adapter plate having an aperture sized for clearance fitting the upper end of the guide thimble. The top nozzle is lowered with the aperture aligned with the upper end of the guide thimble until the adapter plate bears on the load collar. Advantageously, the top spacer grid includes a grid skirt extension having an aperture therein and the top nozzle includes a resilently biased locking pin. During latching the locking pin is held in a deflected position until the grid skirt extension aperture passes into alignment with the locking pin. Thereupon, the locking pin is released into the grid skirt extension aperture. In order to remove the top nozzle, the locking pin is withdrawn from the grid skirt extension aperture and the top nozzle is lifted from the fuel assembly to thereby access the plurality of upstanding fuel rods. Additional objects, advantages and novel features of the invention are set forth in the description which follows and will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages may be realized and attained by means of the invention particularly pointed out in the appended claims.