Patent Number: 046845011
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly in FIGS. 1 and 3, there is shown a nuclear fuel assembly, generally designated 10 for a boiling water nuclear power reactor (BWR), in which the improvement of the present invention is incorporated. The fuel assembly 10 includes an elongated outer tubular flow channel 12 that extends along substantially the entire length of the fuel assembly 10 and interconnects an upper support fixture or top nozzle 14 with a lower base or bottom nozzle 16. The bottom nozzle 16 which serves as an inlet for coolant flow into the outer channel 12 of the fuel assembly 10 includes a plurality of legs 18 for guiding the bottom nozzle 16 and the fuel assembly 10 into a reactor core support plate (not shown) or into fuel storage racks, for example in a spent fuel pool. The outer flow channel 12 generally of rectangular cross-section is made up of four interconnected vertical walls 20 each being displaced about ninety degrees one from the next. Formed in a spaced apart relationship in, and extending in a vertical row at a central location along, the inner surface of each wall 20 of the outer flow channel 12, is a plurality of structural ribs 22. The outer flow channel 12, and thus the ribs 22 formed therein, are preferably formed from a metal material, such as an alloy of zirconium, commonly referred to as Zircaloy. Above the upper ends of the structural ribs 22, a plurality of upwardly-extending attachment studs 24 fixed on the walls 20 of the outer flow channel 12 are used to interconnect the top nozzle 14 to the channel 12. For improving neutron moderation and economy, a hollow water cross 26 extends axially through the outer channel 12 so as to provide an open cruciform inner channel 28 for subcooled moderator flow through the fuel assembly 10 and to divide the fuel assembly into four, separate, elongated compartments 30. The hollow water cross 26 is mounted to the angularly-displaced walls 20 of the outer channel 12. Preferably, the outer, elongated lateral ends of the water cross 26 are connected such as by welding to the structural ribs 22 along the lengths thereof in order to securely retain the water cross 26 in its desired central position within the fuel assembly 10. Also, the water cross 26 has a lower flow inlet end 32 and an opposite upper flow outlet end 34 which each communicate with the inner channel 28 for providing subcoolant flow therethrough. Disposed within the channel 12 is a bundle of fuel rods 36 which, in the illustrated embodiment, number sixty-four and form an 8.times.8 array. The fuel rod bundle is, in turn, separated into four mini-bundles thereof by the water cross 26. The fuel rods 36 of each mini-bundle, such being sixteen in number in a 4.times.4 array, extend in laterally spaced apart relationship between an upper tie plate 38 and a lower tie plate 40. The fuel rods in each mini-bundle are connected to the upper and lower tie plates 38,40 and together therewith comprise a separate fuel rod subassembly 42 within each of the compartments 30 of the channel 12. A plurality of grids or spacers 44 axially spaced along the fuel rods 36 of each fuel rod subassembly 42 maintain the fuel rods in their laterally spaced relationships. The lower and upper tie plates 38,40 of the respective fuel rod subassemblies 42 have flow openings 46 defined therethrough for allowing the flow of the coolant fluid into and from the separate fuel rod subassemblies. Also, coolant flow paths provide flow communication between the fuel rod subassemblies 42 in the respective separate compartments 30 of the fuel assembly 10 through a plurality of openings 48 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 48 serves to equalize the hydraulic pressure betweent he four separate compartments 30, thereby minimizing the possibility of thermal hydrodynamic instability between the separate fuel rod subassemblies 42. Compliant Inserts In Upper Tie Plate Holes Turning now to FIGS. 4 to 9, there is shown several embodiments of a combination of features of the present invention for supporting the upper ends of the fuel rods 36 so as to avoid binding and axially loading thereof which heretofore has frequently resulted in bowing of the fuel rods. As depicted in FIG. 4, each of the fuel rods 36 is one of two types: the tie rod 36a or the standard fuel rod 36b. Each has a pair of end plugs 50a,50b (only the upper one being shown) sealing opposite ends thereof. The upper end plugs 50a,50b of the fuel rods 36a,36b have respective extension members 52a,52b thereon which extends axially outward from the end plugs and have respective diameters less than that of each fuel rod. Also, as seen in FIG. 4, the upper tie plate 38 disposed adjacent the upper ends of the fuel rods 36a,36b has a plurality of holes 54 defined by endless sidewalls 56 formed therethrough between opposite upper and lower sides 58 of the upper tie plate. The holes 54 are arranged in an array which matches that of the fuel rods 36. The extension member 52a of the tie rod 36a is threaded and fastened by a nut 60 so as to limit its movement within one of the holes 54 in the upper tie plate 38, whereas the extension member 52b of the standard fuel rod 36b is slidably received within another one of the holes 54 in the tie plate. In the case of each fuel rod 36, there is a compressed coil spring 61 disposed above the extension member 52 and extending between the respective end plug 50 and the upper tie plate 38. The springs 61 force the tie plate 38 upwardly against the nut 60 on the tie rod 36a. The holes 54 are substantially larger in diameter than the respective end plug extension members 52a,52b of the fuel rods 36a,36b for accommodating insertion of a compliant insert, generally designated 62, in each of the holes 54 of the upper tie plate 38. There are three different embodiments of the insert 62 disclosed, each being modified slightly from the other. However, basically, all embodiments of the compliant insert 62 function to engage both the tie plate 38 and the respective end plug extension member 52 so as to yieldably support the extension member within the given hole 54 in spaced relationship from the hole sidewall 56. Also, each embodiment of the compliant insert 62 includes a plurality of spring members 64. Turning initially to the first embodiment depicted in FIGS. 4 and 5, it will be seen that the spring members 64a making up the compliant insert 62a are separate from one another, being angularly spaced apart approximately 120 degrees about the hole 54. Each of the spring members 64a is made from a strip of resiliently flexible material which is also creep resistant, such as Inconel. Each spring member 64a has opposite upper and lower end portions 66a and a middle portion 68a interconnecting the opposite end portions. The opposite end portions 66a in the form of tabs are disposed along the opposite sides 58 of the tie plate 38 adjacent to the respective tie plate hole 54. In addition, means are provided for securing the opposite end portions 66a of each spring member 64a to the respective sides 58 of the tie plate 38. In the first embodiment of FIG. 4, such means take the form of tackwelds 70 which interconnect the spring member end portions 66a to the tie plate 38. The elongated middle portion 68a of each spring member 64a extends through the tie plate hole 54 between the hole sidewall 56 and the respective one of extension members 52a,52b. Resilient means is defined on each spring member middle portion 68a for engaging and positioning the respective extension member 52a,52b in spaced relationship from the hole sidewall 56. In the first embodiment of FIG. 4, such resilient means is in the form of a single inwardly-protruding dimple 72 formed on each spring member 64a. In view of the above-described arrangement, the spring members 64a of the insert 62a will yield and prevent binding of the respective end plug extension member 52a,52b upon tilting of the tie plate 38 relative to the plug extension member. They will also support the extension member so as to eliminate lateral vibrations thereof, while at the same time allow the end plug extension member to freely slide relative to the upper tie plate. The alternative embodiment of the compliant insert 62b seen in FIGS. 6 and 7 is generally similar to the first embodiment of FIGS. 4 and 5. Therefore, only the difference between the two will be described. The opposite end portions or tabs 66b of the separate spring members 64b are secured to the respective sides 58 of the upper tie plate 38 by means of cutouts or indentations 74 formed therein into which turned ends 76 on the tabs 66b extend. Also, angularly-spaced recesses 78 are formed in the hole sidewall 56 in which the middle portions 68b of the spring members 64b are seated to maintain the circumferential positioning of the spring members in the hole 54. Further, the resilient means of the insert 62b is a combination of the single inwardly-protruding dimple 72 formed on one spring member middle portion 68b and a pair of tandemly-arranged inwardly-protruding dimples 80 formed on one of the other spring member middle portions 68b. The pair of bi-level dimples 80 provide more resistance to fuel rod tilting if that should be desired in a particular case. In another alternative embodiment of the compliant insert 62c seen in FIGS. 8 and 9, the spring members 64c are integrally connected to one another by spaced apart upper and lower ring portions 82. Here, the opposite end portions or tabs 66c are connected to the respective ring portions and circumferentially displaced from the middle portions 68c of the spring members 64c. The tabs 66c are also bendable between axially-extending releasing positions, seen in dotted outline form, and radially-extending securing positions, seen in full line form, in FIG. 9. The integral structure of the spring members 64c allows the insert 62c to anchor itself in the tie plate hole 54. It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.