Patent Number: 047298677
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 to FIGS. 1 to 3, there is shown a nuclear fuel assembly, generally designated 10 for a boiling water nuclear power reactor (BWR), with which the spring retainer apparatus (FIGS. 11 to 17) of the present invention can be employed, as will be described later on. 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 of 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, as seen in FIGS. 1, 2 and 4 and generally designated 26, extends axially through the outer channel 12 so as to provide an open 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 water cross 26 has a plurality of four radial panels 32 composed by a plurality of four, elongated, generally L-shaped, metal angles or sheet members 34 that extend generally along the entire length of the channel 12. The sheet members 34 of each panel 32 are interconnected and spaced apart by a series of elements in the form of dimples 36 formed therein and extending therebetween. The dimples 36 are provided in opposing pairs that contact each other along the lengths of the sheet members 34 to maintain the facing portions of the members in a proper spaced-apart relationship. The pairs of contacting dimples 36 are connected together such as by welding to ensure that the spacing between the sheet members 34 forming the panels 32 of the central water cross 26 is accurately maintained. 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 panels 32 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. Further, the inner ends of the panels together with the outer ends thereof define the inner central cruciform channel 28 which extends the axial length of the hollow water cross 26. Also, the water cross 26 has a lower flow inlet end 38 and an opposite upper flow outlet end 40 which each communicates with the inner channel 28 for providing subcoolant flow therethrough. Disposed within the channel 12 us a bundle of fuel rods 42 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 42 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 44 and a lower tie plate 46. The fuel rods 42 in each mini-bundle are connected to the upper and lower tie plates 44,46 and together therewith comprise a separate fuel rod subassembly 48 within each of the compartments 30 of the channel 12. A plurality of spacers or grids 50 axially spaced along the fuel rods 42 of each fuel rod subassembly 48 are composed of interleaved inner straps 52 and an outer strap 54 which maintain the fuel rods in their laterally spaced relationships. The lower and upper tie plates 44,46 of the respective fuel rod subassemblies 48 have flow openings 56 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 48 in the respective separate compartments 30 of the fuel assembly 10 through a plurality of openings 58 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 58 serves to equalize the hydraulic pressure between the four separate compartments 30, thereby minimizing the possibility of thermal hydrodynamic instability between the separate fuel rod subassemblies 48. Apparatus for Loading Fuel Rods into Grids In the normal manner of loading fuel rods 42 through the fuel assembly grid 50, springs 60 and dimples 62 defined by the inner straps 52 and normally extending into the hollow cells 64 formed by the interleaved straps, as seen in FIGS. 6 and 7, will rub against the fuel rod outer surfaces and cause scratching thereof. Not only is the external appearance of the fuel rods 42 adversely affected, more importantly accelerated corrosion is likely to occur at the scratch sites and debris formed by material from the scratch is released into the coolant flow through the fuel assembly 10. Referring now to FIGS. 8 to 20 in addition to FIGS. 6 and 7, in accordance with the present invention, a spring retainer apparatus, generally designated 66, is provided for facilitating the loading of the fuel rods 42 into the cells 64 of the fuel assembly grid 50 in a manner which minimizes marring or scratching of the outer surfaces of the fuel rods. It will be noted that some of the inner straps 52A are disposed in pairs so as to form the springs 60 in pairs thereof which are positioned in back-to-back relationships between adjacent ones of cells 64. The springs 60 in each pair thereof are configured to normally assume expanded positions, as seen in FIG. 6, in which they are displaced away from one another to engage the fuel rods 42 when received in the respective cells 64. However, using the apparatus 66 of the present invention, the springs 60 in each pair thereof can be deflected to retracted positions, as seen in FIGS. 8 and 10, in which they are displaced toward one another to provide the clearance necessary to allow loading of the fuel rods 42 in the respective cells 64 without engaging the springs 60. More particularly, as seen in FIGS. 8 to 17, the spring retainer apparatus 66 includes a pair of first and second spring retainer assemblies 68A, 68B which are substantially identical. Each of the spring retainer assemblies 68A, 68B includes a pair of elongated holder bars 70 and a handle bar 72 interconnecting the holder bars together at the same one of the opposite ends thereof so that the holder bars 70 are concurrently extended along and aligned with a pair of the straps 52A of the grid 50 which define the pairs of springs 60. The sizes of the cross-sectional dimensions of the holder bars 70 are designed to allow the bars to be extended between and spaced from positions occupied by fuel rods 42 when received in the cells 64 of the grid 50 so that neither the bars 70 nor the springs 60 held in retracted positions by the assemblies 68 will engage nor interfere with the fuel rods 42 during loading thereof. In addition, each of the holder bars 70 of the spring retainer assemblies 68A, 68B supports a number of depending members 74. The number of such members 74 corresponds to the number of pairs of springs 60 defined by the pair of straps 52A aligned with the specific holder bar 70. In the illustrated embodiment, both the members 74 on each bar 70 and the springs 60 aligned with each bar are four in number. Each of the depending members 74 has a terminal end 76 configured to engage and retain the springs 60 of one pair thereof in their retracted positions when the respective holder bar 70 supporting the member 74 is aligned with and moved toward the pair of straps 52A aligned with the holder bar 70. Preferably, each of the members 74 supported by a respective one holder bar 70 is rigidly connected to and extends from the bar in a generally parallel relationship with respect to the other members 74 supported by the bar. Further, each member 74 is an elongated post with the terminal end 76 of the member being a bifurcated end on the post. The bifurcated end 76 on each of the posts 74 defines two spaced pairs of spaced apart fingers 78,80 adapted to receive the pair of springs 60 therebetween and engage the spring pair at two displaced locations therealong, as depicted in FIG. 10, for retaining them in their retracted positions. Many configurations are possible for defining the bifurcated ends 76 of the posts 74, the configurations seen in FIGS. 19 and 20 being merely one example. The important feature, whatever the particular configuration or structure of the fingers 78,80 might be, is that the bifurcated terminal end of each post 74 should define a pocket 82 adapted to receive the pair of springs 60 therein and retain the springs in their retracted positions, as represented in FIG. 20, and a convergently-tapered entrance 84 to the pocket 82 for facilitating insertion of the springs 60 when in their retracted positions into the pocket. In the preferred embodiment seen in FIGS. 19 and 20, the tapered entrance 84 is of sufficient size (i.e., length and width) to cause deflection of the springs 60 from their normal expanded positions to their retracted positions, both of which are depicted in FIG. 20, as the post 74 is moved toward and into contact with the springs 60 at its bifurcated end 76. However, in an alternative embodiment seen in FIG. 22, the tapered entrance 84' to the pocket 82' defined on each post 74' is insufficient in size to cause such deflection of the springs 60. So here, a fixture 86 of dummy fuel rods is first inserted into the cells 64 of the grid 50 to deflect the springs 60 to their retracted positions and then bifurcated terminal ends 76' of the posts 74' are inserted on the retracted springs to retain them in such positions. Finally, the holder bars 70 of the respective first and second spring retainer assemblies 68A, 68B have mating means permitting the assemblies to be superimposed one on top of the other, as seen in FIGS. 8 to 11, in criss-cross fashion and interconnected in alignment with all of the pairs of the springs 60 defined by the straps 52A of the grid 50. Particularly, the mating means on the holder bars 70 are in the form of aligned notches 88,90 defined in the respective holder bars. It will be noted that the notches 88 of the assembly 68A open downwardly, whereas the notches 90 of assembly 68B open upwardly. It will be observed that some of the posts 74 on the holder bars 70 are slightly larger than other of the posts. This is because the sets of the four radial springs 60 in the different quadrants of the grid 50, as seen in FIGS. 7 and 9, are disposed at different levels within the grid due to the manner in which the springs 60 are attached to other of the inner straps 52 which define the dimples 62. 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.