Patent Number: 051732521
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1A, a typical prior art fuel bundle is illustrated in perspective with the major sections between the top and bottom of the bundle removed. The fuel bundle has a lower tie plate 14, an upper tie plate 16, and a plurality of fuel rods F. Fuel rods F extend vertically the length of the fuel bundle from a position of support on lower tie plate 14 to the upper tie plate 16. Unlike the illustration here shown, the fuel bundle is elongated. Typically, it is in the order of 160 inches long with approximately a 5".times.5" cross-section. The fuel rods within the bundle assembly are flexible in the longitudinal direction. A 9.times.9 array of fuel rods is illustrated. Arrays of 10.times.10, 11.times.11, and 12.times.12 are known. It goes without saying that as the arrays become more dense, fuel rod diameter decreases and longitudinal flexibility increases. Referring to FIG. 1B, an entire spacer is shown having a 9 by 9 matrix. The particular spacer illustrated here is provided with a central aperture for a large water rod. It is the construction of such a spacer to which this invention is directed. In particular, this invention allows the entire spacer of FIG. 1B to be constructed before the insertion of any of the springs between the ferrules. Fuel rods F are constructed by placing fuel pellets within tubular metallic cladding. The metallic cladding is thereafter sealed at both ends, making the fuel rods sealed pressure vessels. During fission, the spacing between discrete fuel rods F is important for efficiency of the nuclear reaction as well as the generation of steam. Furthermore, any vibration on the fuel rods F is undesirable as such vibration can induce either rod abrading or cracking with resultant leakage of the radioactive materials within the fuel rods. To assure the proper spacing of the rods and to prevent them from vibrating, a plurality of spacers S are placed along the length of the fuel bundle F. Typically, 5 to 10 such spacers are utilized with 7 spacers being common. The spacers are placed at individual preselected elevations along the length of the fuel bundle. As shown in FIG. 1B, each spacer S consists of a grid of ferrules. The ferrules which make up spacer S in FIG. 1B are described in commonly-owned, co-pending patent application Ser. No. 07/623,828, entitled "Removable Springs for Ferrule Spacer" filed Dec. 6, 1990, now U.S. Pat. No. 5,078,961 issued Jan. 7, 1992. That specification is incorporated herein for all purposes. As an understanding of the construction and operation of the ferrules and springs described therein is necessary for an understanding of this invention, the following description relates to the ferrules described in that application. Accordingly, it what follows, FIGS. 2A, 2B, 3, 4, and 5 will describe the ferrule spacer of that disclosure. Referring to FIGS. 2A and 2B, a ferrule 20 has an "I" shaped aperture 90. Aperture 90 includes upper and lower rectangular sections 92 which sections form the respective upper and lower bars of the "I" section of the aperture 90. Respective stops 28 form the points against which fuel rods are biased. Fabrication of the spring can be easily understood. Spring metal, typically formed of Inconel, is stamped in the shape shown in FIG. 3. The spring includes upper and lower bars 102 with spring legs 48 and rod contacting portions 46 formed therebetween. This spring is bent about axis 104. Referring to FIG. 4, the respective portions of the spring have been bent about axis 104, and the ends 106 are welded together. Such bending causes upper and lower bars 102 to bend in loop configuration back upon themselves. These members are integral with spring legs 48 and expand the effective length of the spring. The rod contacting portions 46 contact the rods, holding them in place against stops 28 (FIG. 2A). Such a spring with an expanded effective length enables compression at the rod contacting portions 46 to occur without appreciable change in the force required for the compression. Such springs with an expanded effective length can be referred to as "softer" springs. Trapping of the spring into apertures 90 and 92 is shown in FIG. 5. Apertures 90 capture the main spring body. Apertures 92 both above and below aperture 90 capture arms 102. Since arms 102 are a part of the spring, these arms are required to extend into the tab receiving slots 112 defined by confronted aperture portions 92 in each of the ferrules. To construct the spacer, ferrules 20 are confronted with the spring shown in FIG. 4 trapped therebetween. When the ferrules are brought together and fastened together, self-centering trapping of the spring occurs. In the description that follows, the present disclosure will be set forth. Over my prior disclosure, it will be understood that spring insertion from the side can now occur. This has the advantage of allowing substantially complete assembly of the spacer before spring insertion, permitting the removal of damaged springs without spacer disassembly, and finally enabling partial spring insertion to compress the spring and yet maintain the spring out of the path of inserted fuel rods so that spring scratching of inserted fuel rods can be avoided upon fuel bundle assembly. Spring 200, fabricated according to the present invention, is illustrated in FIGS. 6A, 6B and 6C. Spring 200 is fabricated from sheet material of a known alloy, typically Inconel. After stamping, the spring blank is folded about a vertical axis and welded together. As shown in FIG. 6A, upper and lower loops 202 have semi-circular arcs at both ends. Ferrule 205, illustrated in FIGS. 8A and 8B, functions with spring 200 to hold the fuel rods in place. Aperture 210 is cut using circular cutters of different radii, the cuts made by these circular cutters being illustrated in FIG. 8A by dashed lines 215, 216, and 217. Rectangular central area 209 of aperture 210 is cut deepest (furthest) into the ferrule circumference (line 215, FIG. 8A). This allows rod contacting projections 201 in spring 200 (FIG. 6C) to pass between two confronted ferrules 205. Tabs 212 above and below central area 209 prevent lateral movement of spring 200 after it has been inserted into aperture 210. FIGS. 9A, 9B and 9C are cross-sections of a spring 200 being inserted into a ferrule 205, the cross-sections being taken, respectively, along lines A--A, B--B, and C--C in FIG. 8B. The shaded portions of the ferrule and spring in each figure show that when the spring is compressed, its insertion into aperture 210 is unobstructed by any projections near aperture 210. The figures clearly demonstrate that the different elevations of the spring will clear the ferrule slot (aperture 210) so that the spring can be inserted. FIG. 10 is a cross-section taken along line A--A in FIG. 8B, showing a spring 200 inserted into ferrule 205, with no compressive force applied to spring 200. Projections 201 extend into the central area of ferrule 205. FIG. 11 shows the same spring and ferrule, with the addition of fuel rods 230. These compress the spring, which in turn forces the fuel rods against supports 235 (FIG. 8A). FIG. 12 illustrates the use of the present invention in compressing of the partially inserted spring to prevent the scratching of fuel rods during fuel bundle assembly. Referring to FIG. 12, spring 200 has been compressed, partially inserted into aperture 210, and released. Spring 200 tries to open to its unloaded position (see FIG. 10) but is restrained by ferrule tabs 212. There is a gap between the spring and the normal position of a fuel rod. Fuel rods can then be inserted with no spring force present, greatly reducing the number and magnitude of scratches on the fuel rods. Dashed circled 251 in FIG. 12 indicates the limit position of a fuel rod as it is being inserted. Projections 201 have been added to the spring so that the edge of spring 200 when in contact the fuel rod has a reduced tendency to cause scratches. After the fuel bundle is completely assembled, the springs can be pushed fully into the apertures, where they will snap into position, pushing against the fuel rods, and holding them in position. The springs can be pushed inward with a tool which fits into the interstices between the ferrules. For example, a tool with a tapered end can enter the spaces between ferrules force the springs into position. If necessary, an additional procedure can be adopted to ensure complete protection against fuel rod scratches. This procedure involves coating each spacer, along with the partially inserted springs, with a gelatin coating. After the bundle is assembled and the springs locked into place, the gelatin is then dissolved in water. This procedure requires only minor modification to existing fuel rod assembly procedures. The spacers must be given a gelatin coating. Guides may be required to lead the fuel rods into the spacer ferrules, and to support the fuel rod weight, as the weight of the fuel rods may wear the gelatin off the stops and springs. A tool mounted on a long rod must be inserted through the interstitial space between the ferrules to nudge the springs into final position. Finally, the fuel bundles must be rinsed in water to remove the gelatin. It should be understood that the current fuel rod loading sequence, loading one row of rods at a time, starting at the bottom, need not be followed. Accordingly, automated machines having random loading sequences of the spacer matrix can be utilized. An alternative method for accomplishing the same end as the gelatin coating utilizes a low melting point metal alloy which can be removed with hot water. The springs are compressed and dipped in the molten alloy. The alloy hardens while the spring is compressed and holds the spring in the compressed state. The springs and the fuel rods are inserted into the spacers. After the fuel rod assembly is complete, it is rinsed in hot water, melting and removing the alloy and releasing the springs to hold the fuel rods. It will be appreciated that this disclosure will admit of modification. Any combination of a spring trapping aperture between a ferrule pair and a trapped spring capable of being inserted to the aperture of the of the ferrule pair is intended to be broadly covered in this disclosure. It will be understood that those having skill in the art by reviewing the disclosed design can carefully dimension springs and ferrules for the insertion process taught herein. The reader is advised that dimensions must be carefully anticipated in light of the density of the fuel rod matrix and the particular ferrule spacer under consideration.