Patent Number: 054815797
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, a representative example of a fuel assembly is shown generally at 10. The assembly includes a plurality of fuel rods 12 forming a bundle. The rods 12 are connected at their upper ends to an upper tie plate 14 and are supported at their lower ends by a lower tie plate grid, generally designated 16, which forms part of a lower tie plate assembly, generally designated 18. Spacers 20 are arranged at a plurality of vertically spaced locations to maintain lateral spacing of the fuel rods 12 relative to one another. The fuel bundle is disposed within a fuel bundle channel 22 whereby coolant water introduced through the bottom nozzle or inlet opening of the tie plate assembly 18 flows upwardly through a flow volume defined by a peripheral wall 24 of the lower tie plate assembly 18, through the lower tie plate grid 16, and then along and about the fuel rods 12 enclosed by the channel 22. As illustrated in FIG. 1, many of the fuel rods 12 are omitted so that water rods 26 and 28 may be seen. The water rods 26 and 28 have lower end plugs which are seated in the lower tie plate grid 16 by any suitable means, such as a threaded connection (not shown). The fuel rods include both full length rods 12 extending between the lower tie plate grid 16 and the upper tie plate 14, and partial length rods 30. The partial length rods 30 extend from the lower tie plate grid 16 to a spacer 20' shown just below the upper ends of the partial length rods 30 in FIG. 1. The spacers 20 located above the partial length rods are provided with enlarged apertures overlying each of the partial length rods. Likewise the upper tie plate 14 includes enlarged apertures overlying each partial length rod. The partial length rods are set forth in both function and purpose in a patent application entitled Two-Phase Pressure Drop Reduction, BWR Assembly Design filed Apr. 4, 1988, No. 176,975, owned by the common Assignee herein. FIG. 2 illustrates a conventional arrangement of fuel rod bosses 32, as viewed from the underside of the upper tie plate 14. In addition to the fuel rod bosses 32, the upper tie plate also provides a pair of diagonally related bosses 34 and 36 for receiving the end plugs of the water rods 28 and 30, respectively. Referring now to FIG. 3, the lower side of an upper tie plate 14' in accordance with this invention includes conventional bosses 32' for the fuel rods, and an enlarged double boss 38 for receiving the end plugs 68 of a pair of water rods 44, 46 (see FIG. 5, 8B) in accordance with this invention. This double boss 38 includes substantially cylindrical boss portions 48 and 50, associated through holes 48A, 50A, respectively, and a connecting web portion 52. The double boss 38 is connected to the other fuel rod bosses 32' in the upper tie plate by a series of relatively narrow webs 54, but it will be appreciated that the double boss extends downwardly below the plane defined by the lowermost edges of fuel rod bosses 32', as seen for example in FIG. 4A. With specific reference to FIGS. 4A and 4B, the double boss 38 includes a center post 56 extending upwardly from a hole 58 at the center point of the web 52, aligned with and laterally between the water rod boss centers 60 and 62. The center post 56 may be assigned for threaded engagement with the upper tie plate. The thickened web 52 also includes a projection 64 extending forwardly of the double boss 38 and formed with a hole 66 on a vertical center axis which is located forwardly of and parallel to the hole 58 and also parallel to the axes of the water rod boss centers 60, 62. Projection 64 extends axially only a relatively small portion of the height of the double boss 38, as best seen in FIG. 4A. As also seen in FIG. 4A the center post 56 is provided with an enlarged diameter removable cap 66. Turning now to FIGS. 5A and 5B, the upper end of a water rod 46 is shown to include an end plug 68 having an upper end formed with a recessed portion or cut-out 70, the shape of which, in plan, is shown in FIG. 5B and the axial extent of which is defined by horizontal shoulders 72, 74. The end plug 68, upon assembly of the upper tie plate 14, will be received in boss hole 50A, for example, and will project above the double boss 38 for cooperation with a latching bar as described further below. Water rod 44 is identical to rod 46 and similar reference numerals are used for the end plug, cut-out, etc. FIGS. 6A through 6C illustrate a latching bar 76 in accordance with a preferred embodiment of the invention, for use with the double boss 38 and water rods 44, 46. The latching bar 76 may be envisioned as a section cut from the center of a cylinder having an upper part 78 of one (smaller) diameter and a lower part 80 of a second (larger) diameter. Thus, the latching bar 76 is formed by flat sides 82 and 84, and curved ends 86, 88 on one side and similarly curved opposite ends 90, 92 on the opposite side. The lower part 80 may be considered as having oppositely extending lower ends 88 and 92. The bar is formed with a vertically extending central through bore 94, having an upper portion 96 with a first relatively larger diameter, and a lower portion 98 with a second, relatively smaller diameter (see FIG. 6C). The latching bar 76 is also formed with a forwardly extending projection 100 (similar to the projection 64) formed with a hole 102, the center line or axis of which is parallel to and forward of the center axis of bore 94. Hole 102 is also located midway between the curved ends 86, 90. The lower surface 104 of the latching bar 76 is provided with a centrally located, elongated rib 106, extending between flat sides 82, 84 and across the center hole 94. Turning now to FIG. 7A, a locking pin 108 is shown which includes a main, cylindrical body portion 110, with an annular groove 112 located between the ends of the pin. The lower end of the pin has an enlarged diameter portion 114, tapering to a rounded point 116. The upper end of the pin has an enlarged, removable head or cap 118. The annular groove 112 is sized to receive a split compression ring 120, shown in FIG. 7B. In a normal assembly procedure, the center post 56 is secured in the hole 58 in the tie plate 14' and the latching bar 76 is then located on the center post, such that the latter extends upwardly through the hole 94. The cap 66 is then reattached to the post 56, thereby securing the latching bar 76 to the tie plate 14'. Locking pin 108 is seated within the hole 102, and it should be noted that cap 118 must be removed so that the pin 108 can be inserted through the hole 102 from below, with enlarged lower end 114 serving as a stop. Once the upper end of the cap protrudes above the upper surface of the latching bar, with ring 120 located as shown in FIG. 8A, cap 118 may be refastened to the pin 108. To install the upper tie plate 14' and to lock the latter to the water rods 44, 46, and with reference also to FIGS. 8A, 8B and 9A, 9B, the upper tie plate 14' is lowered onto the fuel rod bundle including the pair of water rods 44, 46 with respective cut-outs 70 facing each other as best seen in FIG. 8A. A pair of coil springs 122, 124 are seated on horizontal shoulders 123, 125, respectively, at the interface between end plugs 68 and water rods 44, 46. The upper tie plate 14' is moved downwardly over the fuel rods and water rods 44, 46, against the bias of springs 122, 124, and with the end plugs 68 extending upwardly through the boss holes 48A and 50A. Note the angular orientation of the latch bar 76 (in FIGS. 8A and 8B) which is an unlocked assembly position. With reference now to FIGS. 9A and 9B, it will be appreciated that the latch bar 76 can be rotated from the orientation shown in FIGS. 8A and 8B to the orientation shown in FIGS. 9A and 9B such that the lower ends 88, 92 of the latch bar 76 lie within the cut-outs 70 of the end plugs 68. When so located, and upon release of downward pressure on the upper tie plate 14', the coil springs 122, 124 will exert an upward bias on the upper tie plate 14', at the same time biasing the enlarged lower end portion 80 of the latch bar 76 into engagement with shoulders 72', 74' of the cut-outs 70. The upper tie plate 14' is thus secured to the water rods 44, 46 which, in turn, are rigidly secured to the lower tie plate. The locking pin 108 is then pushed downwardly into the aligned hole 102, radially compressing the ring 120 until the ring passes completely through the hole 102, and then springs outwardly below the projection 100 as illustrated in FIG. 9A. Enlarged lower end 114 is now seated within hole 66 in the projection 64. Ring 120 thus prevents accidental detachment of the pin 108, while the pin 108 itself prevents rotation of the locking bar 76 relative to the boss 38 and thus prevents accidental unlocking of the water rods 44, 46 vis-a-vis the upper tie plate 14'. An important feature of the invention is the inclusion of the elongated rib 106 along the lower surface 104 of the latching bar 76. It will be appreciated that the latching bar is thus able to rock back and forth, to the left and to the right as viewed m FIG. 9A and 9B, as it rests on the upper surface of the double boss 38 of the upper tie plate 14'. This rocking action is also facilitated by reason of the fact that the center post 56 has a smaller diameter than bores 96, 98 through the latch bar 76. As a result, radiation growth (in the axial direction) of the respective water rods 44, 46 call occur with equal distribution of the lifting load between the two water rods. In other words, the rocking action of the latch bar 76 distributes the lifting load equally between the two water rods 44, 46 and compensates for unequal initial lengths and unequal changes in length caused by irradiation growth during service. With reference now to FIGS. 10A through 10C, a modified latch bar 106 is shown which is similar in all respects to the latch bar 76, with the few differences noted below To avoid unnecessary duplicative description, only those few differences will be described in detail. The latch bar includes an extended upper portion 128, with a forwardly extending projection 130 at the upper end thereof, including through hole 132. Lower portion 134 is formed with a forwardly extending projection 136 and associated through hole 138, in vertical alignment with through hole 132. The relatively short axial extents of projection 130 and 136 leaves a relatively substantial axial space therebetween, the purpose for which will be described below. With reference to FIGS. 11A-11C, a locking pin 140 for use with latch 126 is shown to include an enlarged head 142 and a shank 144, partially threaded at 146. A threaded nut or ting 148 is adapted for threaded attachment to the shank. The locking pin 140 is initially inserted through the hole 132, at which point coil spring 150 is slipped over the shank 144 followed by threaded attachment of the ring 148. The lower end of shank 144 is then passed through the hole 138 of projection 136 in this way, the locking pin Coy reason of spring 150 acting between projection 130 and ring 148) is biased downwardly. In this alternative embodiment, and with reference also to FIGS. 12A and 12B, after the latch bar 106 is rotated into the locked position against the biasing forces of springs 122, 124, and with the locking pin 140 held in a raised position, the pin is released and allowed to move downwardly into the aligned hole 66 in the projection 64 of the double boss 38. With the pin 140 resiliently urged into the double boss 38, any relative rotation between latching bar 126 and boss 38 is prevented while, at the same time, the biasing force of spring 150 prevents accidental separation of the pin 140. With reference now to FIGS. 13A through 13C, a third alternative latch bar construction 156 is disclosed wherein, in place of a forward projection (of the type shown at 100 in FIG. 6A and at 130 and 136 in FIG. 10A), a pair of horizontal stops 158 and 160 extend laterally from the flat face 162 of a lower enlarged portion 164 of the latch bar. In this embodiment, a spring finger locking pin 166 (FIG. 14) may be inserted into an elongated hole (not shown) in the projection 168 formed in the modified double boss 170 of the upper tie plate 172 This is done, of course, only after the latch bar 156 is rotated to the locking position shown in FIGS. 15A and 15B. Then, when the locking pin 166 is inserted into the upper tie plate 172, it will extend vertically between the stops 158 and 160, which prevents any significant rotation and thus unlocking of the latch bar 156. Locking pin 166 is formed with resilient spring fingers 174, 176 with enlarged ends 178, 180 which protrude from the projection 168 when fully inserted (see FIG. 15A), thereby preventing accidental removal of the pin by a simple lifting or sliding movement. In other words, spring fingers 174, 176 must be squeezed together before the enlarged ends 178, 180 can pass through the projection 168 in a disassembly direction. With reference now to FIGS. 16A-16D, another latch bar is shown at 182. The bar 182 includes an upstanding center portion 184, counterbored at 186 to a shoulder 188, with the bore continuing at a reduced diameter through the remaining thickness of the bar. A pair of laterally extending, generally horseshoe-shaped hooks 190, 192 extend laterally away from the center portion 184, with the hooks 190, 192 having rounded water rod engagement surfaces facing in opposite directions. With specific reference to FIG. 16A, it may be seen that the water rods 194, 196 are formed with annular grooves or cut-outs 196, 198 which are simply reduced diameter portions with beveled entry surfaces 200, 202, respectively. With the latch bar in place on the upper tie plate 204, the former is rotated into the locking position shown in FIG. 16A, and bolt 206 and associated coil spring 208 are employed to secure the latch bar to the tie plate. Insofar as the bolt 206 is threaded into the tie plate 204, and not the latch bar itself, spring biased axial movement of the bar relative to the bolt 204 is permitted to facilitate rotation of the latch bar 182 into the locked position. FIG. 16D illustrates a variation of the latch bar 182. Specifically, the latch bar 208 is formed with squared water tube engagement surfaces within oppositely facing hooks 210, 212 for use with water rods having a square cross-section 214 in the cut-out or grooved portions. This arrangement provides for even greater locking surface area as between the latch bar 208 and the associated water rods. This arrangement also provides for locked angular orientation of the water rods. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.