Patent Number: 046648747
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views. 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 FIG. 1, there is shown an upper end of a reconstitutable fuel assembly, being generally designated by the numeral 10, on which the reusable locking tube insertion and removal fixture of the present invention, generally indicated at 12, is employed. Basically, the fuel assembly 10, being of conventional construction, includes an array of fuel rods 14 held in spaced relationship to one another by a number of transverse support grids 16 (only one being shown) spaced along the fuel assembly length. Each fuel rod 14 includes nuclear fuel pellets (not shown) and is sealed at its opposite ends. The fuel pellets composed of fissile material are responsible for creating the reactive power of the nuclear reactor core in which the assembly 10 is placed. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work. The reconstitutable fuel assembly 10 also includes a number of longitudinally extending guide tubes or thimbles 18 along which the grids 16 are spaced and to which they are attached. The opposite ends of the guide thimbles 18 extend a short distance past the opposite ends of the fuel rods 14 and are attached respectively to a bottom nozzle (not shown) and a top nozzle 20. To control the fission process, a number of control rods (not shown) are reciprocally movable in the guide thimbles 18 located a predetermined positions in the fuel assembly 10. Specifically, a rod cluster control mechanism (not shown) interconnected to the control rods and associated with the top nozzle 20 is operable to move the control rods vertically in the guide thimbles 18 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Top Nozzle Attaching Structure As illustrated in FIG. 1, the top nozzle 20 comprises a housing 22 having a lower adapter plate 24 surrounded by four interconnected, upstanding side walls 26 with raised sets of pads 28,30 (only one pad in each set being shown) located respectively at pairs of diagonal corners 32,34 formed by the side walls 26. The control rod guide thimbles 18 have their uppermost end portions 36 coaxially positioned within the control rod passageways 38 formed through the adapter plate 24 of the top nozzle 20. For gaining access to the fuel rods 14, the adapter plate 24 of the top nozzle 20 is removably connected to the upper end portions 36 of the guide thimbles 18 by an attaching structure, generally designated 40. As partly seen in FIGS. 1, 3 and 4, and better seen in FIGS. 5 to 8, the top nozzle attaching structure 40 of the reconstitutable fuel assembly 10 includes a plurality of outer sockets 42 (only one being shown) defined in the top nozzle adapter plate 24 by the plurality of passageways 38 (also only one being shown) which each contains an annular circumferential groove 44 (only one being shown), a plurality of inner sockets 46 (only one being shown) defined on the upper end portions 36 of the guide thimbles 18, and a plurality of removable reusable locking tubes 48 (only one being shown) inserted in the inner sockets 46 to maintain them in locking engagement with the outer sockets 42. Each inner socket 46 is defined by an annular circumferential bulge 50 on the hollow upper end portion 36 of one guide thimble 18 only a short distance below its upper edge 52. A plurality of elongated axial slots 53 are formed in the upper end portion 36 of each guide thimble 18 to permit inward elastic collapse of the slotted end portion to a compressed position so as to allow the annular bulge 50 thereon to be inserted within and removed from the annular groove 44 via the adapter plate passageway 38. The annular bulge 50 seats in the annular groove 44 when the guide thimble upper end portion 36 is inserted in the adapter plate passageway 38 and has assumed an expanded position. In such manner, the inner socket 46 of each guide thimble 18 is inserted into and withdrawn from locking engagement with one of the outer sockets 42 of the adapter plate 24. Finally, each reusable locking tube 48 is inserted from above the top nozzle 20 into its respective locking position in the hollow upper end portion 36 of one guide thimble 18 forming one inner socket 46. When the locking tube 48 is inserted in its locking position, it retains the bulge 50 of the inner socket 46 in the latter's expanded locking engagement with the annular groove 44 and prevents the inner socket 46 from being moved to its compressed releasing position in which it could be withdrawn from the outer socket 42. In such manner, each locking tube 48 maintains its respective one inner socket 46 in locking engagement with the outer socket 42, and thereby the attachment of the top nozzle 20 on the upper end portion 36 of each guide thimble 18. Each locking tube 48 has at least a pair of small dimples 60 (FIGS. 5 to 8) preformed on the exterior thereof during manufacture and thus prior to insertion of the tube 48 to its locking position. The dimples 60 are so preformed by any suitable method, such as be die forming, by being coined or by spot welding, and so configured to have a generally pyramidal shape such that the metal forming the dimples substantially resists yielding and dimensional change regardless of the number of insertions and withdrawals of the tube 48 into and from the locking position. Thus, the whole locking tube per se yields, rather than the dimples 60, and then springs back to its original shape. Also, the dimples 60 are located along the exterior of the tube 48 and have outer tips diametrically displaced from one another across the tube at a distance greater than the inside diameter of the guide thimble upper edge 52 such that when the tube is inserted to the locking position, as seen in FIGS. 5 to 7, the dimples extend into the annular bulge 50 in the guide thimble upper end portion 36 which in turn fits into the annular groove 44 defined in the passageway 38 of the adapter plate 24. In such manner, the dimples 60 provide a positive interference fit with the guide thimble upper end portion 36 above the annular bulge 50 therein and with the upper portion of the adapter plate passageway 38 which prevents inadvertent withdrawal of the locking tube 48 from the locking position. Fixture for Inserting and Removing Reusable Locking Tubes For effectuating inspection, removal, replacement and/or rearrangement of fuel rods 14 contained in the reconstitutable fuel assembly 10, the assembly must be removed from the reactor core and lowered into a work station (not shown) by means of a standard fuel assembly handling tool (not shown). In the work station, the fuel assembly is submerged in coolant and thus maintenance operations are performed by manipulation of remotely-controlled submersible equipment. One component of such equipment is the fixture 12 of the present invention for removing and reinserting the reusable locking tubes 48 as the first and third steps in removing and replacing the top nozzle 20. Another component of such equipment is the fixture (not shown) forming the invention illustrated and described in the third patent application cross-referenced above. After the locking tubes have been removed in the first step by the fixture of the present invention, the fixture of the cross-referenced application is used for removing the top nozzle 20 of the reconstitutable fuel assembly 10 in the second step. Then after the locking tubes have been reinserted in the third step, the latter fixture is again used for replacing the top nozzle 20 back on the guide thimbles 18. Referring to FIGS. 1 to 8, there is shown the fixture 12 useful in inserting and removing the reusable locking tubes 48 into and from their locking positions within the top nozzle 20. The fixture 12 basically includes locking tube engaging means, generally designated 62, and actuating means, generally indicated 64. The locking tube engaging means 62 includes a lower traveling plate 66 and a plurality of hollow flexure tubes 68 attached to and projecting downward from the lower plate 66. The lower plate 66 has a generally square configuration and a plurality of openings 70 arranged in a pattern which matches that of the plurality of guide thimbles 18 and adapter plate passageways 38 of the fuel assembly 10. Each flexure tube 68 is anchored in one of the openings 70 defined through the lower plate 66 and has a lower axially segmented sleeve portion 72 which terminates in a lower segmented rim 74. An annular exterior shoulder 76 on the flexure tube 68 above its lower segmented portion 72 provides a stop such that when the shoulder 76 is disposed on an upper surface 78 of the adapter plate 24, the length of the lower sleeve portion 72 of the flexure tube 68 compared to that of the locking tube 48 is such that its segmented rim 74 will be positioned just below a lower end 80 of the locking tube 48. The segmented sleeve portion 72 is normally in a circumferentially collapsed condition in which its segmented rim 74 has an outside diameter less than the inside diameter of the reusable locking tube 48. Thus, the segmented rim 74 is normally contracted or collapsed inwardly toward the central axis of the flexure tube 68 sufficiently to allow the flexure tube 68 to be inserted into and withdrawn from the locking tube 48. The actuating means 64 includes an upper traveling plate 82 and a plurality of elongated solid actuating rods 84 attached to and projecting downward from the upper plate 82. The upper plate 82 has a generally square configuration matching that of the lower plate 66 and a plurality of taped holes 86 arranged in a pattern which matches that of the openings 70 in the lower plate 66. Each actuating rod 84 is anchored in one of the tapped holes 86 in the upper plate and has a shaft portion 86 which extends through one of the flexure tubes 68 and terminates in a lower enlarged nose 88 having a conical configuration. When the upper and lower plates 82,66 are positioned in contact next to one another, as seen in FIG. 1, the length of the actuating rod 84 compared to that of the flexure tube 68 is such that the enlarged nose 88 of the rod 84 is displaced a short distance below the segmented rim 74 of the tube 68 (see FIGS. 5 and 6). The outside diameter of the enlarged nose 88 of the actuating rod 84 is larger than the inside diameter of the segmented rim 74 of the flexure tube 68 such that as seen in FIG. 3, when the upper plate 82 is moved upwardly away from the lower plate 66, in an exemplary embodiment through a distance of approximately 0.35 inch, the inner annular tapered surface 90 of the enlarged nose 88 engages and expands the segmented rim 74 to a circumferentially expanded condition as the nose is forcibly inserted into the flexure tube 68, as seen in FIG. 7. The rim 74 is expanded to have an outside diameter greater than that of the inside diameter of the locking tube 48. Then, when the upper and lower plates 82,66 are moved together away from the adapter plate 24, as seen in FIG. 4, the rim 74 engages the lower end 80 of the locking tube 48 and carries the tube 48 with it, withdrawing the tube 48 from the remainder of the attaching structure 40 in the top nozzle 20, as seen in FIG. 8. Conversely, when the upper plate 82 is moved toward the lower plate 66, the enlarged nose 88 disengages and withdraws from segmented rim 74, allowing the rim 74 to contract back to its normal collapsed condition. The fixture 12 also includes mounting means, aligning means and biasing means, being generally designated 92, 94 and 96, respectively. The mounting means 92 includes a mounting plate 98 and a central shaft 100 rotatably journalled by a bushing 102 in the mounting plate 98. The lower portion 104 of the central shaft 100 is externally threaded and threadably engaged in a center tapped hole 106 in the upper plate 82. The lower end 108 of the lower threaded portion 104 of the central shaft 100 extends through a bore 110 in the center of the lower plate 66. In such arrangement, the central shaft 100 supports the upper plate 82, but not the lower plate 66, from the mounting plate 98. The mounting plate 98 has a generally square configuration approximately equal in size to that of the fuel assembly top nozzle 10. Four corner legs 112 on the bottom of the mounting plate 98 rest against the corner pairs of pads 28,30 of the top nozzle 20 such that the mounting plate 98 is stationarily supported on the top nozzle while operations are performed, as will be described later below, to remove and reinsert the reusable locking tubes 48. Two diagonally disposed ones of the four corner legs 112 of the mounting plate and one pad pair 28 of the top nozzle 20 include means for releasably locking the mounting plate 98 on the top nozzle 20. The releasable locking means includes a pair of hollow expandable split sleeves 114 fixedly mounted in the pair of diagonal corner legs 112 and a pair of wedge pins 116 mounted for rotational and axial movement in the respective sleeves 114. The sleeves 114 are adapted for insertion within respective bores 118 defined in the pair of diagonal corner pads 28 of the top nozzle housing 22. The wedge pins 116 having nuts 120 on their upper ends adapted to receive a suitable long-handled socket tool (not shown) for rotating the pins 116. When the pins 116 are rotated and thereby axially moved in a first direction, the sleeves 114 are caused to expand into frictional engagement with the bores 118 which secures the mounting plate 98 on the top nozzle housing 22. Conversely, when the pins 116 are rotated and thereby axially moved in an opposite second direction, the sleeves 114 are allowed to contract and release their frictional engagement with the bores 118, allowing removal of the mounting plate 98 from the top nozzle. The aligning means 94 of the fixture 12 includes a pair of elongated pins 122 which extend between and interconnect the stationary mounting plate 98 and the lower and upper traveling plates 66,82. Aligned holes 124,126,128 are formed in the respective plates 66,82,98 for receipt of the pins 122. Upper and lower retaining rings 130,132 are attached to opposite ends of the pins 122 for preventing the pin ends from slipping through the holes 124,128 of the respective lower and stationary plates 66,98. Each of the holes 124 through the lower plate 66 have a larger diameter counterbore 134 which opens toward the upper surface 78 of the adapter plate 24 and defines a ledge 136. Also, each of the pins 122 has a shouler 138 defined at the transition between a lower smaller diameter shaft portion 140 which extends through the holes 124,126 of the lower and upper plates 66,82 and an upper larger diameter shaft portion 142 which extends from the upper plate 82 through the hole 128 in the mounting plate 98. Thus, the aligning pins 122 not only keep the plates 66,82,98 aligned with one another, but also the lower plate ledge 136 and upper side of the upper plate 82 together in combination with the lower retaining ring 132 and pin shoulder 138 define the maximum limit of movement of the upper plate 82 away from the lower plate 66. Such maximum limit is designed to equate to the point at which the enlarged nose 88 on the actuating rod 84 expands the diameter of the segmented rim 74 on the flexure tube 68 to its circumferentially expanded condition in which it extends under the lower end 80 of the locking tube 48. The biasing means 96 of the fixture 12 includes a plurality of pairs of opposing pockets or recesses 144,146 formed respectively in the facing surfaces of the lower and upper plates 66,82 and a plurality of coil springs 148, one of which is disposed in each pair of the opposing recesses. The springs 148 thus interengage the plates 66,82 and normally bias them for movement away from one another so as to displace them at their aforementioned maximum limit. However, the springs 148 are yieldable for allowing the plates to move toward one another to the adjacent contacting positions, as seen in FIG. 1. Additionally, the fixture 12 includes a pair of spring-loaded shafts 150 which extend between the mounting plate 98 and the lower and upper plates 66,82. The plates 66,82,98 have respective holes 152,154,156 therein for receipt of the shafts 150. Each hole 154 in the upper plate 82 has a larger diameter counterbore 158 which opens facing toward the lower plate 66 and defines a ledge 160. The bottom ends 162 of the shafts 150 are externally threaded and adapted to threadably engage the internal threads in the holes 152 in the lower plate 66. Also, each of the shafts 150 has a shoulder 164 defined at the transition between a lower smaller diameter shaft portion 166 which extends through the holes 152,154 of the lower and upper plates 66,82 and an upper larger diameter shaft portion 168 which extends from the upper plate 82 through the hole 156 in the mounting plate 98. Further, a spring 170 is captured about each of the shafts 150 between the upper side of the mounting plate and a washer 172 retained on the upper end of the shaft 150 below a hex head 174 thereon. The spring 170 biases the shaft in a direction away from the lower plate. Still further, each shaft 150 is slidable axially relative to the plates 66,82,98 and against the bias of the spring 170 so that the bottom end 162 of the shaft 150 can be brought into the threaded hole 152 and threaded therein for attaching and maintaining the upper plate 82 against the lower plate 66 between the shoulder 164 and the attached bottom end 162 of the shaft 150. The hex head 174 is adapted to receive a suitable long-handled tool (not shown) for forcing the shaft 150 downward to overcome the bias of the spring 170 and to rotate the shaft to thread and unthread its bottom end 162. A retainer ring 176 on the lower shaft portion 166 prevents the shaft 150 from slipping out of the hole 154 through the upper plate 82 when the bottom end 162 of the shaft 150 is unthreaded and detached from the lower plate 66 and the spring 170 then causes the shaft 150 to move axially away from the lower plate. When the shafts 150 are detached from the lower plate 66, the biasing springs 148 in the recesses 144,146 of the plates 66,82 try to force the plates 66,82 apart to their maximum limit and thus will provide the positive plate separation necessary to cause the enlarged noses 88 on the actuating rods 84 to align with the segmented rims 74 on the flexure tubes 68, expanding the rims beneath the lower ends 80 of the reusable locking tubes 48 for removal of the tubes. Thus, when the central shaft 100 is rotated, by a suitable long-handled socket tool (not shown) connected to its hex head 178, so as to move the upper plate 82 upwardly away from the lower plate 66, the biasing springs 148 assist in their positive separation. Once the maximum limit of the upper plate 82 away from the lower plate 66 is reached, further rotation of the central shaft 100 raises both the upper and lower plates 82,66 together away from the adapter plate 24. Also, central shaft 100 has an externally threaded section 180 below its hex head 178 adapted to connect with a long-handled fixture handling tool (not shown) for installing and removing the mounting plate 98 onto and from the top nozzle 20. The procedures for removing and reinstalling the reusable locking tubes 48 are as follows. Removal of the locking tubes 48 from the removable top nozzle 20 for reconstitution of the fuel assembly 10 is initiated by lowering the fixture 12 (see in phantom in FIG. 1) toward the top nozzle when the fuel assembly is housed in a submerged work station. The lowering of the fixture 12 is accomplished using a long-handled tool (not shown) connected to the threaded section 180 of the central shaft 100 to guide the wedge pins 116 into the bores 118 of the top nozzle housing 22. Note that the upper and lower plates 82,66 are attached together, as seen in FIG. 1, by threading the bottom ends 162 of the shafts 150 into the tapped holes 152 in the lower plate 66 before lowering of the fixture 12. Once the mounting plate 98 is resting on the top nozzle 20, using a suitable long-handled socket tool connected to the nuts 120 on wedge pins 116, the fixture 12 can be locked on the top nozzle. Next, another long-handled socket took (not shown) is used to engage the hex head 178 of the central shaft 100 and rotate it, causing the attached upper and lower plates 82,66 to lower until the enlarged shoulders 76 on the flexure tubes 68 rests on the adapter plate upper surface, as seen in FIG. 1. At this point, the flexure tubes 68 (and actuating rods 84) extend through the reusable locking tubes 48 and the segmented bottom rims 74 of the flexure tubes 68 are located below the lower ends 80 of the locking tubes 48. Also, the enlarged noses 88 of the actuating rods 84 protrude below the flexure tube rims 74, as seen in FIG. 6. The flexure tubes 48 remain in their collapsed conditions which enabled them to have clear passage through the hollow locking tubes. Now, a long-handled socket tool (not shown) is used to engage the hex head 174 on each of the spring-loaded shafts 150 which are threaded at their bottom ends 162 into the holes 152 in the lower plate 66. Turning the shafts 150 the appropriate direction, the threads are disengaged and the spring 170 raises the shaft 150 away from the lower plate 66. Following next, a long-handled socket tool (not shown) is again connected to the hex head 178 on the central shaft 100 and the shaft is turned so as to cause the upper plate 82 and the actuating rods 84 attached thereon to rise. When the upper plate 82 travels through its maximum displacement or limit (for instance 0.35 inch) away from the lower plate 66 at which point the lower retaining rings 132 on the aligning pins 122 contact the ledges 136 in the counterbores 134 in the lower plate 66, as seen in FIG. 3, the lower plate 66 is then moved upward also. At the point where both plates 66,82 begin to travel upward, as seen in FIG. 4, the segmented rims 74 on the flexure tubes 68 are expanded beneath the locking tubes 48 by the noses 88 on the actuating rods 84, as seen in FIGS. 7 and 8, thus engaging and withdrawing the locking tubes 48 simultaneously from the removable top nozzle adapter plate 24. The central shaft 100 is turned until the end of mechanical travel is reached at which elevation the locking tubes are disengaged from the remainder of the top nozzle attaching structure 40 and the top nozzle 20 is then free to be raised off the upper end portions 36 of the guide thimbles 18. For reinstallation of the reusable locking tubes 48 back into the top nozzle attaching structure 40 after the fuel assembly 10 has been reconstituted and the top nozzle 20 has been replaced on the guide thimbles 18 of the fuel assembly 10, a long-handled socket tool (not shown) is engaged with the hex head 178 on the central shaft 100. The central shaft 100 is then turned in the appropriate direction to lower the two plates together and the captured locking tubes 48 until the enlarged shoulders 76 on the flexure tubes 68 rests against the adapter plate 24. At this point, the locking tubes 48 have been reinserted into their corresponding attaching structures 40. Turning of the central shaft 100 is continued until the upper plate 82 rests against the lower plate 66. Now, the rims 74 on the flexure tubes 68 have contracted from under the locking tubes 48. The hex head 174 on the shafts 150 are now engaged and the weight of the tool is used to compress the springs 170, forcing the shafts downward so that the bottom ends 162 can be threaded into the holes 152 in the lower plate 66. When the shafts 150 have been so threaded to the lower plate 66, the upper plate 82 is attached to the lower plate 66 so as to prevent expansion of the flexure tubes 68 by the actuating rods 84 during removal thereof which happens next. With a suitable long-handled socket tool engaged to the hex head 178 on the central shaft 100, the shaft is turned, raising the plates together and the flexure tubes 68 and actuating rods 84 therewith out of the locking tubes 48 and clear of the top nozzle adapter plate 24. Finally, using suitable tools (not shown) the wedge pins 116 are released and the fixture 12 unlocked from the top nozzle 20, and the fixture 12 is then removed from the reconstituted fuel assembly 10. It is thought that the present 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.