Patent Number: 052079805
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like references 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 a prior art nuclear fuel assembly, generally designated 10. Being the type use in a pressurized water nuclear reactor (PWR), the prior art fuel assembly 10 basically includes a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 14 which project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 axially spaced along the guide thimbles 14 and an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16. Also, the assembly 10 has an instrumentation tube 20 located in the center thereof and an upper end structure or top nozzle 22 attached to the upper ends of the guide thimbles 14. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 18 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 16 spaced along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets 24 and the opposite ends of the rod are closed by upper and lower end plugs 26, 28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the pellets 24 to maintain the pellets in a tight, stacked relationship within the rod 18. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the nuclear reactor. 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. To control the fission process, a number of control rods 32 are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Specifically, the top nozzle 22 includes a rod cluster control mechanism 34 having an internally grooved cylindrical member 36 with a plurality of radially extending flukes or arms 38. Each arm 38 is interconnected to a control rod 32 such that the control mechanism 34 is operable to move the control rods 32 vertically in the guide thimbles 14 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Referring now to FIGS. 2 and 3 as well as FIG. 1, it can be seen that the art top nozzle 22 of the prior art fuel assembly 10 includes an enclosure or housing 40 formed by a transversely extending lower adapter plate 42 and an upper annular flange 44 with an upstanding sidewall 46 extending between and integrally interconnecting the adapter plate 42 and flange 44 at their respective peripheries. The lower adapter plate 42 has a main central portion 42A provided with a first plurality of holes 48 to permit the flow of coolant upward through the top nozzle 22 and a second plurality of holes 50 to receive the upper ends of the guide thimbles 14 and where they are attached to the lower adapter plate 42. The upper annular flange 44 defines a central top opening 52 in the top nozzle 22 through which is disposed the rod cluster control assembly 34 being operable to insert and withdraw the control rods 32 into and from the guide thimbles 14 of the fuel assembly 10 through the second plurality of lower adapter plate holes 50. Also, a plurality of spring assemblies 54 are suitably clamped to the upper annular flange 44 to constitute a hold-down device for the fuel assembly 10. Each spring assembly 54 is composed of a set of leaf springs 54A disposed in a stack relation, and fastened in operative position on the top nozzle upper flange 44 at each of one pair of opposite diagonal corners 22A of the top nozzle 22 by using a spring clamp 56 which includes a corner block 58 and a spring screw 60. The spring assemblies 54 cooperate in a conventional manner with an upper core plate 62 of the reactor core located above the fuel assembly 10 to prevent hydraulic lifting of the fuel assembly 10 caused by upward coolant flow while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Prior Art Guide Pins Referring to FIGS. 1 and 4-7, there is illustrated a first embodiment of a guide pin of the prior art, generally designated 64, mounted from the upper core plate 62. The other pair of diagonal corners 22B of the top nozzle 22 of FIGS. 2 and 3 have upwardly projecting abutments 66 formed on the upper annular flange 44 and defining holes 68 which mate with the prior art guide pins 64 mounted in and projecting below the upper core plate 62. The guide pins 64 disposed between the upper core plate 62 and top nozzle 22 of each fuel assembly 10 provide proper alignment and engagement of the fuel assembly 10 with the upper core plate 62 so that the guide thimbles 14 of the fuel assembly 10 will extend vertically in alignment with the control rods 32 for receiving the control rods 32 from above. Referring to FIGS. 4-7, the first embodiment of the prior art guide pin 64 has a generally circular crosssection, and includes an elongated lower body portion 64A with a conical or bullet-shaped lower end nose 64B, a threaded upper end portion 64C, and an upper shaft portion 64D of reduced diameter which interconnects the lower body portion 64A and the threaded upper end portion 64C. The upper shaft portion 64D has a smaller diameter than the lower body portion 64A so as to facilitate a tight fitting (shrink fit) relation with the upper core plate 62 through a bore 70 in the upper core plate 62, with an upwardly-facing annular shoulder 64E defined on the upper end of the lower body portion 64A abutting against a lower surface 62A of the upper core plate 62 about the bore 70. Also, the upper core plate 62 has a recess 72 defined in an upper surface 62B which receives an attachment nut 74. The nut 74 has an internally-threaded central hole 76 by which it is threaded onto the threaded upper end portion 64C of the guide pin 64. The attachment nut 74 also has a pair of pilot holes 78 offset from opposite sides of the central hole 76 which mate with alignment pins on a tool (not shown) used in threading the nut 74 on the upper end portion 64C of the guide pin 64. To install the guide pin 64, the nut 74 is tightened relative to the bottom of the upper core plate recess 72 to thereby clamp the upper core plate 62 between the nut 74 and the shoulder 64E on the lower body portion 64A of the guide pin 64. After installation, the attachment nut 74 is permanently attached, such as by tack welding, to the upper core plate 62. Referring to FIGS. 8-10, there is illustrated a second embodiment of a guide pin of the prior art, generally designated 80, mounted from the top nozzle 22. The second embodiment of the prior art guide pin 80 has a generally circular cross-section, and includes an elongated upper body portion 80A with a conical or bullet-shaped upper end nose 80B, a threaded lower end portion 80C, and a lower shaft portion 80D of reduced diameter which interconnects the upper body portion 80A and the threaded lower end portion 80C. The lower shaft portion 80D has a smaller diameter than the upper body portion 80A so as to facilitate installation of the guide pin 80 in the hole 68 of one of the top nozzle corner raised abutments 66. At its end, the upper body portion 80A an enlarged diameter collar 80E integrally formed thereabout which rests on the top surface 66A of the abutment 66 extending about the hole 68 therein. The threaded lower end portion 80C of the guide pin 80 is threaded into the internally threaded abutment hole 68 and thereafter a lock pin 82 is driven into a hole 84 drilled radially through the abutment 66 and into the lower shaft portion 80D, as seen in FIG. 8. From the above descriptions, it will be understood that the first and second embodiments of the prior art guide pins 64, 80 are intended to be permanently attached the respective upper core plate 62 and top nozzle 22. Guide Pin Assemblies of the Invention Turning now to FIGS. 11-29, there is illustrated a first embodiment of a replacement (and removable) guide pin assembly in accordance with the present invention, being generally designated 86, for installation in one of the corner abutment holes 68 of the top nozzle 22. The first embodiment of the replacement guide pin assembly 86 includes an elongated guide pin body 88, a ferrule 90, a lock screw 92, and a lock pin 94. Referring to FIGS. 11-14, the guide pin body 88 of the replacement guide pin assembly 86 has a bullet-shaped upper end nose 88A, a lower expandable base 88B, and an elongated middle cylindrical shaft portion 88C of generally uniform or constant diameter extending between and integrally connected with the upper end nose 88A and lower expansion base 88B. The bullet-shaped nose 88A facilitates insertion of the middle shaft portion 88C of the guide pin body 88 into the bore 70 of the upper core plate 62 which is the same bore used in case of the prior art guide pins. The middle cylindrical shaft portion 88C of the guide pin body 88 will extend within the bore 70 and thereby interface with the upper core plate 62. An annular flange 96 is formed on the guide pin body 88 at the juncture of the lower expansion base 88B and the middle shaft portion 88C thereof. The annular flange 96 projects radially outwardly from the guide pin body 88 and has a downwardly-facing, outwardly and upwardly inclined, shoulder 96A which seats on a complementarily-shaped internal annular surface 98 defined in the top surface 66A of the top nozzle abutment 66 surrounding the upper end of the abutment hole 68. The surface 98 provides a reference elevation for the guide pin assembly 86. The lower expandable base 88B of the guide pin body 88 has an upper cylindrical base portion 100, a lower cylindrical thin-walled skirt portion 102, and a middle expandable wall portion 104, which portions extend within the top nozzle abutment bore 68, thereby interfacing with the top nozzle 22. The upper base portion 100 of the lower expandable base 88B has a central internally-threaded bore 100A. The middle expandable wall portion 104 has a plurality of circumferentially spaced vertical slots 106 defining between them a plurality of flexible wall segments 108 extending between and interconnecting the upper base portion 100 and the lower skirt portion 102. The interior surfaces 108A of the wall segments 108 are inclined upwardly and inwardly with respect to a longitudinal axis A of the guide pin body 88. Referring to FIGS. 11-20, the ferrule 90 and lock screw 92 of the replacement guide pin assembly 86 interfit within the lower expandable base 88B of the guide pin body 88. The ferrule 90 has a central opening 90A and a frusto-conical exterior configuration defining an exterior surface 90B inclined upwardly and inwardly relative to the longitudinal axis A of the guide pin body 88. The exterior surface 90B of the ferrule 90 is thus complementary to and interfaced with the interior surfaces 108A of the wall segments 108 of the lower expandable base 88B. The ferrule 90 is installed into the interior of the lower expandable base 88B of the guide pin body 88 from its lower end and upwardly along the longitudinal axis A of the guide pin body 88. The ferrule 90 thus interfits with the lower expandable base 88B of the guide pin body 88 and is capable of imparting a radially and outwardly directed force on the lower expandable base 88B to expand of the base within the hole 68 of the top nozzle 22 and thereby to secure the guide pin body 88 to the top nozzle 22 in response to a predetermined displacement of the ferrule 90 relative to the guide pin body 88 along its longitudinal axis A. The lock screw 92 has cylindrical shank 92A and head 92B integrally attached to the lower end of the shank 92A. The upper portion of the shank 92A has a reduced diameter (or relief) section 92C (compared to the rest of the shank) which defines an annular stop 92D on the top end of the shank 92A. The shank 92A and stop 92D fit upwardly through the central opening 90A of the ferrule 90 (once the latter has already been installed in the lower expandable base 88B) and into the internally-threaded bore 100A in the upper base portion 100 of the lower expandable base 88B. A portion 92E of the shank 92A immediately below the reduced diameter section 92C is externally-threaded to allow threading of the lock screw 92 into the guide pin body 88. The lock screw 92 thus provides a means which interfits with the ferrule 90 and threads into the guide pin body 88 to produce the required predetermined displacement of the ferrule 90 to cause expanding of the lower expandable base 88B. Referring to FIGS. 21-25, the purpose of the reduced diameter section 92C on the shank 92A, which defines an annular cavity 110 between the shank 92A and the upper base portion 100, is to accommodate the presence of the lock pin 94 in order to hold the guide pin body 88, ferrule 90, and lock screw 92 together as a unit before installation in the abutment hole 68 in the top nozzle 22. The lock pin 94 will thus preclude the lock screw 92 from being unthreaded and withdrawn inadvertently from the guide pin body 88 which would result in the ferrule 90 and lock screw 92 becoming loose parts. The lock pin 94 is forced into a hole 112 predrilled through the upper base portion 100 of the lower expandable base 88B of the guide pin body 88. The hole 112 is predrilled generally perpendicular to and offset from the longitudinal axis A of the guide pin body 88 so as not to intersect with the reduced diameter section 92C on the shank 92A and instead intersect with the annular cavity 110. Thus, the lock pin 94 when installed through the hole 112 will underlie the annular stop 92D on the top end of the shank 92A so as to prevent removal of the lock screw 92 without first removing the lock pin 94. Referring to FIGS. 26-29, there is illustrated the first embodiment of the replacement guide pin assembly 86 of the present invention at successive stages of the procedure for installing the guide pin assembly 86 in the top nozzle 22. FIG. 26 shows guide pin assembly 86 before insertion into one of the corner abutment holes 68 of the top nozzle 22. It will be observed that the lock pin 94 has been installed so as to hold the guide pin body 88, ferrule 90, and lock screw 92 together as a unit. FIG. 27 depicts the guide pin assembly 86 after insertion in the corner abutment hole 68 of the top nozzle 22, but before final torquing or tightening of the lock screw 92 into the guide pin body 88 and crimping of a lower peripheral edge 102A of the lower skirt portion 102 of the lower expandable base 88B under the outer peripheral edge 92F of the lower head 92B of the lock screw 92. It will be noted that a small gap 114 exists between the top of the ferrule 90 and the upper base portion 100 of the lower expandable base 88B of the guide pin body 88. The width of this gap 114 represents the amount of predetermined displacement that the ferrule 90 must undergo in order to impart the required radially and outwardly directed force to expand the lower expandable base 88B the desired amount to secure the guide pin body 88 in the top nozzle hole 68. FIGS. 28 and 29 show the guide pin assembly 86 after torquing of the lock screw 92 completely into the guide pin body 88 and crimping of the lower peripheral edge 102A of the lower skirt portion 102 of the lower expandable base 88B under the outer peripheral edge 92F of the lock screw lower head 92B. To torque the lock screw 92, a socket 116 for receiving an appropriate tool (not shown) is provided in the lower surface of the head 92B. During threading and torquing of the lock screw 92 into the guide pin body 88, the ferrule 90 is displaced the desired predetermined amount along the longitudinal axis A of the guide pin body 88 such that the forementioned gap 114 between the ferrule 90 and guide pin body 88 becomes closed. The purpose of the gap 114 is to provide a predetermined interference fit between the guide pin assembly 86 and the top nozzle 22. As the gap 114 becomes closed by the upward displacement of the ferrule 90, the sliding contact between the complementary inclined surfaces 108A, 90B of the wall segments 108 and the ferrule 90 imparts a radially and outwardly directed force on the slotted flexible wall segments 108 of the middle expandable wall portion 104 of the lower expandable base 88B sufficient to cause them to expand outwardly into engagement with the interior surface of the abutment opening 68 in the top nozzle 22. In such manner the replacement guide pin assembly 86 is secured by an interference fit to the top nozzle 22. As seen in FIG. 29, the lock screw head 92B has a plurality of hemispherical-shaped relief pockets 92G formed in spaced circumferential relation from one another about the peripheral edge 92F of the head 92B. The lower peripheral edge 102A is crimped into the more readily accessible ones of relief pockets 92G. The function of the relief pockets 92G and the crimping of the skirt edge 102A into some of them is to prevent loosening of the lock screw 92 during normal service. Should it be desirable to remove the guide pin assembly 86, a reverse torque can be applied by the socket tool to overpower the skirt crimp and loosen the lock screw 92. Referring to FIGS. 30-39, there is illustrate a second embodiment of a replacement (and removable) guide pin assembly in accordance with the present invention, being generally designated 118, for installation in one of the corner abutment holes 68 of the top nozzle 22. The second embodiment of the replacement guide pin assembly 118 includes an elongated guide pin body 120, an expandable insert 122, a ferrule 124, and an end cap 124. Referring to FIGS. 30-32, the guide pin body 120 of the replacement guide pin assembly 118 has a bullet-shaped upper end nose 120A, a lower attachment base 120B, and an elongated middle cylindrical shaft portion 120C of generally uniform or constant diameter extending between and integrally connected with the upper end nose 120A and lower attachment base 120B. The bullet-shaped nose 120A facilitates insertion of the middle shaft portion 120C of the guide pin body 120 into the bore 70 of the upper core plate 62 which is the same bore used in case of the prior art guide pins. The middle shaft portion 120C of the guide pin body 120 extends within the bore 70, thereby interfacing with the upper core plate 62. The lower attachment base 120B has upper and lower cylindrical mounting sections 128, 130 and a middle mounting section 132 being externally threaded to threadably receive the ferrule 124 as described below. The upper, middle and lower mounting sections 128, 132, 130 are of reduced diameters relative to the middle shaft portion 120C and relative to one another in that order. Referring to FIGS. 30 and 33-35, the expandable insert 122 of the guide pin assembly 118 has an upper cylindrical base portion 134, a lower cylindrical skirt portion 136, and a middle expandable wall portion 138, which portions extend through the top nozzle abutment bore 68 and thereby interface with the top nozzle 22. The expandable insert 122 inserts within the top nozzle hole 68, interfits about the lower attachment base 120B of the guide pin body 120, and is capable of expanding radially outwardly relative to the longitudinal axis A of the guide pin body 120 to provide an interference fit with the top nozzle 22. The upper base portion 134 has a central bore 134A which receives the upper mounting section 128 of the lower attachment base 120A of the guide pin body 120. The middle expandable wall portion 138 has a plurality of circumferentially spaced vertical slots 140 defining between them a plurality of flexible wall segments 142 extending between and interconnecting the upper base portion 134 and the lower skirt portion 136. The interior surfaces 142A of the wall segments 142 are inclined upwardly and inwardly with respect to a longitudinal axis B of the guide pin body 120. An annular flange 144 is formed about the upper peripheral edge of the upper base portion 134 of the expandable insert 122. The annular flange 144 projects radially outwardly from the expandable insert upper base portion 134 and has a downwardly-facing, outwardly and upwardly inclined, shoulder 144A which seats on the complementarily-shaped internal annular surface 98 defined in the top surface 66A of the top nozzle abutment 66 surrounding the upper end of the abutment hole 68. As before, the surface 98 provides a reference elevation for the guide pin assembly 118. Referring to FIGS. 30, 33 and 36-39, the ferrule 124 and end cap 126 of the replacement guide pin assembly 118 fit within the middle wall portion 138 and lower skirt portion 136 of the expandable insert 122 and fit on the middle threaded section 132 and lower section 128 of the lower attachment base 120B of the guide pin body 120. The ferrule 124 has a central opening 124A that is internally-threaded for threading of the ferrule 124 onto the externally-threaded middle section 132 of the lower attachment base 120B of the guide pin body 120. The ferrule 124 also has a frusto-conical exterior configuration defining an exterior surface 124B inclined upwardly and inwardly relative to the longitudinal axis B of the guide pin body 120. The exterior surface 124B of the ferrule 124 is thus complementary to and interfaced with the interior surfaces 142A of the wall segments 142 of the expandable insert 122. The ferrule 124 is installed into the interior of the expandable insert 122 from its lower end and upwardly along the longitudinal axis B of the guide pin body 120. The ferrule 124 interfits with the expandable insert 122 and threads over the middle shaft portion 120C of the guide pin body 120. The ferrule 124, as it is threaded through a predetermined displacement along the longitudinal axis B of the guide pin body 120 toward the upper base portion 134 of the expandable insert 122, will impart a radially and outwardly directed force on the middle expandable wall portion 138 of the expandable insert 122 to expand it within the hole 68 into an interference fit with the top nozzle 22 and thereby to secure the guide pin body 120 to the top nozzle 22. Thus, it is the threading of the ferrule 124 itself on the lower attachment base 120B of the guide pin body 120 which produces the required predetermined displacement of the ferrule 90 to cause expanding of the expandable insert 122. In addition, the ferrule 124 has a plurality of radially outward projecting tabs 124C formed on the exterior surface 124B of the ferrule which are spaced circumferentially from one another. The tabs 124C project into the slots 140 in the expandable middle wall portion 138 of the expandable insert 122 so as to prevent rotation of the ferrule 124 relative thereto as the guide pin body 120 is threaded and torqued into the ferrule 124. The end cap 126 inserts over the lower section 130 of the lower attachment base 120B of the guide pin body 120. The end cap 126 is then attached thereto, such as by tack welding, to preclude disassembly of the basic parts of the guide pin assembly 118 from one another. As in the case of the first embodiment, it will be noted that prior to torquing of the guide pin body 120 to the ferrule 124, a small gap 145 (see FIG. 30) exists between the top of the ferrule 124 and the upper base portion 134 of the expandable insert 122. During torquing, the aforementioned gap 145 between them. The width of this gap 145 becomes closed represents the amount of predetermined displacement that the ferrule 124 must undergo in order to impart the required radially and outwardly directed force to expand the expandable insert 122 the desired amount to secure the guide pin body 120 in the top nozzle hole 68. During threading and torquing of the ferrule 124 onto the lower attachment base 120B of the guide pin body 120, the ferrule 124 is displaced the desired predetermined amount along the longitudinal axis B of the guide pin body 120 such that the forementioned gap 145 between the ferrule 124 and expandable insert 122 becomes closed. The purpose of the gap 145 is to provide a predetermined interference fit between the guide pin assembly 118 and the top nozzle 22. As the gap 145 becomes closed by the upward displacement of the ferrule 124, the sliding contact between the complementary inclined surfaces 142A, 124B of the wall segments 142 and the ferrule 124 imparts a radially and outwardly directed force on the slotted flexible wall segments 142 of the middle expandable wall portion 138 of the expandable insert 122 sufficient to cause them to expand outwardly into engagement with the interior surface of the abutment opening 68 in the top nozzle 22. In such manner the replacement guide pin assembly 118 is secured by an interference fit to the top nozzle 22. Referring to FIGS. 30-32, in order to torque the guide pin body 120 relative to the ferrule 124, the guide pin body 120 has a plurality of torque grooves 146 near the base of its middle shaft portion 120C to accommodate suitable installation tooling (not shown). During torquing of the guide pin body 120, the ferrule 124, in addition to causing expansion of the wall segments 142 into engagement with the top nozzle 22, also acts like a "nut" since it does not rotate due to the presence of the anti-rotation tabs 124C. Also, a locking ring or disc 148 is attached to the guide pin body 120 at the juncture of the middle shaft portion 120C and the lower attachment base 120B. To preclude inadvertent loosening of the parts of the guide pin assembly 118 during reactor operation, the locking ring 148 is crimped locally (at two or three locations) into hemispherical relief pockets 150 provided in the top surface 122A of the expansion insert 122. The guide pin assemblies 86,118, being designed to preclude loose parts, can be installed remotely underwater on irradiated top nozzles. However, because of the evolution of fuel assembly design, most reactors are now operating with cores having fuel assemblies with both welded and removable top nozzles. Should a reactor experience damaged guide pins at a fuel assembly with a removable top nozzle, the irradiated nozzle could be removed and replaced with a top nozzle having these replacement guide pin assemblies 86, 118. Installation of the replacement guide pin assemblies 86, 118 in the new top nozzle could be accomplished in either the factory or field. Equipment and procedures used for fuel assembly top nozzle reconstitution can be readily applied for this repair. To accommodate the replacement guide pin assemblies 86, 118 removal of the damaged guide pin (in its entirety) from the reactor internals upper core plate is necessary. Removal of the guide pin can be readily accomplished by machining out the center of the shank (by either conventional or EDM techniques). Machining out the center of the shank will diminish the pre-load contact stress and facilitate removal of the damaged guide pin. The design of the replacement guide pin body can be specified to maintain fuel assembly top nozzle/upper internals alignment equivalent to that of the original equipment. 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.