DEVICE FOR GRIPPING AND DETACHING THE TOP NOZZLE SUBASSEMBLY FROM A RECONSTITUTABLE FUEL ASSEMBLY

Abstract: The invention relates to a device for gripping and detaching a top nozzle subassembly of a reconstitutable nuclear fuel assembly from upper guide-thimble extensions. The device has a central spider (64) with radial spokes (78) each carrying a locating lug (66) insertable into a coolant-flow opening of the hold-down plate of the top nozzle subassembly, a collar (68) adapted to seat against the top surface of the hold-down plate, and a spring compressing rod (70) extending through openings in the lug and collar and threadedly engageable with the adapter plate of the top nozzle subassembly. When in use, the rods (70) first are rotated to depress the hold-down plate and, hence, the hold-down springs, thereby to relieve tension on the guide-thimble extensions. Then, the latter are cut, whereafter the rods (70) are rotated in the opposite direction, thereby enabling the hold-down springs to push the hold-down plate upwards and off the cut ends of the guide-thimble extensions. An anvil (100) and impact slide (98) serve to force release of the hold-down plate from the cut extensions, if necessary.

Description: The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, to a device and method of gripping and detaching a top nozzle as an assembled unit from a reconstitutable fuel assembly. In most nuclear reactors, the reactor core is composed of a large number of elongate fuel assemblies which receive support and alignment from upper and lower transversely extending core support plates. Conventional designs of these fuel assemblies include a plurality of fuel rods and control-rod guide thimbles held in an organized array by a plurality of grids spaced along the fuel assembly length and attached to the control-rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles to form an integral fuel assembly. The guide thimbles extend slightly above and below the ends of the fuel rods. Fuel assemblies also employ hold-down devices to prevent the force of upward coolant flow from lifting the fuel assembly into damaging contact with the upper core support plate, while allowing the changes in fuel assembly length due to core induced thermal expansion and the like. During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks, resulting primarily from internal stresses, thus making it possible for radioactive fission products to seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of the integral assembly of the guide thimbles attached to the top and bottom nozzles, it is difficult to detect and remove the failed rods. In view of the high costs associated with replacing fuel assemblies, considerable interest has arisen in reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. In European patent publication No. 0138606 of April 24, 1985, there is described a reconstitutable fuel assembly with a top nozzle subassembly including a hold-down device as an integral part thereof. Basically, the top nozzle subassembly is made up of a lower adapter plate, an upper hold-down plate, and coil springs disposed about the upper ends of the guide thimbles and sandwiched between the two plates. The lower adapter plate is slidably mounted on the guide thimbles, its downward movement being limited by retainers on the guide thimbles. The upper hold-down plate has a passageway matched with each guide thimble, with an internal ledge defined in a lower portion of the passageway. The guide thimble is received in the passageway so as to mount the hold-down plate on the thimble for slidable movement therealong. An upper retainer is attached on the upper end of the guide thimble and disposed within the passageway of the hold-down plate. The upper retainer cooperates with the internal ledge of the passageway to limit the upward movement of the hold-down plate along the guide thimble. The construction is such that the upper end of the guide thimble, along with the upper retainer, is permitted to reciprocate within the passageway of the hold-down plate, thus allowing for thermal growth of the guide thimble. The coil springs are held in a state of partial compression between the lower adapter plate and the upper hold-down plate. It is the principal ojbect of the invention to provide a device enabling the top nozzle subassembly of a reconstitutable fuel assembly to be removed and remounted in a facile manner. The invention accordingly provides a device adapted first to be operated so as to depress the hold-down plate, and to hold it and all of the hold-down springs depressed until all of the guide thimbles are severed, and thereafter to be adjusted so as to allow the springs to gradually expand to their free (i.e. uncompressed) length in a controlled manner. The device includes a mechanism which ensures that all of the upper retainers can be easily removed from the guide thimbles after severance and substantially all at the same instance. The device is adapted to grip the top nozzle subassembly and revote it from the fuel assembly as a unit. Furthermore, it is relatively easy to manipulate by remote control and take advantages of pre-existing features in the top nozzle hold-down plate and adapter plate to achieve its capabilities. More specifically, the invention resides in a device for gripping and detaching a top nozzle subassembly from upper end portions of guide thimbles forming part of a reconstitutable fuel assembly, said top nozzle subassembly comprising a lower adapter plate, an upper hold-down plate with coolant-flow openings formed therethrough, and a plurality of hold-down springs held under partial compression between the adapter and hold-down plates, said adapter and hold-down plates being slidably mounted on the guide-thimble upper end portions, and the latter including lower retaining means cooperable with the adapter plate to limit downward sliding movement thereof, and upper retaining means cooperable with the hold-down plate to limit upward sliding movement thereof, characterised by: (a) a central spider adapted to be positioned in overlying relationship with respect to said upper hold-down plate; (b) a plurality of locating lugs disposed radially outward from said spider and arranged for alignment with the insertion into said coolant-flow openings of the upper hold-down plate), each of said locating lugs having an axial bore extending therethrough; (c) a plurality of collars connected to said spider, each collar having connected thereto one of said locating lugs and being adapted to bear upon said hold-down plate when the locating lug is inserted in the flow opening associated therewith; and (d) a plurality of elongate members extending through, and rotatable in, the axial bores of the respective locating lugs, said elongate members being threadedly engageable with the adapter plate such that rotation of the elongate members in one direction effects movement of the hold-down plate toward the adapter plate, and thereby further compresses the hold-down springs while relieving tension on the guide thimble upper end portions in preparation for severing them at locations immediately below the upper retaining means, and rotation of the elongate members effected, after said severing, in the opposite direction enables the hold-down plate to move away from the adapter plate under the action of said hold-down springs and to be released from the parts of the guide-thimble upper end portions remaining below the locations of severance. The central spider includes an elongate central hub having upper and lower ends, a plurality of spokes having inner and outer ends with each spoke being connected at its inner end to the lower end of the central hub, extending radially outwardly therefrom and connected at its outer end to one of the collars, and a handle connected to the upper end of the central hub. The gripping and detaching device includes further an impact-transmitting rod mounted in the central spider hub for movement toward and away from the upper hold-down plate with the rod being attachable at its lower end to the hold-down plate, and means carried on the rod being actuatable to deliver an upwardly directed driving force on the upper hold-down plate via the rod in order to cause positive release of the hold-down plate from the unsevered guide thimble upper end portions. More specifically, the actuatable means includes an impact-generating slug having a central bore through which the impact-transmitting rod extends for mounting the slug for slidable movement along the rod, and an impact-receiving anvil fixed on the impact-transmitting rod across the path of slidable movement of the slug along the rod. A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which : Figure 1 is a partially sectioned, elevational view, with parts broken away for clarity, of a reconstitutable fuel assembly having a top nozzle subassembly of the type to which the gripping and detachinbg device of the invention can be applied; Figure 2 is an enlarged top plan view of the fuel assembly as seen along line 2-2 of Figure 1.mm. Figure 3 is an enlarged sectional view of the fuel assembly as taken along line 3-3 of Figure 1; Figure 4 is a fragmentary perspective view of the gripping and detaching device embodying the invention; Figure 5 is a sectioned view, on a smaller scale, of the top nozzle as taken along line 5-5 of Figure 2, and of the gripping and detaching device shown in position but not yet locked to the top nozzle subassembly; and Figure 6 is a sectioned view similar to Figure 5 but showing the device in its operative position and after lifting of the top nozzle subassembly from the guide thimbles. In the following description, like reference characters designate like or corresponding parts throughout the several views, and terms such as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are employed as words of convenience not to be construed as limiting terms. Referring now to the drawings, and particularly to Figure 1, the reconstitutable fuel assembly, generally designated with the numeral 10, comprises a lower end structure or bottom nozzle 14 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), control-rod guide tubes or thimbles 16 longitudinally extending upward from the bottom nozzle 14, an organized array of fuel rods 18 held in spaced relationship with respect to one another by transverse grids 20 spaced along the fuel assembly length and attached to the guide thimbles 16, an instrumentation tube 22 at the center of the fuel assembly 10, and an upper end structure or top nozzle 24 attached to the upper ends of the guide thimbles 16. The guide thimbles 16 together with the bottom and top nozzles 14, 24 form a unitary assembly capable of being conventionally handled without damage to the assembly parts. To form the fuel assembly 10, the transverse grids 20 are attached to the guide thimbles 16 at predetermined axial locations, and the fuel rods 18 are inserted from below through the grids 20. The lower nozzle 14 is then suitably attached, such as by machine screws 26, to the lower ends of the guide thimbles. Finally, the top nozzle 24 is attached to the upper end portions 28 of the guide thimbles 16. Control rods (not shown) reciprocally movable in the control rod guide thimbles 16 of the fuel assembly 10 serve to control the fission process. Top Nozzle Subassembly With Hold-Down Means Referring now to Figures 1 to 3, the top nozzle subassemble 24 includes a lower adapted plate 30 and an upper hold-down plate 32. The lower adapter plate 30, as seen in Figure 3, is formed of a plurality of cross-laced ligaments or bars 34 defining coolant flow openings 36, and it is provided with a number of through-holes 38 corresponding in number to the guide thimble upper end portions 28. The bars 34 overlie the upper ends of the fuel rods 18 such that the fuel rods are axially restrained or captured between the bottom and top nozzles 14, 24. The holes 38 are of sufficient dimensional size and are in a pattern matched with the arrangement of the guide thimbles 16 such that the adapter plate 30 can be slidably mounted on the guide thimble upper end portions 28. Downward movement of the adapter plate 30 is limited by a plurality of lower ring-shaped retainers 40 disposed on the respective guide thimbles 16 at a location axially spaced above the upper ends of the fuel rods 18. The upper hold-down plate 32 (Figure 2) is also slidably mounted on the guide thimble upper end portions 28. It has a plurality of passageways 42 each having an internal ledge 44 formed therein in a lower portion of the passageway. Each passageway 42 has a generally constant larger diameter upper segment and a generally constant diameter lower segment, with the latter defining the internal ledge 44 and being slidably engageable with the guide thimble upper end portions 28. As seen in Figure 2, the hold-down plate 32 is also provided with a number of relatively large coolant flow openings 46 and several smaller coolant flow openings 48 positioned at predetermined locations about the plate 32. Axially spaced above the lower retainer 40 is another, upper retainer 50 mounted on each guide thimble upper end portion 28 and disposed within the upper segment of the respective passageway 42. The upper retainer 50 is in the form of a collar having an internal annular groove 52 in the upper portion thereof and is attached to an upper end 54 of the guide thimble 16 by circumferentially bulging the wall of the guide thimble upper end portion 28 into the internal groove 52 of the upper retainer 50. The attached upper retainers 50 cooperate with the internal ledges 44 to limit the upward movement of the hold-down plate 32 along the guide thimble upper end portion 28. The top nozzle subassembly 24 further includes a coil spring 56 disposed on each of the guide thimble upper end portions 28 and interposed between the lower surface of the upper hold-down plate 32 and the upper surface of the lower adapter plate 30. Preferably, the coil springs 56 are held in compression between the upper hold-down plate 32 and the lower adapter plate 30 to preload the fuel assembly 10. For proper alignment of the coil springs 56 and to prevent them from contacting their respective guide thimbles 16, a spring seat 58 is provided on the lower end of each of the coil springs 56, setting in a recess formed in the upper surface of the lower adapter plate 30. In defining an enclosure for channeling the coolant flow upwardly at the upper region of the fuel assembly 10, upstanding sidewalls 60 are formed on the peripheral edges of the lower adapter plate 30. The upper hold-down plate 32 is slidable along the guide thimble upper end portions 28 within the enclosure sidewalls 60. Top Nozzle Gripping and Detaching Device Turning now to Figures 4 to 6, there is shown the device, generally designated 12, for gripping and detaching the top nozzle 24 as a subassembly from the guide thimble upper end portions 28 of the reconstitutable fuel assembly 10. The device 12 utilizes a plurality of internally threaded anchors 62 (Figures 2 and 3) defined in the lower adapter plate 30 between the coolant flow openings 36 and guide thimble holes 38, and a plurality of large coolant flow openings 46 defined in the upper hold-down plate 32 between its passageways 42 and vertically aligned with the anchors 62. The gripping and attaching device 12 basically includes a central spider 64, a plurality of locking lugs 66 disposed radially outwardly from the spider 64, a plurality of collars 68 attached on the locating lugs 66 and connected to the central spider 64, and a plurality of elongated spring compression studs 70 associated with the locating lugs 66. The central spider 64, which during use (as seen in Figures 5 and 6) is disposed in overlying relation the upper hold-down plate 32, includes an elongated central hub 72 having upper and lower ends 74, 76 (Figure 4), a plurality of spokes 78 being connected at their inner ends to the lower end 76 of the central hub 72, extending radially outwardly therefrom, and being connected at their outer ends to the collars 68, and a handle 80 connected to the upper end 74 of the central hub 72 for use in handling the device 12. The locating lugs 66 are arranged and sized for alignment with the insertion into the large coolant flow openings 46 in the upper hold-down plate 32. Also, each of the locating lugs 66 and its upper collar 68 have elongated central bores 82, 84 aligned with one another and with one of the anchors 62 in the lower adapter plate 30 when the lug 66 is inserted into its respective flow opening 46. As clearly seen in Figures 5 and 6, each collar 68 overlying one of the locating lugs 66 has a diameter larger than its respective lug 66 and the flow opening 46 into which the lug is inserted so that the collar will overlie the flow opening and bear on the upper surface 86 of the hold-down plate 32 when its lug is inserted in its respective flow opening. The elongated spring compression studs 60 of the gripping and detaching device 12 are received through and rotatable within the respective aligned central bores 82, 84 of the locating lugs 66 and collars 68. Each stud 70 has a socket element 88 on the upper end thereof for grasping to rotate the stud. The socket element 88 overlies and bears against the respective collar 68 of the one locating lug 66 through which the stud 70 is mounted. Each stud 70 also has an externally-threaded segment 90 defined at its lower end which adapts the stud for threaded engagement with one of the internally-threaded anchors 62 in the lower adapter plate 30. When the spider 64 is installed on the top nozzle 24 with its locating lugs 66 inserted into respective large coolant flow openings 46 in the upper hold-down plate 32, rotation of each of the studs 70 in one predetermined direction will thread its threaded end segment 90 into one anchor 62 and when the socket element 88 bears on the upper surface 86 of the hold-down plate 32 continued threading of the stud into the adapter plate will cause movement of the adapter plate and upper hold-down plate toward one another. Such movement of the plates 30, 32 increases the compression of the hold-down springs 56 disposed therebetween and concurrently relieves the tension imposed on the upper retainers 50 via the ledges 44 within the hold-down plate passageways 42. With tension on the upper retainers 50 so relieved, severing of the guide thimble upper end portions at locations immediately below the upper retainers 50 can now take place, for example, via the method illustrated and described in the third application cross-referenced above. Once severing of the guide thimbles 16 is completed, rotation of the studs 70 in the predetermined opposite direction results in unthreading of each stud from its anchor 62 and thereby decreases the compression of the hold-down springs 56 which then cause the lower adapter plate 30 and upper hold-down plate 32 to move away from one another. Upon such movement, the ledges 44 of the hold-down plate 32 will engage the severed upper retainers 50 and ordinarily cause them to release from the remaining unsevered guide thimble upper end portions 28. However, if the interference fit between the upper retainers 50 and the upper tips of the guide thimbles just below the location of severing is tight enough, the upper retainers may not release and a more positive way is needed to secure their removal. To satisfy this need, the device 12 additionally includes an impact-transmitting rod 92 mounted in the central spider hub 72 for movement toward and away from the upper hold-down plate 32. The rod 92 is externally-threaded at its lower end 94 for securement in an internally-threaded blind hole 96 in the upper surface 86 of the hold-down plate. Means carried on the rod 92 in the form of an impact-generating slug 98 and an impact-receiving anvil 100 are actuatable together to deliver an upwardly directed driving force on the upper hold-down plate 32 via the rod 92 to ensure a positive release of the upper retainers 50 from the unsevered guide thimble upper end portions 28. Specifically, the impact-generating slug 98 has a central bore 102 through which the rod 92 extends for mounting the slug for slidable movement along the rod. The impact-receiving anvil 100 is fixed on the impact-transmitting rod 92 across the rod 92. Thus, when the slug 98 is struck against the stationary anvil 100, the force of the impact is transmitted from the anvil to the hold-down plate 32 via the interconnecting rod 92. To summarize its operation, the device 12 is first lowered down onto the top nozzle 24 to the position seen in Figure 5 where the locating lugs 66 are inserted into the large coolant flow openings 46 in the upper hold-down plate 32. Then, the four spring compression studs 70 are threaded into the internally-threaded anchors 62 in the lower adapter plate 30 by using an extension socket wrench engaged with the socket elements 88 on the studs. The studs 70 are then tightened down enough to relieve the tension load on the guide thimble upper end portions 28 and the upper retainers 50 attached thereto. The guide thimbles 16 are then severed just below the bulges 104 which should free the top nozzle subassembly from the fuel assembly skeleton. The spring compression studs 70 are loosened to allow the hold-down springs 56 to raise the hold-down plate 32 which should pull the upper retainers 50 off the unsevered upper end portions 28 of the guide thimbles 16. If the upper retainers 50 do not become free, the impact rod 92 is then threaded into the hole 96 in the upper hold-down plate 32 and the impact slug 98 is used like a hammer to impact the plate 32 in an upward direction via the rod 92 and anvil 100. This will then free the upper retainers from the guide thimble upper end portions 28 and permit the top nozzle subassembly 24 to be lifted off the fuel assembly skeleton, as seen in Figure 6, using the handle 80 or a crane attached to the handle.

Claims: 1.A device for gripping and detaching a top nozzle subassembly from upper end portions of guide thimbles forming part of a reconstitutable fuel assembly, said top nozzle subassembly comprising a lower adapter plate, an upper hold-down plate with coolant-flow openings formed therethrough, and a plurality of hold-down springs held under partial compression between the adapter and hold-down plates, said adapter and hold-down plates being slidably mounted on the guide-thimble upper end portions, and the latter including lower retaining means cooperable with the adapter plate to limit downward sliding movement thereof, and upper retaining means cooperable with the hold-down plate to limit upward sliding movement thereof, characterized by: (a)a central spider (64) adapted to be positioned in overlying relationship with respect to said upper hold-down plate (32); (b)a plurality of locating lugs (66) disposed radially outward from said spider (64) and arranged for alignment with and insertion into said coolant-flow openings (46) of the upper hold-down plate (32), each of said locating lugs (66) having an axial bore (82) extending therethrough; (c)a plurality of collars (68) connected to said spider (64), each collar (68) having connected thereto one of said locating lugs (66) and being adapted to bear upon said hold-down plate (32) when the locating lug (66) is inserted in the flow opening (46) associated therewith; and (d)a plurality of elongate members (70) extending through, and rotatable in, the axial bores (82) of the respective locating lugs (66), said elongate members (70) being threadedly engageable with the adapter plate (30) such that rotation of the elongate members in one direction effects movement of the hold-down plate (32) toward the adapter plate (30), and thereby further compresses the hold-down springs (56) while relieving tension on the guide-thimble upper end portions (28) in preparation for severing them at locations immediately below the upper retaining means (50), and rotation of the elongate members (70) effected, after said severing, in the opposite direction enables the hold-down plate (32) to move away from the adapter plate (30) under the action of said hold-down springs and to be released from the parts of the guide-thimble upper end portions (28) remaining below the locations of severance. 2.A gripping and detaching device according to claim 1, characterized in that said central spider (64) includes an elongated upright central hub (72), a handle (80) connected to the central hub adjacent the upper end (74) thereof, and a plurality of spokes (78) connected to the central hub (72) adjacent the lower end (76) thereof and extending radially outward therefrom, each of said spokes having one of said collars (68) secured thereto. 3.A gripping and detaching device according to claim 1 or 2, characterized in that said central spider (64) supports an impact-transmitting rod (92) for movement of the rod toward and away from the upper hold-down plate (32), said impact-transmitting rod (92) being attachable at its lower end (94) to said hold-down plate (32) and carrying impact-producing means (98,100) operable to deliver to the impact-transmitting rod and, through the latter, to the hold-down plate (32) upwardly directed driving impacts causing positive release of the hold-down plate (32) from said remaining parts of the guide-thimble upper end portions (28). 4.A gripping and detaching device according to claim 3, characterized in that the impact-producing means (98, 100) comprises a rigid anvil (100) disposed on the impact-transmitting rod (92), and a slug (98) slidably mounted on the impact-transmitting rod so as to be axially movable thereon against the anvil (100). 5.A gripping and detaching device according to claim 1, 2, 3 or 4, characterized in that each of said collars (68) has an axial bore (84) in alignment with the axial bore (82) of the associated locating lug (66), each of said elongated members (70) extending through the axial bores of the collar (68) and locating lug (66) associated therewith. 6.A gripping and detaching device according to claim 5, characterized in that each of said elongate members (70) has on the upper end thereof a socket element (88) for engagement with a rotating tool, said socket element (88) being disposed adjacent and bearing against an upper surface of the respective collar (68) associated therewith.

Inventors: GJERTSEN, ROBERT KENNETH, WILSON, JOHN FRANCIS
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G21C3
IPC codes: G21C19, G21C3, G21C21