Patent Number: 046845038
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. 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 perspective view of a reconstitutable nuclear reactor fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. Basically, the fuel assembly 10 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 end of the guide thimbles 14 in accordance with the present invention which will be fully described below. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conveniently 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 (not shown) and is closed at its opposite ends by upper and lower end plugs (not shown). The fuel pellets composed of fissile material are responsible for creating the reactive power of the reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the guide thimbles 14 and along the fuel rods 18 of the fuel assembly in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods (not shown) are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Since the control rods are inserted into the guide thimbles 14 from the top of the fuel assembly 10, the placement of the components forming the top nozzle 22 and their attachment to the guide thimbles 14 must accommodate the movement of the control rods into the guide thimbles 14 from above the top nozzle 22. For mounting the top nozzle 22, each of the guide thimbles 14 has an upper end portion in the form of a separate tubular extension 24. The guide thimble extension 24 is attached at its bottom portion 26 to the upper end 28 of the main guide thimble body 30 by a circumferential bulge fit wherein the bottom portion 26 has a coaxial, inside circumferential groove 32 and the upper end 28 has a coaxial, outside circumferential ridge 34 bulge-fitted in the groove 32. The reason for using the separate guide thimble extension 24, instead of a longer guide thimble body 30, is that just the extension 24 needs to be replaced and not the entire thimble 14 should any problems in threading be encountered during manufacture or use of the extension 24. The guide thimble body 30 would be severed just below the bottom portion 26 of the old extension 24 to remove the damaged extension 24. Then a new longer extension would be attached to the slightly shortened guide thimble body 30 by circumferentially bulging a new groove/ridge set into the interfitted guide thimble body and replacement thimble extension. Improved Attaching Structure for Removably Mounting the Top Nozzle The separate components making up the top nozzle 22 which are mounted on the guide thimble extension 24 include an upper hold-down plate 36, a lower adapter plate 38 and a plurality of hold-down springs 40 disposed between the upper and lower plates 36,38. The upper hold-down plate 36 has a plurality of apertures 42 defined therein, while the lower adapter plate 38 has a plurality of openings 44. As described earlier, to carry out reconstitution of the fuel assembly 10, the top nozzle 22 must first be removed and then, after the fuel rods 18 of the assembly have been inspected and the failed fuel rods removed and replaced, the top nozzle is reattached to the guide thimble extensions 24 of the assembly. In FIG. 1, and in greater detail in FIG. 2, there is seen the preferred embodiment of the improved attaching structure, generally designated 46, for removably mounting the components of the top nozzle 22 as a unitary subassembly on the guide thimble extensions 24. Consequently, the top nozzle 22 does not fall apart each time it is removed from the guide thimbles 14 for reconstitution of the fuel assembly 10. The improved attaching structure 46 basically includes a plurality of coupling members in the form of a plurality of sleeves 48 which interfit the upper hold-down plate 36, the lower adapter plate 38 and the hold-down springs 40 disposed therebetween in such manner as to capture and retain the plates 36,38 and springs 40 together as a unitary subassembly. Each coupling sleeve 48 has spaced apart upper and lower portions 50,52 with a central passageway 54 extending therebetween for receiving one of the guide thimble extensions 24 therein to removably mount the coupling sleeve 48 on the thimble extension 24. Particularly, means on the bottom portion 26 of the thimble extension 24 in the form of an outwardly projecting annular ledge 56 supports the coupling sleeve 48 at its lower portion 52. Further, the improved attaching structure 46 includes a detachable member in the form of an annular cap 58 releasably applied to the upper portion 50 of the coupling sleeve 48 and disengagably engaged with a top portion 60 of the thimble extension 24 so as to removably lock the coupling sleeve 48 to the guide thimble extension 24. In such a subassembly, during the period of reactor operation when the fuel assembly 10 is resident in the reactor core, the hold-down plate 36 can slidably move along the coupling sleeves 48 toward the lower adapter plate 38 and against the yieldable biasing of the hold-down springs 40 to accommodate upward lifting of the fuel assembly 10 through a short distance by the pressure of the coolant flowing upwardly along the length of the fuel assembly or by thermal expansion of the guide thimbles 14, while preventing damaging contact of the fuel assembly 10 with an upper core support plate (not shown) secured to the reactor. Then, when reconstitution of the fuel assembly 10 is desired, each of the detachable caps 58 can be conveniently and easily removed from the coupling sleeves 48 and thimble extensions 24, freeing the coupling sleeves 48 from their attachments to the thimble extensions 24. The coupling sleeves 48 and the upper hold-down plate 36, the lower adapter plate 38 and the hold-down springs 40 retained thereon can be removed therewith still as a unitary subassembly from the upper extensions 24 of the guide thimbles 14. Several features are included in the improved attaching structure 46 which allow the coupling sleeves 48 interfitted with the upper and lower plates 36,38 and the springs 40 to capture and retain them in the unitary subassembly. The first feature has already been mentioned, that being, the annular ledge 56 on the bottom portion 26 of the thimble extension 24 which supports an individual one of the coupling sleeves 48 at its lower portion 52. A second feature is means in the form of pluralities of first and second complementary threaded sections 62,64 for attaching the lower adapter plate 38 in a fixed positional relationship to each of the coupling sleeves 48. Each of the first threaded sections 62 in the adapter plate 38 actually defines one of the openings 44 in the adapter plate 38, while each of the second threaded sections 64 on the coupling sleeves 48 are defined about the lower portions 52 thereof. Both the thimble extensions 24 and the coupling sleeves 48 which slidably fit over, and seat on the ledge 56 of, the thimble extensions 24 extend together through the openings 44 in the adapter plate 38. However, while the thimble extensions 24 are not attached to the lower adapter plate 38, the coupling sleeves 48 are so attached via their second annular threaded sections 64 which are threaded into the respective first annular threaded sections 62 of the adapter plate 38. A third feature of the improved attaching structure 46 takes the form of pluralities of first and second overlapping means 66,68 which define upper stops which limit the upward travel of the upper hold-down plate 36 away from the adapter plate 38 due to the bias provided by the compressed hold-down springs 40 encircling the coupling sleeves 48 and positioned between the upper and lower plates 36,38. Two alternative forms of the first and second overlapping means 66,68 are illustrated in the drawings. In the first form depicted in FIGS. 2 and 3, the first and second overlapping means 66,68 are each respectively a recessed slot 70 formed in the coupling sleeve 48 and a pin 72 threadably connected in an internally threaded hole 74 formed in an inner annular edge 76 which defines one of the apertures 42 in the hold-down plate 36 and surrounds the coupling sleeve 48. It is readily apparent that more than one slot 70 and pin 72 may be used. In the second form depicted in FIG. 4, the first and second overlapping means 66,68 are each respectively an outwardly projecting annular shoulder 78 formed on the upper portion 50' of the coupling sleeve 48' and the inner annular, aperture-defining, edge 76 in the hold-down plate 36. Finally, a fourth feature of the improved attaching structure 46 relates to first and second matable means 80,82 for disengagably engaging each detachable cap 58 to the top portion 60 of one of the thimble extensions 24, and first and second interfering means 84,86 for releasably attaching each detachable cap 58 to the upper portion 50 of one of the coupling sleeves 48. The annular cap 58 in being engaged and attached respectively to the guide thimble extension 24 and the coupling sleeve 48 provides a connecting bridge therebetween which removably locks them together at the upper portion 50 of the coupling sleeve 48 and the top portion 60 of the thimble extension 24. A slightly different arrangement of the matable means 80,82 is shown in FIG. 2 as compared to the arrangement shown in FIGS. 3 and 4. On the one hand, in FIG. 2 the first matable means 80 is an internally threaded section 88 on the cap 58 and the second matable means 82 is an externally threaded section 90 on the top portion 60 of the guide thimble extension 24. On the other hand, in FIGS. 3 and 4 the first matable means 80 is an externally threaded section 92 on the cap 58 and the second matable means 82 is an internally threaded section 94 on the top portion 60' of the guide thimble extension 24'. The form of the interfering means 82,84 is the same for the detachable cap 58 in each of the three alternatives of the attaching structure 46. The cap 58 has an outer rim 96 which fits over the edge 98 of the corresponding upper portions 50,50' of the respective coupling sleeves 48,48' so as to overlie a series of dimples 100 formed therein. After the cap 58 is threadably engaged with the respective thimble extensions 24,24', the areas of the rim 96 opposite the dimples 100 are deformed by a suitable tool into complementary dimples 102 which extend into the dimples 100. The locking connection provided by the dimples 100,102 can be overridden by forcibly counter-rotating the cap 58, such as by use of a screwdriver type tool employed in a slit 104 in the top of the cap 58. Also, the cap 58 has a central bore 106 which communicates with the bores 108,110 of the thimble extension 24 and the guide thimble body 30 so that a control rod (not shown) can slidably enter the bores 106,108, 110 from above. To reconstitute the fuel assembly 10, the cap 58 is counter-rotated to overcome the dimple locking mechanism. With the cap 58 removed, the top nozzle 22 is simply lifted free of the rest of the fuel assembly 10 without disturbing the hold-down device (hold-down plate 36, adapter plate 38, and hold-down springs 40). When reassembly of the top nozzle 22 to the fuel assembly 10 is required, the top nozzle 22 is lowered with the coupling sleeve 48 sliding down over the guide thimble extension 24 and stopping against the ledge 56. The cap 58 is threaded to the guide thimble extension 24 which secures the top nozzle coupling sleeve 48. Then, one or more unused areas of the cap's rim 96 are deformed into underlying dimples 100 to lock the cap 58 against rotating loose. The procedures are performed remotely, usually under many feet of radiation-shielding water. 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.