Spring retention system for upper tie plate and fuel bundle channel in a nuclear reactor assembly

Abstract: Leaf-type springs (22) are inserted into slots (21) in an upper tie plate (12) of a nuclear fuel assembly (10) and bear against interior surfaces of the fuel channel (14) to center the upper tie plate (12) within the fuel channel (14). More robust similarly configured leaf springs (24) are secured in slots (23) in the upper tie plate (12) and are cantilevered for bearing engagement through slots in the fuel assembly channel (14) against the reactor top guide (16) to bias the fuel assemblies toward the control rod (18). To mount each spring (22,24) to the upper tie plate, the base of each leaf spring includes a central beam (36,54) and a pair of outermost beams (38,56) spaced from the central beam (36,54), all beams lying in a common plane. Laterally outwardly directed tabs (42,62) are provided on the lower ends of the outer beams (35,56). Stops (40,60) are provided intermediate the ends of the outer beams (35,56). By inserting the unstressed, unloaded base of the springs into the slots, the outer beams are displaced inwardly into a stressed condition. Once the tabs (42,62) extend past the lower surface of the tie plate (12), the beams return to an unloaded, unstressed condition with the tabs (42,62) and stops (40,60) locking the springs to the tie plate (12). The distal ends of the springs bear against the channel (14) or top guide (16), as applicable, to locate the upper tie plate (12) within the channel (14) and bias the fuel assembly (10) and channel (14) toward the control rod (18), respectively.

Description: TECHNICAL FIELD The present invention relates to apparatus for locating the upper tie plate of a nuclear fuel bundle in a fuel bundle channel, and for locating the channel in a top guide assembly of a nuclear reactor. BACKGROUND Nuclear fuel assemblies typically include a fuel bundle comprised of a plurality of fuel rods, upper and lower tie plates coupled to the fuel rods, and a fuel bundle channel for receiving the fuel bundle and upper tie plate. A multiplicity of these assemblies, together with control rods, typically comprise a boiling water reactor core. In certain fuel bundle assemblies, the fuel bundle is inserted downwardly into the channel, with the lower tie plate of the bundle resting on top of the channel assembly's nose piece. The upper tie plate is not directly fastened to the channel. Rather, the upper tie plate is positioned and located within the fuel channel by a plurality of identical upwardly projecting, flat, cantilevered springs attached to the sides of the upper tie plate. Similar additional upwardly projecting cantilever-type springs are employed to position or locate the upper end of the fuel assembly in the multi-bundle cell. These latter springs bias or push the upper end of the fuel assembly toward the centrally located, normally cruciform-shaped control rod such that the fuel assemblies are symmetrically located about the control rod. The movement toward the control rod is constrained by an interaction between adapter blocks in the upper tie plate handle and stub beams attached to the reactor top guide. In both cases, the springs have been attached to the upper tie plate by small countersunk head-threaded fasteners screwed into the sides of the upper tie plate. Fasteners of this type, however, have failed in operation, resulting in a number of deleterious effects, including accelerated corrosion in the boiling water reactor environment. These failures are believed to be a result of the high stresses applied to the fasteners under ordinary loading conditions and also to differential growth of dissimilar metals. While one solution to the prob!em may be to weld the fasteners to the tie plate, welding requires extra. manufacturing steps and costs. According to the invention, there is provided an apparatus for locating an upper tie plate relative to a reactor top guide assembly or a fuel bundle channel, in a nuclear reactor having fuel assemblies, the upper tie plate and a fuel bundle channel for receiving a fuel bundle of the fuel assemblies and upper tie plate, the reactor top guide assembly forming a multi-bundle cell for the fuel assemblies, said apparatus comprising: a plurality of slots formed in the upper tie plate adjacent margins thereof; springs disposed in said slots, respectively, said slots and said springs being disposed about said tie plate such that outer surfaces of said springs bear against an interior surface of one of said reactor top guide assembly and said fuel bundle channel to locate the upper tie plate relative thereto; each said spring including a spring body having at least a pair of beams spaced one from another and cantilevered from said body in an unstressed condition and movable toward one another into a stressed condition, at least one of said beams having an outwardly directed tab, said beams and said tab of each spring being inserted into respective slots with said beams movable toward one another into the stressed condition and movable away from one another into said unstressed condition upon passage of the tab through said slot, said tabs engaging the tie plate to lock the springs to the tie plate with the outer spring surfaces thereof projecting from said tie plate to bear against an interior surface of one of said reactor top guide assembly and said fuel bundle channel. In one embodiment of the invention, there is provided a novel and improved apparatus for securing the springs to the upper tie plate in a manner which eliminates or minimizes the foregoing and other problems, and enables effective spring-biased retention of the upper tie plate within the fuel channel and the fuel channel within the reactor top guide assembly. To accomplish this, a plurality of generally rectangular shaped slots are provided adjacent the margins of the upper tie plate and which slots extend between its upper and lower surfaces. Flat, leaf-type springs are provided, which at one end are received in the slots and project cantilever fashion from the slots to engage the fuel channel and top guide assembly, respectively. While different dimensional and strength considerations apply to the two different sets of springs for locating the tie plate within the fuel bundle channel and biasing the channel toward the control rod, the mounting of the two different sets of springs to the upper tie plate is fundamentally identical. Each spring includes a spring body having an upper cantilevered portion for engaging against the channel or top guide, as applicable. Each spring is preferably provided at its lower end with a central cantilevered beam and a pair of cantilevered outer beams which straddle and are spaced from the central beam on opposite sides thereof. The three beams lie in a common plane. The combined widths of the pair of outer beams and the center beam, as well as the width of the slots between the beams, is approximately equal to or slightly less than the width of the slot in the upper tie plate. The outer beams carry two laterally outwardly projecting tabs along their outer surfaces and in the plane of the spring. The bottom tabs are chamfered along their outer surfaces to facilitate insertion of the spring into the slot. An additional pair of tabs or stops in the plane of the spring and spaced from the distal tabs a distance slightly greater than the width of the upper tie plate between its upper and lower surfaces project laterally outwardly from the outer beams to afford an upper stop for the spring. Each spring may be inserted into the slot of the upper tie plate by squeezing the two outer spring beams, i.e., stressing the outer beams, toward the larger center beam and inserting all three beams into the slot. The springs are pushed downwardly through the slot until the bottom tabs pass the bottom surface of the upper tie plate, at which time the outer beams spring laterally outwardly and return to their normal unstressed position. In that unstressed position, the tabs bear against the undersurface of the tie plate to retain the spring in the slot, while the stops bear against the upper surface of the upper tie plate to prevent further downward movement of the spring through the slot of the upper tie plate. When the fuel bundle is inserted into the channel, the first set of springs deflect inwardly to engage the interior surface of the channel and locate, e.g., center, the upper tie plate relative to the channel. When the fuel assembly is inserted into the reactor core, the more robust second set of springs pass through openings in the channel to engage in a stressed condition the interior surface of the top reactor guide. While the cantilevered springs are themselves stressed in use, the connection between the springs and the tie plate is unstressed. That is, the beams return to an unloaded state after installation, enabling reliable retention of the springs in the slots in a substantially unloaded condition. Stated differently, the insertion stresses are not present once the fuel assemblies are loaded. Accordingly, the present invention seeks to provide a novel and improved apparatus for spring retention of the upper tie plate within the fuel channel and biasing the fuel assembly toward the control rod in a nuclear reactor core. The invention will now be described in greater detail, by way of example, with reference to the drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a fragmentary cross-sectional view of a portion of a fuel assembly disposed in a nuclear reactor top guide; FIGURE 2 is a cross-sectional view thereof taken generally about on line 2-2 in Figure 1; FIGURES 3A and 3B are side and end elevational views of the present invention illustrating springs for locating the upper tie plate in the fuel bundle channel; and FIGURES 4A and 4B are views similar to Figures 3A and 3B, respectively, and on a similar scale, illustrating springs for locating the fuel bundle assembly relative to the top guide assembly of the nuclear reactor. BEST MODE FOR CARRYING OUT THE INVENTION Referring now to Figure 1, there is illustrated a pair of fuel assemblies, generally designated 10, including an upper tie plate 12 and a fuel bundle channel 14 surrounding the tie plate and fuel rods within the fuel bundle. It will be appreciated that the upper tie plate 12 has a plurality of openings for receiving the upper ends of fuel rods and plugs for guide rods, the fuel rods and guide rods extending below the upper tie plate 12 to a lower tie plate, not shown. The fuel bundle channel 14 extends the full length of the fuel bundle between the upper and lower tie plates, encasing the fuel rods and enabling flow of water upwardly through the fuel bundles. The upper ends of the fuel bundles are situated in a reactor top guide 16, which is comprised of a lattice-type beam structure, typically encompassing four fuel bundles 10 disposed about a central (generally cruciform in plan) control rod indicated by reference numeral 18. The fuel bundle, including the upper and lower tie plates, fuel rods, water rods and guide rods, include an upwardly projecting handle 19 (Figure 2). As will be appreciated, the configuration of the upper and lower tie plates, as well as the fuel channel 14 and reactor top guide 16, are conventional in construction, except as otherwise noted. Channel and guide springs are used to locate the upper tie plate relative to the channel 14 and to bias the fuel assemblies 10 in the reactor top guide toward, and hence locate them relative to, the control rod 18. With that purpose, and referring to Figure 1, there is provided a plurality of generally elongated rectilinear slots at spaced locations about the margins of the upper tie plate and which slots extend linearly between the upper and lower surfaces of tie plate 12. As illustrated in Figure 1, two such slots 21 lie spaced from one another along the sides of the upper tie plate 12 remote from the center of the control rod 18. Also, a slot 23 is provided on each of the sides of the upper tie plate and which slots 23 lie closely adjacent the diagonally inner and outer corners of the fuel bundle. As illustrated in Figure 1, and by way of example, four channel guide springs 22 are disposed in slots 23 adjacent the diagonally inner and outer corners of the fuel assembly relative to the control rod 18. As discussed hereafter, the springs 22 are stressed and, hence, locate, e.g., center, the upper tie plate within channel 14. Also as noted in Figure 1, a pair of more robust but similarly shaped (as described hereinafter) guide springs 24 are provided along the outer sides of the upper tie plate 12 for projection through openings in channel 14 for bearing engagement against the interior surface of the reactor top guide 16. The location of springs 24 and the forces generated by their biased engagement against the reactor top guide 16 provide a net force in a diagonally inward direction toward control rod 18. This diagonally directed force is constrained by the interaction between adapter blocks 26 in the upper tie plate handle and short stub beams 28 attached to the midpoint of each side of the four-bundle core cell defined by the reactor top guide 16. Referring now to Figures 3A and 3B, there is illustrated a typical channel spring 22. Each channel spring 22 comprises a flat leaf spring having a main body 32, an upper portion 33 bent or deflected out of the plane of and to one side of spring body 32, as illustrated in Figure 3A, and a reversely-curved distal end portion 34. The outer surface 35 of upper portion 34 comprises a bearing surface for engaging an interior surface of the channel 14. The lower or base portion of spring 22 preferably includes three beams, i.e., a central beam 36 and a pair of outermost beams 38, although it will be appreciated that a pair of beams may suffice. Beams 36 and 38 lie in a common plane and the outermost beams 38 are spaced from the central beam 36 by slots 66. The slots 66 extend throughout the base portion of spring 22 substantially up to the point where the leaf spring deflects laterally outwardly. The beams 38 are movable from their unstressed, unloaded condition illustrated by the full lines in Figure 3B inwardly toward one another and the central beam 36 as illustrated by the dashed lines in Figure 3B. In this latter condition, the beams 38 are stressed and biased for outward movement. All such movements lie in the plane of the leaf spring. That is, the inward and outward movement of beams 38 relative to central beam 36 lies within the plane containing all of beams 36 and 38. A pair of stops 40 project laterally outwardly from beams 38, and in the plane of beams 36 and 38, to provide stops for engagement against the upper surface of the upper tie plate 12. At the lower or distal end of the outer beams 38, laterally outwardly projecting tabs 42 in the plane of the beams are also provided. The lower surfaces of tabs 42 are chamfered at 44 to facilitate insertion of the beams into the tie plate slots, as described below. It will be appreciated that the distance between the outer edges of the outermost beams 38 in their unstressed natural condition is equal to or slightly less than the width of the slots into which the springs are disposed in final assembly, whereas the stops 40 and tabs 42 have a lateral extent greater than the width of the slots. All springs 22 and 24 are preferably formed of a high nickel alloy, such as Inconel-X-750. Referring to Figures 4A and 4B, each top guide spring 24 is formed similarly as but is not identical to the channel spring 22. While the top guide spring 24 is formed of a similar material, it is a heavier or more robust construction, i.e., thicker and dimensionally larger. However, the base portions of the channel and top guide springs are configured identically, although dimensionally different. Particularly, the top guide spring includes a body portion 46 having an upper portion 48 deflected outwardly, terminating at one end in a reversely curved distal end portion 50 having an outer surface 52 for bearing against an interior surface of the reactor top guide 16. Similarly as the channel spring, the base portion of the top guide spring 24 includes a central beam 54 and a pair of outermost beams 56, which straddle and are spaced from the central beam 54 by slots 58. The outer beams 56 are movable from their unloaded, unstressed condition, illustrated by the full lines in Figure 4B, to an inwardly displaced position, illustrated by the dashed lines, and in the plane of spring 24 where the beams are stressed and loaded for laterally outward biased movement. The outer beams 56 also carry laterally outwardly projecting stops 60 intermediate their ends and lower tabs 62. The stops 60 and tabs 62 lie in the plane of the beams 54 and 56. Similarly as with the channel springs 22, the lower edges of the tabs 62 are chamfered at 64. In using the springs, the outermost beams 38 of the channel springs 22 can be squeezed together manually or by using a suitable tool, such as pliers, either directly or remotely, the proximal end of the spring is then inserted into the appropriate slot in the upper tie plate through the upper surface thereof. After inserting the spring into the slot, the externally applied compression of the outermost beams is removed and the spring is pushed downwardly into the slot until the bottom tabs 42 pass below the lower face of the tie plate. Because the outermost beams 38 are stressed upon passing through the slot, the beams will spring back to their unloaded, unstressed condition once the lower tabs clear the bottom surface of the tie plate. Consequently, the tabs 42 and stops 40 bear against the lower and upper surfaces, respectively, of the tie plate, retaining the spring in the tie plate. Importantly, the spring mounting after installation is in an unloaded and unstressed condition. Of course, upon locating the fuel assembly within the fuel assembly channel, the surfaces 35 of the springs 22 will bear against interior surfaces of the channel to locate the upper portion of the fuel assembly within the channel. Similarly, the top guide springs may be inserted into the appropriate slots in the tie plate by compressing the outermost beams 56 and pushing the springs through the slots until the tabs 62 emerge below the bottom tie plate. The beams 56 thus move back to their unloaded, unstressed condition, locking each spring to the tie plate between the stops 60 and tabs 62. When inserting the springs, the chamfered surfaces 44 and 64 facilitate the initial insertion of the springs into the slot. Upon locating the fuel assembly and channel within the top reactor guide structure, the surfaces 52 of springs 24 bear against interior surfaces of the guide structure to locate and bias the fuel assembly channel toward the control rod 18. As a result of this configuration of the springs, the maximum stress in the beams during spring insertion occurs at the top end of the spring slots away from the maximum stress area when the bundle is in the channel assembly. Importantly, the insertion stresses are not present in the fuel assembly after the springs have been locked into place. Further, the springs can be readily removed by using a tool such as pliers without damage to the upper tie plate or the spring. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: An apparatus for locating an upper tie plate (12) relative to a reactor top guide assembly (16) or a fuel bundle channel (14), in a nuclear reactor having fuel assemblies (10), the upper tie plate (12) and a fuel bundle channel (14) for receiving a fuel bundle of the fuel assemblies and upper tie plate (12), the reactor top guide assembly (16) forming a multi-bundle cell for the fuel assemblies, said apparatus comprising: a plurality of slots (21; 23) formed in the upper tie plate (12) adjacent margins thereof; springs (22; 24) disposed in said slots (21, 23), respectively, said slots and said springs being disposed about said tie plate (12) such that outer surfaces (35, 52) of said springs bear against an interior surface of one of said reactor top guide assembly (16) and said fuel bundle channel (14) to locate the upper tie plate (12) relative thereto; each said spring (22; 24) including a spring body (32, 46) having at least a pair of beams (38; 56) spaced one from another and cantilevered from said body in an unstressed condition and movable toward one another into a stressed condition, at least one of said beams (38; 56) having an outwardly directed tab (42; 62), said beams (38; 56) and said tab (42; 62) of each spring being inserted into respective slots (21; 23) with said beams (38; 56) movable toward one another into the stressed condition and movable away from one another into said unstressed condition upon passage of the tab (42; 62) through said slot, said tabs engaging the tie plate (12) to lock the springs (22; 24) to the tie plate (12) with the outer spring surfaces thereof projecting from said tie plate (12) to bear against an interior surface of one of said reactor top guide assembly (16) and said fuel bundle channel (14). Apparatus according to Claim 1 wherein said springs (22; 24) includes a first set of springs (22) having outer surfaces bearing against an interior surface of said fuel bundle channel (14), and a second set of springs (24) bearing against an interior surface of said reactor top guide assembly (16). Apparatus according to Claim 1 or 2, wherein each spring includes a tab (42; 62) carried by another of said pair of beams (38; 56), said tabs being engageable against said tie plate (12) to lock the spring (22; 24) to the tie plate (12). Apparatus according to Claim 1 or 2, wherein each said spring (22; 24) includes a central beam (36; 54) lying between and spaced from said pair of beams (38; 56), said pair of beams being movable toward said central beam (36; 54) upon said springs (22; 24) being inserted into said slots (21; 23) and into said stressed condition and away from said central beam into said unstressed condition upon passage of the tab through said slot. Apparatus according to Claim 1 or 2, including a stop (40; 60) on each said spring (22; 24) spaced opposite from said tab (42; 62) for engaging an upper surface of said tie plate (12) and locking the spring (22; 24) from movement toward the lower surface of said tie plate (12). Apparatus according to any one of claims 1 to 5, wherein each of said springs is a flat leaf spring. Apparatus according to Claim 6, wherein each spring (22, 24) is bent to provide an outwardly deflected portion including said outer spring surface thereof in an unstressed condition and movable inwardly into a stressed condition when bearing against the interior surfaces of said reactor top guide assembly (16) and said fuel bundle channel (14), respectively.

Inventors: CURULLA MICHAEL VINCENT, ELKINS ROBERT BRUCE, JOHANSSON ERIC BERTIL, MEFFORD CARL RAYFORD, SPELL JERRY THOMAS, ZULOAGA JR JAIME ANTONIA, CURULLA, MICHAEL VINCENT, ELKINS, ROBERT BRUCE, JOHANSSON, ERIC BERTIL, MEFFORD,CARL RAYFORD, SPELL, JERRY THOMAS, ZULOAGA JR.,JAIME ANTONIA
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1995
Main classification: G21C3
IPC codes: G21C3, G21C21