Patent Number: 054886449
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, there is illustrated a nuclear fuel assembly, generally designated 20, including a plurality of fuel elements or rods 21 supported between an upper tie plate 22 and a lower tie plate 23. Fuel rods 21 pass through a plurality of fuel rod spacers 24a and 24b which provide intermediate support to retain the elongated rods 21 in spaced relation and to restrain them from lateral vibration. In one embodiment, seven spacers are elevationally evenly spaced along the fuel assembly. While an 8.times.8 array of fuel rods is disclosed, it will be appreciated that the invention hereof is applicable to arrays of different numbers, for example, matrices of 10.times.10 arrays. Each fuel rod 21 is formed of an elongated tubular cladding material with the nuclear fuel and other materials sealed in the tube by upper and lower end plugs 26 and 27, respectively. The lower end plugs 27 are registered and positioned in bores formed in the lower tie plate 23 for supporting the fuel bundle. The upper end plugs 26 are disposed in cavities 31 in the upper tie plate 22. Typically, the fuel rod assembly includes a channel 33 of substantially square cross-section sized to form a sliding fit over the upper and lower tie plates 22 and 23 and the spacers 24 so that the channel 33 may readily be remounted and removed. The lower tie plate 23 includes a nose piece 38 adapted to support the fuel assembly 20 in a socket in a core support plate (not shown) in the reactor pressure vessel. The end of the nose piece is formed with openings 39 to receive coolant/moderator for flow upwardly along and among the fuel rods. Turning now to FIG. 2, there is illustrated a spacer constructed in accordance with the present invention having the spring assemblies and ferrule combinations hereof. The spacer is comprised of a plurality of cylindrical ferrules 50 and spring assemblies 52, both of which are described in detail hereinafter. Each spacer 24 thus comprises a plurality of ferrules 50 arranged in a square matrix in which each ferrule receives a fuel rod and maintains the fuel rods spaced and restrained relative to one another. The spring assemblies 52 bias the fuel rods in a lateral direction against stops 54 whereby the fuel rods are maintained in a predetermined position relative to one another and in the spacer 24. Referring now to FIGS. 3-7, the spring assembly 52 will be described. Referring to FIG. 3, each spring assembly 52 includes two identical spring bodies, one of which is illustrated in FIG. 3. Thus, in FIG. 3, spring body 56a includes, in an unstressed condition, a central elongated leaf 58, a pair of outer leaves 60 straddling central leaf 58 and spaced laterally therefrom, and end portions 62 joining the ends of the central and outer leaves 58 and 60, respectively, at opposite ends of the spring body 56a. Central leaf 58 includes an arcuate projection 64 intermediate its ends and preferably medially thereof. Central leaf 58 also includes leg portions 66 which join the intermediate projection 64 and the end portions 62. Outer leaves 60 include intermediate, preferably medial, portions 68 which project in a direction opposite from the central portion 64 of the central leaf 58. Outer leaves 60 also include leg portions 70 joining the central portions 68 and the end portions 62 to one another. As best seen in FIG. 5, the leg portions 66 of central leaf 58 and leg portions 70 of outer leaves 60, as well as the end portions 62 of the spring body 56a lie in a common plane, designated A--A in FIG. 5 fin an unstressed condition of the spring assembly. Thus, the arcuate projection 64 of the central leaf 58 projects to a forward side of the plane A--A. More particularly, central portion 68 projects outwardly of the plane A--A of the spring body to a fuel rod contacting front side of the plane as described hereinafter. Conversely, the projecting intermediate portions 68 of the outer leaves 70 project rearwardly to the opposite side of the plane A--A, as best illustrated in FIG. 5. Also as indicated upon comparison of FIGS. 3 and 5, end portions 62 have end tabs 74 which project to the forward or fuel rod contacting front side of the plane A--A for overlying the outer surface of a ferrule, as described below. As best illustrated in FIGS. 4 and 5, it will be appreciated that the spring bodies 56a and 56b are disposed in back-to-back relation one to the other. For convenience in handling the spring assembly and assembling the spring bodies and ferrules to form a spacer, the spring bodies 56a and 56b are secured in back-to-back relation, preferably by spot welds 75 along the intermediate portions 68 of outer leaves 70 which abut one another in assembly. In such assembly, the arcuate projections 64 of the central leaves project to opposite sides of the spring assemblies, i.e., toward adjacent fuel rods in adjacent ferrules, as described below. The intermediate portions 68 of the outer leaves 60 lie in back-to-back relation one to the other. With this configuration, the load applied to the springs through the arcuate projections 64 follows a load path through the end portions 62 and through the outer leaves 60 to the back-to-back contact area of projections 68. The central and outer leaves thus act as springs in series, providing a low spring constant, i.e., a soft spring, while affording some flexing in the end portions 62. Particularly note that in the assembly, the plane of each spring body lies essentially parallel to the direction of coolant flow, thus presenting only edges of the spring assembly to the flow. Also, there is only one projection, i.e., the central projection 64 of central leaf 58 which lies in the path of fuel rod insertion into the ferrule. FIG. 8 represents the configuration of the spring assembly when in a stressed condition in use. Referring now to FIGS. 9 and 10, there is illustrated a ferrule 50 according to the present invention. Particularly, ferrule 50 includes a generally cylindrical or tubular element 80 open at its opposite ends and having a rectangular or rectilinear opening or aperture 82 formed through a side wall thereof. As will be appreciated from a review of FIG. 2, the ferrules are secured to one another in a rectilinear array of ferrules, generally a square matrix thereof, and may be coupled one to the other, for example, by welding. In FIG. 10, the spring body 56a is illustrated in position within the opening 82 in bearing engagement against the fuel rod, it being appreciated that, in this form of the invention, only one-half of the spring assembly is illustrated. It will be seen with respect to FIG. 10 that the arcuate projection 64 extends into the ferrule 80 through the rectilinear slot 82 for engagement with the fuel rod 21, biasing it against the stops 54 at the opposite side of the ferrule. The projections 68 lie generally outside the cylindrical confines of the ferrule 80 and the tabs 74 overlie the outer surface of ferrule 80. Referring to FIG. 11, the spring assembly of FIG. 4 is illustrated between adjoining ferrules 80. Thus, as illustrated, the intermediate portions 68 of the outer leaves 60 of the back-to-back disposed spring bodies 56a and 56b are welded one to the other and lie outside of the confines of both ferrules. The spring projections 64, of course, lie within the confines of the ferrules and engage fuel rods 21 within the ferrules 80. The tabs 74 overlie the outer surfaces of the ferrules and locate the spring assembly laterally. The ferrules 80 are secured to one another, preferably by welding. With this spring assembly and ferrule combination disposed in the spacer, for example, as illustrated in FIG. 2, it will be appreciated that the planes A--A of the spring assembly bodies lie generally parallel to the coolant flow so that water droplets impinge on the edges of the spring. The spring bodies are advantageously stamped from sheet metal, while a rectilinear or rectangular slot 82 is formed in the ferrule 80, thus reducing manufacturing costs. Note also that the stiffness of the spring assembly is low because each half of the spring assembly consists of two springs in series. The deflection range is large and, with the exception of the projections 64, the spring assemblies do not have space limiting features. Referring now to the embodiment hereof illustrated in FIGS. 12-18, there is provided a spring formed of sheet metal which acts on a single fuel rod. As illustrated in FIGS. 12-14, the spring includes a spring body 88 having a central leaf 90 joined at opposite ends to end portions 92. A pair of outer leaves 94 are laterally spaced from and straddle the central leaf 90, the outer leaves likewise being connected at their opposite ends to the end portions 92. The central leaf 90 has an arcuate projection 96, preferably medially of the spring, and which projects to a forward side of the spring for engagement against a fuel rod. The spring also includes outer leaves 94 which are projected out of the plane of the spring to a rearward side of the spring as illustrated in FIG. 13. The outer leaves 94 also have a rearwardly extending intermediate, preferably medial projections 98 for bearing against the outer surface of the adjoining ferrule in the spacer. Referring now to FIG. 15, there is illustrated a ferrule 100 comprised of a generally cylindrical element open at its opposite ends and having a generally rectangular or rectilinear opening or aperture 102 through a side wall thereof. Ferrules of this type are arranged in the spacer as illustrated in FIGS. 16 and 18, with the rectangular slot in opposition to a solid wall portion of the adjoining ferrule. As illustrated in FIG. 16, the central portion of the spring 88 lies inwardly of the rectangular slot 102 of ferrule 100, with the medial projection 96 projecting inwardly to engage the fuel rod within the spacer. The end portions 92 bear against the side margins of the rectilinear opening 102. The rearwardly projecting portions 98 of the outer leaves 94 bear against the outer surface of the adjoining ferrule 100. With the ferrules 100 secured to one another, for example, by welding, it will be appreciated that the spring 88, similarly as in the first embodiment, provides a load path through the central leaf 90 to the end portions 92 and through the outer leaves 94 to the contact area between the projections 98 and the outer surface of the adjoining ferrule. Thus, the central and outer leaves act as springs in series, providing a low spring constant. To facilitate insertion of the fuel rods into the ferrules of the spacer, a bar or pin 106 can be inserted between the spring 88 and the ferrule 100, as illustrated in FIG. 17. Thus, the pin 106 may be employed to compress the spring and hence deflect the projections 96 and 98 more towards the central plane of the spring whereby the central portion 96 of the center leaf is spaced back from the position of the fuel rod within the ferrule. By spacing the central spring projection from the fuel rod, the fuel rod may be readily inserted into the spacer, without any force being applied thereto by the spring. A complete fuel rod insertion operation is illustrated in FIG. 18, wherein it will be seen that the pins 106 can be inserted through suitable openings in the peripheral bands surrounding the spacer. Also, it will be noted that the pins 106 compress more than one spring and typically several springs. The pins 106, of course, are withdrawn from the assembly once the fuel rods are inserted into the spacers. 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 spirit and scope of the appended claims.