Patent Number: 054886449
Section: summary

TECHNICAL FIELD The present invention relates to spring assemblies, a combination of spring assemblies and adjacent ferrules for locating nuclear fuel rods in the ferrules and spacers comprised of the spring assemblies and ferrules, all for use in a nuclear reactor. BACKGROUND In nuclear reactors, for example, a boiling water reactor, nuclear fuel rods are grouped together in an open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. The fuel rods are supported between upper and lower tie plates in side-by-side parallel arrays. Spacers are employed at predetermined elevations along the fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation. A typical spacer for this purpose is disclosed in U.S. Pat. No. 5,002,726, of common assignee herewith. In that spacer, ferrules are arranged in a matrix, with each ferrule comprising a generally cylindrical element secured to an adjacent ferrule and having a generally C-shaped slot in the ferrule wall in registration with a corresponding slot in an adjoining ferrule. A closed-loop spring is disposed about the tabs of the C-shaped openings of adjoining ferrules and has medial portions projecting into the cylindrical spaces defined by the ferrules for engaging nuclear fuel rods within the adjacent ferrules and biasing the rods against stops adjacent opposite sides of the ferrules. While this ferrule arrangement has proven satisfactory, there are problems associated with springs and ferrules of this and other types. For example, the coolant flow in a nuclear fuel rod bundle is upward and consists of a steam/water mixture. High-velocity water droplets impinge on the bottom of looped springs of this type and may cause erosion of the spring material. Further, looped springs of this type are comparatively expensive to manufacture. The ferrules similarly are expensive to manufacture because of the complex shape of the side cutout and tab necessary to mount the spring. Additionally, "soft assembly" is difficult with the spring ferrule arrangement. "Soft assembly" is a process in which fuel rods having a coating or protective surrounding sleeves are inserted into the spacers. The presence of a coating or sleeve requires clearance between the bulbous end portions of the looped springs and the fuel rod and this is difficult to obtain with these looped springs. DISCLOSURE OF THE INVENTION The present invention minimizes or eliminates the foregoing and other problems associated with spring assemblies and ferrule combinations in spacers for nuclear fuel bundles and provides a novel and improved spring assembly, a spring assembly and ferrule combination and nuclear fuel rod bundle spacer having various advantages in construction, mode of operation and use. Particularly, the spring assembly in one embodiment of the present invention comprises two identical spring bodies stamped from sheet metal and disposed in back-to-back relation between adjacent ferrules. Each spring body lies in a plane in an unstressed condition and has a central leaf, a pair of outer leaves spaced from the central leaf and an end portion joining ends of the central and outer leaves at each of the opposite ends of the spring body. The central leaf includes an intermediate portion projecting forwardly of the plane of the spring body to a fuel rod contacting front side of the plane. The pair of outer leaves of each spring body have intermediate portions projecting rearwardly of the plane of the spring body. Preferably, the end portions of the spring bodies have tabs projecting laterally beyond the outer leaves and in a forward direction. The spring bodies may be secured to one another in back-to-back relation to one another by spot welds along the rearwardly projecting intermediate portions of the outer leaves. The joining of the spring bodies prior to assembly is for convenience in handling and assembling the springs and ferrules to form the spacer. The welds Carry no loads. The ferrules of the present invention are generally cylindrically shaped elements formed of sheet metal. Each ferrule has a generally rectilinear opening through a side wall of the ferrule. Pairs of stops are provided in the ferrule walls at locations generally opposite the rectilinear side wall opening. The ferrules are generally secured to one another by welding with the rectilinear openings in lateral registration with one another and spring assemblies therebetween. When the spring assemblies are disposed between adjoining ferrules, the intermediate portion of the central leaf on one spring body projects forwardly into the interior of a ferrule for biasing the fuel rod in that ferrule against the stops in that ferrule. Similarly, the intermediate portion of the central leaf of the other spring body projects into the interior of the adjacent ferrule to bias the fuel rod in that ferrule against stops in that ferrule. With this arrangement, the load path on the spring assembly is through the central leaf to the end portions and through the outer leaves to their contact areas with one another. Thus, the inner and outer leaves act as springs in series, providing a low spring constant (soft spring). There is also flexing in the end portions. With a soft spring as here, variations in dimensions have a small effect on the spring force so that tight tolerances are not required. It will be appreciated that with the foregoing design of the spring assembly and ferrule combination, the plane of the spring is parallel to the coolant flow so that water droplets impact essentially only on the end edges of the spring. Additionally, the spring may be stamped from sheet metal and formed, the stamping and forming operations being very simple processes. Further, the slot in the ferrule is a simple rectangle as compared with the complex shape of the slots of ferrules of prior designs. Additionally, the stiffness of the spring is low because each spring body consists of two springs in series. The deflection range is also quite large. As indicated previously, the prior art ends of looped springs are bulbous and therefore limit the space available for receiving coated or sleeved fuel rods. The present spring, however, does not have such space-limiting features. In a further form of the present invention, there is provided a spring for location between a pair of adjacent ferrules wherein one of the ferrules has a generally rectilinear opening through a side wall thereof which lies in opposition to a side wall of an adjacent ferrule. The spring includes a spring body lying in a plane having a central leaf with an intermediate portion projecting to a forward side of the spring. A pair of outer leaves are spaced laterally from the central leaf and project to the opposite or rearward side of a plane passing through the spring body. The outer leaves have intermediate projections which project further beyond the plane of the spring body. In use, the spring body is disposed between the adjacent pair of ferrules, with the central leaf located within the rectilinear opening with its intermediate projection engaging the fuel rod for biasing the fuel rod against stops at the opposite side of the ferrule. The outer leaves lie between the adjacent ferrules and the intermediate portions of the outer leaves bear against the outer surface of the adjacent ferrule, providing a spring reaction force whereby the central leaf biases the fuel rod in the one ferrule. To enable soft assembly of the fuel rods within a spacer comprising the springs of this latter form of the invention, the peripheral band about each spacer is provided with openings. Rods or pins are receivable within the openings for engaging the central leaf on its forward side and the edges of the rectilinear opening of the ferrule whereby the central projection can be displaced by compressing the central leaf in a direction away from the fuel rod. Consequently, the fuel rod may be inserted into the ferrule without interference from the projection of the central leaf of the spring. In a preferred embodiment according to the present invention, there is provided a spring assembly for location between a pair of adjacent ferrules in a nuclear fuel bundle having a plurality of nuclear fuel rods passing through the ferrules and biasing the fuel rods of adjacent ferrules against stops in the ferrules, the spring assembly in an unstressed condition, comprising first and second spring bodies lying in respective planes, each spring body having a central leaf, a pair of outer leaves spaced from the central leaf, and an end portion at each of opposite ends of the spring body joining ends of the central and outer leaves, the central leaf of each spring body having an intermediate portion projecting forwardly of the plane of the spring body to a fuel rod contacting forward side of the plane, the pair of outer leaves of each spring body having intermediate portions projecting rearwardly of the plane of the spring body. The spring bodies are disposed in back-to-back relation to one another with the intermediate portions of the outer leaves engaging one another, the planes lying parallel to and spaced from one another and the end portions lying in registration with one another whereby the intermediate portions of the central leaves project to opposite sides of the assembly for engagement with fuel rods in next-adjacent ferrules. In a further preferred embodiment according to the present invention, there is provided a spacer assembly for maintaining a matrix of nuclear fuel rods in spaced-apart relation between upper and lower tie plates, said spacer assembly comprising a matrix of ferrules for receiving the fuel rods in the spacer, each ferrule having a plurality of fuel rod contacting points for abutting the fuel rods, adjacent pairs of the ferrules in the matrix comprising cylindrical members each having an axis and open opposite ends, a generally rectilinear opening in a side wall thereof, the adjacent pairs of the ferrules being disposed in side-by-side relation to one another with the rectilinear openings in lateral registration with one another, with spring assemblies for location between the pairs of adjacent ferrules for biasing the fuel rods in the pair of ferrules into engagement with the contact points. Each spring assembly comprises first and second spring bodies lying in respective planes, each spring body having a central leaf, a pair of outer leaves spaced from the central leaf, and an end portion at each of opposite ends of the spring body joining the ends of the central and outer leaves, the central leaf of each spring body having an intermediate portion projecting forwardly of the plane of the spring body to a fuel rod contacting forward side of the plane, the pair of outer leaves of each spring body having intermediate portions projecting rearwardly of the plane of the spring body, the spring bodies being disposed in back-to-back relation to one another, with the intermediate portions of the outer leaves engaging one another, the planes lying parallel to and spaced from one another and the end portions lying in registration with one another. The spring assemblies are disposed between adjacent pairs of the ferrules with the intermediate portions of the central leaves lying in the rectilinear openings and the intermediate portions of the outer leaves lying between the adjacent pairs of ferrules. In a still further preferred embodiment according to the present invention, there is provided a spring for location between a pair of adjacent ferrules in a nuclear fuel bundle having a plurality of nuclear fuel rods passing through the ferrules and biasing a fuel rod of one of the adjacent ferrules against stops in the one ferrule, the spring assembly in an unstressed condition, comprising a spring body lying in a plane and having a central leaf, a pair of outer leaves spaced laterally from the central leaf, and an end portion at each of opposite ends of the spring body joining ends of the central and outer leaves, the central leaf of the spring body having an intermediate portion projecting forwardly of the plane thereof to a fuel rod contacting forward side of the plane, the pair of outer leaves of the spring body lying rearwardly of the plane of the body and having intermediate portions projecting rearwardly of the outer leaves. The spring body is adapted for disposition between the adjacent ferrules with the intermediate portion of the central leaf projecting through an opening in the one ferrule for biasing the fuel rod therein against the stops and the intermediate portions of the outer leaves bearing against an outer surface of another ferrule of the adjacent ferrules. In a still further preferred embodiment according to the present invention, there is provided a spacer assembly for maintaining a matrix of nuclear fuel rods in spaced-apart relation between upper and tie plates, said spacer assembly comprising a matrix of ferrules for receiving the fuel rods in the spacer, each ferrule having a plurality of fuel rod contacting points for abutting the fuel rods, adjacent pairs of the ferrules in the matrix comprising cylindrical members each having an axis, open opposite ends and a generally rectilinear opening in a side wall thereof, the adjacent pairs of the ferrules being disposed in side-by-side relation to one another with the rectilinear opening in the one ferrule lying in lateral registration with a side wall of another ferrule of the adjacent pairs thereof. A spring body is provided lying in a plane and having a central leaf, a pair of outer leaves spaced laterally from the central leaf, and an end portion at each of opposite ends of the spring body joining ends of the central and outer leaves, the central leaf of the spring body having an intermediate portion projecting forwardly of the plane thereof to a fuel rod contacting forward side of the plane, the pair of outer leaves of the spring body lying rearwardly of the plane of the body and having intermediate portions projecting rearwardly of the outer leaves. The spring body is disposed between the adjacent ferrules, with the intermediate portion of the central leaf disposed in the rectilinear opening for biasing the fuel rod in the one ferrule against the stops and the intermediate portions of the outer leaves disposed between the adjacent ferrules and engaged against an outer surface of another ferrule of the adjacent ferrules. Accordingly, it is a primary object of the present invention to provide a novel and improved spring assembly, spring assembly and ferrule combination, and a spacer comprising the spring assembly and ferrule combination having the aforesaid and other various advantages in construction, operation and use as compared with prior art spring assemblies, spring and ferrule combinations and spacers containing such springs and ferrules.