Patent Number: 046684662
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus and method for measuring the spring force imposed on a fuel rod when disposed through a cell in the support grid of the fuel assembly. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above the below the ends of the fuel rods. The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multipicity of interleaved straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the grid straps to impart strength and rigidity to the grid. In view that the opeation of the reactor core involves irradiation of the fuel rods, it is desirable that the supporting forces imposed by the grid on the fuel rods fall within such a range that the grid adequately supports the fuel rods but does not promote lengthwise distortion thereof over time. Also, in view that the grids are irradiated along with the fuel rods they support, the grids inherently degrade somewhat over time as a result. In particular, notwithstanding improvements in grid construction, such as illustrated and described in U.S. Pat. No. 4,474,730 issued Oct. 2, 1984 and assigned to the assignee of the present invention, over the life of the fuel assembly the metal of the interleaved straps forming the grid, and thereby the springs and dimples formed therein, are subject to stress relaxation due to irradiation. In light of the above conditions, it is necessary to initially carry out grid spring force measurements during the manufacture of the grid. Then, although the strap material does not ordinarily suffer a significant loss of resiliency when exposed to intense and prolonged irradiation, it is also desirable to carry out periodic checks of the forces imposed by the springs on the fuel rods during the life of the grid in order to monitor and detect the degree of degradation of the grid in terms of its ability to continue to provide adequate fuel rod support. One present technique for initially measuring grid forces during manufacture is to select a typical spring taken from a strip of formed strap material and characterize it by force/deflection curves. Other techniques include the use of a solid bar acting on a grid spring in a grid assembly to provide similar force/deflection curves which take into account some of the spring/cell boundary conditions. From the above techniques, grid spring forces for a given size fuel rod are interpolated from a set of curves based on statistically averaged values. However, with these techniques it is not possible to evaluate by direct means the interaction of the dimples and strap joints opposing the forces imposed by the grid spring, nor the flexibility of the grid strap as an individual cell wall of a total grid assembly containing numerous cells. Another techniques for measuring grid spring forces is illustrated and described in U.S. Pat. No. 4,246,783 to Steven et al. The Steven et al measuring device uses a flexible beam on the force measuring plug having a diameter equal to the diameter of the fuel rod to be inserted into the spacer gird. A strain gage is disposed on the flexible beam and electrically picks up bending of the flexible beam as the device is inserted into the grid cell. However, it is necessary to have available different plugs with the proper dimensions for measuring different sizes of fuel rod diameters and different spacer geometries. Consequently, a need exists for a technique to measure grid spring forces in a manner which more nearly replicates real-life conditions, that is to say, the actual forces experienced by fuel rods as they are supported within the cells of the grid by the springs and dimples and to be able to carry out such measurements with relative ease, speed and accuracy, SUMMARY OF THE INVENTION The present invention provides a grid cell spring force measurement apparatus and method designed to satisfy the aforementioned needs. The apparatus of the present invention includes a pair of hinged bars being adaptable to simulate fuel rods of different outside diameter sizes. The apparatus further utilizes a strain gauge attached to one of the bars to sense the force applied to the spring and electrical contacts on the bars which go from a "closed" to "open" condition at the instance the applied force becomes marginally greater than the spring reaction force causing the spring to deflect. Further, the apparatus electrically records the strain gauge reading at this instance and hence provides a determination of the spring force for the preselected fuel rod outside diameter. By repeating the procedure for different preset dimensions of the pair of bars of the apparatus, the spring rate of the spring/dimple cell system in the grid can be determined. In such manner, rapid readout of forces is obtained with minimum apparatus setup requirements and thus individual grid cells can be characterized with repeatability, speed and accuracy not previously attainable using prior techniques. Accordingly, the present invention is directed to an apparatus and method for measuring the spring force imposed on a fuel rod when disposed through a cell in a support grid of the fuel assembly which contains at least one spring-like element, comprising the operative steps of: (a) generating an increasing force at a first location external of the grid cell; (b) transferring the increasing force from the first location and applying the increasing force at a second location displaced from the first location and internal of the grid cell to the spring-like element disposed in the cell; and (c) measuring the level of the increasing force at the instance the application of the force causes deflection of the spring-like element to occur. More particularly, the spring force measurement apparatus, comprises (a) a pair of front and rear elongated members, each having a mid-section and upper and lower end portions extending in opposite directions from the mid-section, the members being pivotally connected together at their mid-sections such that as the upper end portions of the members being juxtaposed in spaced apart relation to one another are moved toward and away from each other the lower end portions of the members also being juxtaposed in spaced apart relation to one another are moved away from and toward each other; (b) means disposed on the lower end portion of at least one of the members and being operable to coact with the lower end portion of the other of the members to preset a minimum displacement between the members at the respective lower end portions thereof and thereby a minimum combined cross-sectional dimension of the members at their lower end portions such that when the lower end portions of the members are inserted into a grid cell they simulate a fuel rod disposed through the cell having a predetermined outside diameter; (c) means coupling the upper end portions of the members together and being operable to apply a progressively increasing force so as to draw the upper end portions toward one another and thereby, via the pivotal connection of the members, push the lower end portions apart from one another when deflection of the spring-like element positioned within the grid cell in engagement with at least one of the lower end portions of the members occurs; (d) means coupled to at least one of the members for sensing the level of the increasing force being applied to the spring-like element within the grid cell; (e) means coupled to at least one of the members for sensing when the deflection of the spring-like element occurs; and (f) means coupled to the force level sensing means and the spring deflection sensing means for indicating the level of force at the instance the deflection of the spring-like element occurs. Still further, the measuring apparatus comprises an adjustable stop attached to one of the members for engaging the grid so as to provide correct positioning of the lower end portions of the members in the grid cell for application of the increasing force to the spring-like element in the cell. Also, the measuring apparatus includes guide means coupled between the upper end portions of the elongated members to assist in maintaining alignment of the members with one another as they are pivotally moved relative to one another. Finally, the measuring apparatus has limit means connected to the upper end portion of one of the elongated members and extending transversely toward the upper end portion of the other of the elongated members for engagement therewith upon relative pivotal movement of the elongated members toward one another, the limit means being adjustable for presetting the minimum displacement between the upper end portions of the elongated members and thereby defining a maximum force which can be applied at the lower end portions of the elongated members to the sprng-like element in the grid cell. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.