Patent Number: 046684662
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views. 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 an elevational view of a fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. The fuel assembly 10 basically 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 ends of the guide thimbles 14. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conventionally 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 24 and the opposite ends of the rod are closed by upper and lower end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the pellets 24 to maintain the pellets in a tight, stacked relationship within the rod 18. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the nuclear reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods 32 are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Specifically, the top nozzle 22 includes a rod cluster control mechanism 34 having an internally threaded cylindrical member 36 with a plurality of radially extending flukes or arms 38. Each arm 38 is interconnected to a control rod 32 such that the control mechanism 34 is operable to move the control rods 32 vertically in the guide thimbles 14 to thereby control the fission process in the fuel aassembly 10, all in a well-known manner. Grid Cell Spring Force Measurement Apparatus For precisely maintaining the spacing between the fuel rods 18 in the reactor core and preventing both lateral and longitudinal movement thereof, the grids 16 are conventionally designed to impose spring forces on the fuel rods 18 directed from around the circumference of the individual rod radially inwardly toward the longitudinal axis of the rod. Referring now to FIGS. 2 to 4, it is seen that each of the grids 16 includes a multiplicity of interleaved inner straps 40 having an egg-crate configuration designed to form cells, indicated as 42, a majority of which individual accept one fuel rod 18 (for purposes of clarity only one cell 40 is shown in FIG. 2 with a fuel rod 18 disposed through it) and a minority of which accept one control rod guide thimble 14. The cells 42 of each grid 16 which accept and support the fuel rods 18 at a given axial location therealong typically use relatively resilient springs 44 and relatively rigid protrusions or dimples 46 formed into the metal of the interleaved inner straps 40 to generate the spring forces needed to hold the fuel rods therein. Also, the inner straps 40 are generally flexible such that they bow somewhat when the fuel rods 18 are disposed through the grid cells 42. In the illustrated embodiment, there are two springs 44 on two adjacent sides of each cell 42 containing a fuel rod 18 and two dimples 46 on each of two adjacent sides of the cell facing each spring. The springs 44 and dimples 46 of each grid cell 42 frictionally engage or contact the respective fuel rod 18 extending through the cell. Additionally, outer straps 48 are attached together and peripherally enclose the grid inner straps 40 to impart strength and rigidity to the grid 16. Thus, the actual spring force imposed on a given fuel rod 18 results from interaction with one another of the resilient springs 44, rigid dimples 46 and flexible interleaved straps 40 comprising the cell 42 which receives the fuel rod. In order to properly characterize the holding capability of an individual grid 16, it is this spring force that must be measured. Turning finally to FIGS. 5 to 7, for measuring the spring force, resulting from the combined action of the system of resilient springs 44, rigid dimples 46 and flexible interleaved grid straps 40, imposed on a given fuel rod 18 when disposed through one cell 42 in one of the support grids 16 of the fuel assembly 19, the present invention provides a grid cell force measuring apparatus, generally designated 50. The measuring apparatus 50 includes a pair of front and rear elongated members 52,54 having respective mid-sections 56,58 and respective upper and lower end portions 60,62 and 64,66 extending in opposite directions from the respective mid-sections 56,58. The members 52,54 are pivotally connected together at their mid-sections 56,58 such that as the upper end portions 60,64 of the members, being juxtaposed in spaced apart relation to one another, are moved toward and away from each other the lower end portions 62,66 of the members, also being juxtaposed in spaced apart relation to one another, are moved away from and toward each other. More particularly, the elongated members 52,54 of the measuring apparatus 50 take the form of a pair of front and rear bars. The upper and lower end portions 64,66 of the rear bar 54 extend in opposite directions from the mid-section 58 thereof and in generally linear alignment with one another, whereas the upper and lower end portions 60,62 of the front bar 52 extend in opposite directions from the mid-section 56 thereof but in a transversely offset relationship. Due to such offset relationship, the upper end portion 60 of the front bar 52 is spaced farther or remote from the upper end portion 64 of the rear bar 54 while the lower end portion 62 of the front bar 52 is spaced closer or adjacent to the lower end portion 66 of the rear bar 54. Additionally, a pair of transversely spaced tabs 68,70 are attached to the rear bar 54 at its mid-section 58 and extend generally parallel to one another and outwardly from a side thereof facing the front bar 52. The tabs 68,70 have respective aligned holes 72,74 defined therethrough, whereas a hole 76 is defined through the midsection 56 of the front bar 52. A pivot pin 78 extends through the aligned holes 72,74 in the spaced tabs 68,70 on the rear bar 54 and through the hole 76 in the front bar 52 so as to mount the front bar on the tabs for pivotal movement relative to the rear bar. Further, the measuring apparatus 50 includes force generating means, generally designated 80, coupling the upper end portions 60,64 of the front and rear elongated bars 52,54 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 bars, push the lower end portions 62,66 apart from one another. In particular, the force generating means 80 includes a shaft 82 rotatably connected to one of the upper end portions 60,64 of the bars 52,54, such as the upper end portion 60 of the front bar 52, and threadably connected to the other thereof, such as the upper end portion 64 of the rear bar 54. A knob 84 is attached to an end of the shaft 82 disposed adjacent the front bar 52 for facilitating rotation of the shaft through manual turning of the knob in either of two opposite directions in order to move the upper end portions 60,64 of the elongated bars 52,54 toward and away from each other. Thus, the rotatable shaft 82 and knob 84 of the force generating means 80 are used to generate an increasing force at a first location along the elongated bars 52,54 which will be external of the given one grid cell 42 when the lower end portions 62,66 of the bars 52,54 are inserted in the cell to the position seen in FIG. 7 for carrying out the measuring procedure. The elongated bars then serve as means for transmitting that increasing force from the first location therealong and applying the force at a second location displaced from the first location and internal of the one grid cell 42. The elongated bars 52,54 of the measuring apparatus 50 can be adjusted to simulate fuel rods of various diameters. Toward this end, means in the form of a set screw 86 is attached to the lower end portion 62 or 66 of the one of the elongated bars 52,54 and can be adjusted to preset the displacement between the lower end portions of the bars. Specifically, by rotating the set screw 86, which in the illustrated embodiment is threadably attached to the lower end portion 66 of the rear bar 54, and by simultaneously adjusting the shaft 82 on the upper end portion 60 of the front bar 52, the set screw 86 is operable to coact with the lower end portion 62 of the front bar 52 to preset a minimum displacement between the bars at the respective lower end portions thereof and thereby a minimum combined cross-sectional dimension of the bars at their lower end portions. Then, when the lower end portions of the bars are inserted into a given one grid cell 42, such as seen in FIG. 7, they will simulate a fuel rod 18 disposed through the cell having a predetermined outside diameter. Other features of the measuring apparatus 50 comprise an adjustable stop 88, guide means 90 and limit means 92. The adjustable stop 88 includes a strip 94 having an elongated slot 96 and being attached to one of the bars, such as the mid-section 58 of the rear bar 54 along the rearwardly facing side thereof, by a bolt 98 inserted through the slot. The strip 94 extends downwardly a short distance and has a lower terminal end 100 for engaging the top of a grid strap 40, as seen in FIG. 7, to provide correct positioning of the lower end portions 62,66 of the bars 52,54 in the one grid cell 42 for applying the increasing force to one of the springs 44 in the cell. The position of the terminal end 100 of the strip 94 can be vertically adjusted by untightening the bolt 98 and then sliding the strip 94 relative thereto. The guide means 90, being coupled between the upper end portions 60,64 of the front and rear bars 52,54 to assist in maintaining alignment of the bars with one another as they are pivotally moved relative to one another, includes a guide pin 102 and a guide bore 100. The guide pin 102 is anchored in the upper end portion 60 or 64 of one of the elongated bars 52,54, such as the rear bar 54, and extends transversely toward the upper end portion of the other elongated bar, such as the front bar 52. The guide bore 104 is formed through the upper end portion of the other of the elongated bars, such as the front bar 52 for slidably receiving the guide pin 96 therethrough as the bars 52,54 are pivotally moved relative to one another. The limit means 92, being a set screw threadably received through the upper end portion of one of the elongated bars, such as the front bar 52, extends transversely toward the upper end portion of the other of the elongated bars, such as the rear bar 54 for engagement therewith upon relative pivotal movement of the bars toward one another. The set screw 92 is adjustable for presetting the minimum displacement between the upper end portions 60,64 of the elongated bars 52,54 and thereby defining a maximum force which can be applied at their lower end portions 62,66 to the resilient spring 44 in the grid cell 42. Finally, the grid force measuring apparatus 50 includes means for sensing and recording the spring force of the grid cell 42. First, means in the form of a strain gauge 106 is attached to the lower end portion of one of the elongated bars, such as the front bar 52, a short distance below its mid-section 56. The strain gauge 106 senses the level of the increasing force being applied to the spring 44 within the one grid cell 42 into which the bars 52,54 are inserted. Although the bars 52,54 are made of metal, the lower end portion 62 of the front bar 52 is thin enough in cross-section so as to have sufficient flexure to give a meaningful strain gauge readout. Also, the measuring apparatus 50 has means, generally designated 108, in the form of a pair of electrical contacts being coupled between the lower end portions 62,66 of the front and rear bars 52,53 and capable of breaking contact with one another when application of the increasing force to the spring 44 in the cell 42 causes deflection of the spring to occur. The pair of electrical contacts 108 includes a circuit element in the form of an electrically conducting plate 110 attached to and electrically insulated from the lower end portion of one of the elongated bars, such as the front bar 52, and the set screw 86 attached to the lower end portions of the other of the elongated bars, such as the rear bar 54. As mentioned earlier, one function of the set screw 86 is to preset the displacement between the lower end portions 62, 66 of the elongated bars 52,54. The second function is to provide electrical contact with the circuit element 110 when the elongated bars 52,54 are initially inserted into the grid cell 42. Lastly, as seen in FIG. 7, the measuring apparatus 50 has means in the form of a readout 112, preferably a suitable digital type, coupled across the terminals 114,116 of the strain gauge 106, being preferably in a bridge arrangement, and in parallel with the contacts 108. The readout 112 indicates the level of force at the instance deflection of the spring 44 occurs. Specifically, a pair of lead lines 118,120 couples the readout in series with the strain gauge terminals 114,116. However, the contacts 108, i.e., the circuit element 110 and set screw 86, are also connected by lead lines 122,124 in series with the strain gauge terminals. When the contacts 108 are closed, the terminals 114,116 of the strain gauge 106 are effectively short circuited and no force level signal is conducted to the readout 112. However, when the force applied to the grid cell spring 44 is marginally greater than the spring force, the spring 44 deflects and the electrical contacts 108 go from a closed to an open condition. Then the strain gauge terminals 114,116 are no longer shorted and a signal is generated by the strain gauge 106 and received and recorded by the readout 112 at the instance the contacts 108 are opened. This provides a determination of the spring force for the preselected fuel rod 18 outside diameter. In FIG. 7, the pivotally connected elongated bars 52,54 of the measuring aparatus 50 are shown placed vertically in the grid cell 42 to be measured, with the rear bar 54 in contact with both grid dimples 46 and the front bar 52 in contact with the grid spring 44, With the bars preset by the set screw 86 at the selected fuel outside diameter dimension and rotatable shaft 82 in a loosened condition, the depth of insertion into the grid cell is preset by the adjustable positioning stop 88. By turning the knob 84, the shaft 82 is gradually tightened until electrical contact between the set screw 86 and the insulated conducting circuit element 110 is broken. The contact break is monitored electrically such that at the instance the break occurs the strain gauge 106 reading is recorded by readout 112. This reading determines the spring force for one spring 44 and its associated pair of dimples 46 at the preset dimension across the bars 52,54. The same steps are repeated to measure the spring force for the other spring and pair of dimples in the same cell. The two readings are added together to get the total spring force in the given cell. By increasing or decreasing the preset dimension between the bars 52,54 by, for example, increments of two thousands of one inch, the bars may be reinserted into the cell 42 and the spring force again determined. Using differences in spring force for the incremental changes in preset dimensions, the spring rate characteristic of the cell can be derived over the total required range. 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.