Abstract:
A secure connection between the bottom nozzle of a nuclear fuel assembly and the control rod guide thimbles having two or four crimped pockets in the side wall of a bottom nozzle leg counterbore which are formed by drilling two or four blind holes through the top surface of the bottom nozzle. After attaching the bottom nozzle to the fuel assembly with a crimp bolt a pneumatic tool is then inserted into a flanged cup feature on the crimp bolt head and the flanged portion of the cup feature of the crimp bolt is deformed into the crimp pockets. The crimp pockets are contained within the leg counterbore and prevent the screw from rotating or falling out of the assembly in the event of preload loss or screw fracture.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to an improved connection between the lower nozzle of the fuel assemblies and the control rod guide thimbles. 
         [0003]    2. Background Information 
         [0004]    In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by structural frame works referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower traversely extending core support plates. These upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof. In the most common configuration, the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically resting on the lower support plate. Generally, in most reactors, a fluid coolant such as water, is directed upwardly through apertures in the lower core support plate and along the various fuel assemblies to receive the thermal energy therefrom. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles; thereby forming an integral fuel assembly. The respective top and bottom nozzles extend slightly above and below the ends of the fuel rods, capturing the rods therebetween. 
         [0005]    Generally, in each fuel assembly, there are a number of grids axially spaced along the fuel assembly length and traversely extending across the assembly. Convention designs of these grids include a plurality of interleaved straps of egg-crate configuration designed to form a plurality of cell openings, with each cell opening adapted to receive therethrough one of the fuel rods. A peripheral strap, being of the same height of the interleaved straps, encloses the interleaved straps to impart strength and rigidity to the grid. The purpose of these grids is twofold. One purpose is for the lateral support or positioning of the fuel rods so as to prevent localize neutron flux peaking and thereby permit operation of the reactor closer to its design power limit. The other purpose is for the mounting of deflecting vanes to promote mixing of the upwardly flowing coolant along the fuel rods to average the enthalpy/temperature rise for maximizing the power output of the reactor core. Normally, for lateral support or positioning of the fuel rods, each cell opening includes an arrangement of spring fingers and dimple protrusions which provide a six point contact of the fuel rods. For deflecting the coolant flow, some or each of the cell openings of the grids are provided with cantilevered deflecting vanes for deflecting the coolant. All of these prior art grids extend completely across the fuel assembly and separately surround each of the fuel rods contained in the assembly. Furthermore, the construction of each of these grids is such that its outer peripheral strap is of a height equal to a height of its inner straps. 
         [0006]    The power output of a nuclear reactor is limited by the rate at which heat can be removed from the reactor core, and the rate of heat transferred determines the temperatures developed in a reactor core. Therefore, the maximum reactor operating power is limited by some enthalpy and/or temperature value in the reactor core. The variation of the neutron flux in the reactor core causes the fuel assemblies in the core to operate at different power levels, and this variation occurs even among the fuel rods within a single fuel assembly. The reactivity and, in-turn, the power output of a nuclear reactor is limited by the amount of structural material in the reactor core, as the structure material parasitically absorbs neutrons which could otherwise be used in the fission process. Furthermore, a reduction of the structural material in the fuel assembly reduces the pressure drop and thereby increases the power output. Still further, it is well known, that the burn up rate for the different fuel rods contained in a given fuel assembly varies. And still further, the output of the given fuel assembly can be enhanced by the use of different fissionable materials, as well as, by the amount of fissionable material, such as through the use of different diameter sized fuel rods. With these considerations in mind, designers are constantly striving to improve upon the power output of the various fuel assemblies which make up a core to increase the total power output of the reactor, while at the same time, striving to improve on the construction of the assemblies so as to facilitate the assembly of the fuel assembly and to reduce the repair and maintenance cost associated with operating the reactor. Some of these costs are associated with the fixed components of the fuel assembly forming the fuel assembly skeleton. The skeleton comprises essentially the upper and lower nozzle with the guide thimbles and instrumentation tube rigidly connected therebetween and the spaced, tandem arrangement of grids fixedly connected to the guide thimbles between the upper and lower nozzle. Currently, in one design of a fuel assembly, the control rod guide thimbles are secured to the lower nozzle through a screw that extends through the lower nozzle into the guide thimble. The guide thimble screw is retained in position by a circular locking disc that mates with the slot in the thimble screw head and is welded to the inside of the bottom nozzle leg counterbore through which the screw extends. The welding procedure requires the ability of a skilled operator and inspection of the finished product is very difficult because of the deep recess in the bottom nozzle leg counterbore. This weld procedure is performed after the fuel assembly is loaded with fuel rods, thus raising the level of difficulty in completing the task as well as increasing the difficulty of recovery should an improper weld be performed. 
         [0007]    Accordingly, an improved attachment mechanism is desired to secure the control rod guide thimble to the bottom nozzle that does not require welding. 
         [0008]    Furthermore, a new attachment mechanism is desired that will fasten the control rod guide thimbles to the bottom nozzle that can easily be re-worked if a defective connection is identified. 
         [0009]    Additionally, a new connection between the bottom nozzle and control rod guide thimbles is desired that requires fewer components and can be more easily inspected. 
       SUMMARY OF THE INVENTION 
       [0010]    Thus, in view of the above mentioned design considerations, the present invention provides a nuclear reactor fuel assembly with an improved control rod guide thimble to lower nozzle connection that has fewer components and is easier to implement, re-work and inspect. 
         [0011]    The connection is formed by an elongated fastener bolt having a head and a shank. The shank of the fastener bolt extends from the head concentric with the axis of the elongated dimension of the fastener bolt and has a profile designed to engage a female latch profile on the inside of the mating end of the control rod guide thimble. The upper surface of the fastener bolt head is fitted with a torque contour recess such as a drive hex that can be engaged by a torque tool to place the shank in engagement with the female latch profile on the interior of the control rod guide thimble. The upper surface of the fastener bolt head also has a raised, deformable rim. To secure the connection between the bottom nozzle and the control rod guide thimble the shank is inserted through a hole in the bottom nozzle to engage the female latch profile in the control rod guide thimble and fasten the control rod guide thimble to the bottom nozzle. At least a portion of the deformable rim on the head of the fastener bolt is crimped outwardly of the central axis of the fastener bolt to engage a recess in the bottom nozzle that locks the fastener bolt from disengaging from the female latch profile of the control rod guide thimble. 
         [0012]    In one preferred embodiment the female latch profile is a female thread on the interior of the bottom end of the control rod guide thimble that mates with a male thread on the shank of the fastener bolt. In another preferred embodiment the bottom nozzle has an underside portion that has a series of counterbores of varying diameters that are in line with the control rod guide thimble fastened to the bottom nozzle. The underside portion aligned with the control rod guide thimble, has a first counterbore having the first diameter at least as large as the diameter of the head of the fastener bolt. The first counterbore extends up into the bottom nozzle, parallel to the axis of the fuel assembly a first distance with the central line of the counterbore aligned with the axis of the control rod guide thimble. A second counterbore extends coaxially with the first counterbore from the first distance to a second distance wherein the second counterbore has a diameter at least as large as the diameter of the head of the fastener bolt. A third counterbore extends coaxially with the second counterbore from the second distance to a third distance. The third counterbore has a nominal diameter at least as large as the diameter of the fastener bolt head with portions of the circumference of the third counterbore having a larger diameter than the second counterbore, wherein the intersection of the portions of the second counterbore and the portions of the circumference of the third counterbore having the diameter larger than the second bore forms a ledge and the first distance, second distance, and the third distance are collectively less than the height of the lower nozzle. A fourth bore extends coaxially with the third counterbore from the third distance through an upper surface of the bottom nozzle, in line with the control rod guide thimble fastened to the bottom nozzle. The diameter of the fourth bore is smaller than the diameter of the head of the fastener bolt and the crimped portion of the deformable rim and the head of the fastener bolt are captured between the ledge and the fourth bore. 
         [0013]    In another preferred embodiment, the portions of the circumference of the third counterbore having a diameter larger than the second counterbore are formed by spaced drilled holes around the outer circumference of the fourth bore, that extend into the surface of a wall of the third counterbore and form the recesses in the bottom nozzle that locks the fastener bolt from disengaging from the female latch profile of the control rod guide thimble. Preferably, the holes comprise a pair of diametrically spaced holes positioned around the outer periphery of the fourth bore that extend into the surface wall of the third counterbore. The holes are preferably spaced ninety degrees apart from each other around the periphery of the fourth bore. Most desirably the counterbore is formed in a leg of the bottom nozzle and the fastener bolt is provided with a drain hole extending through the head and shank along its major axis. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  is a perspective view of a conventional fuel assembly incorporating the improved connection between the bottom nozzle and the control rod guide thimble of this invention; 
           [0016]      FIG. 2  is a cross sectional view of the bottom nozzle leg with the control rod guide thimble crimp screw of this invention installed; 
           [0017]      FIG. 3  is a top view of the upper plate of the bottom nozzle at a location just above a corner leg, which shows the various diameters of the counterbores through the leg; 
           [0018]      FIG. 4  is a perspective view of the crimping tool employed to crimp the bolt head rim of this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to the drawings, particularly to  FIG. 1 , the fuel assembly  10  is shown comprising an upper end structure on nozzle  12 , a lower end structure on nozzle  14 , a generally square array of substantially parallel elongated fuel elements or rods  16 , a plurality of elongated hollow tubular members  18  containing control elements  20  strategically located among the array of fuel element  16 , a top grid member  22 , a bottom grid member  24 , and a plurality of intermediate grid members  26  longitudinally spaced along the tubular members  18 , which are also referred to as control rod guide thimbles or guide tubes. An instrumentation tube is located at the center of the fuel assembly, although not shown. 
         [0020]    As shown in  FIG. 1  the upper end nozzle  12  is a generally square shaped plate having elongated coil springs extend upwardly in an axial direction. In the reactor core the springs  28  are seated against the upper core plate and function to hold down the fuel assembly while permitting axial growth of the fuel assembly with changes in temperature within the core. During operation of the reactor, a coolant is pumped upwardly from the lower support plate up through perferations in the bottom nozzle  14 , over the fuel rods  16 , through the top nozzle  12  and out the upper core plate not shown. The springs  28  hold down the assembly against the lower core support plate under the influence of these hydraulic forces. 
         [0021]    The bottom nozzle  14  is also generally square in shape and has a plurality of openings therein to permit the circulation of the reactor coolant. The upper ends and lower ends of the tubular control rod guide thimbles  18  are rigidly connected to the top nozzle  12  and the bottom nozzle  14 , respectfully. The connection to the bottom nozzle is particularly difficult due to the alignment of the control rod guide thimbles with legs  15  extending from the underside of the bottom nozzle square shaped plate  14 . 
         [0022]    As shown in  FIG. 1 , each one of the grid structures comprises a plurality of straps which are interleaved to provide a structural network similar to an “egg-crate”, that forms a plurality of generally square opening or cells through which the fuel rods  16  and the control rod guide thimbles  18  pass. The grid straps are provided with resilient fingers which engage the fuel rod  16 . The guide thimbles are rigidly connected to the grid cells through which they pass and the straps generally prevent lateral displacement of the fuel rods  16  and control rod guide thimbles  18 . 
         [0023]    As stated above, the hollow tubes  18  serve as guide thimbles for the cylindrical control elements or rods  20 . In order to reduce the quantity of high neutron capture material in the fuel assembly, the control rod guide thimbles  18  are composed of a relatively low neutron capture material, such as a zirconium based alloy known as Zircaloy. The basic fuel assembly structure consist of a skeleton made up of individual Zircaloy control rod guide thimbles held by Inconel or stainless steel grids with stainless steel nozzles at the top and the bottom of the structure. 
         [0024]    In the past, one means of attaching the bottom nozzle  14  to the lower end of the control rod guide thimbles  18  is described in U.S. Pat. No. 3,992,259 issued Nov. 16, 1976. The current design for retaining the guide thimble screw in one type of fuel assembly design that is currently employed commercially, consists of a circular locking disc that mates with a slot in the thimble screw head and is welded to the inside of the bottom nozzle leg counterbore. The welding procedure requires the ability of a skilled operator and inspection of a finished product is very difficult because of the deep recess in the bottom nozzle leg counterbore. This weld procedure is performed after the fuel assembly is loaded with fuel rods, thus raising the level of difficulty in completing the task as well as increasing the difficulty of recovery should an improper weld be performed. The instant invention is a substantial improvement on such a current practice that requires fewer components, is much more easily inspected, does not require any complicated set-up fixturing or welding equipment or a skilled weld operator and makes rework a lot easier to accomplish with no risk to major fuel assembly components. The bottom nozzle to control rod guide thimble connection of this invention is illustrated in  FIGS. 2 and 3 .  FIG. 2  illustrates a cross section of a bottom nozzle leg  15  showing a cross section of the elongated fastener bolt  32  of this invention that is employed to attach the bottom nozzle to the control rod guide thimble  18 , shown in phantom  FIG. 2 . 
         [0025]    The fastener bolt  32  has a head  34  and a shank  36  that extends axially in the fastener bolts elongated dimension. The shank of the fastener bolt has a profile along its circumferential surface that is designed to engage a mating female latch profile on the inside lower surface of the control rod guide thimble. In the preferred embodiment the latch profile is the thread  38  that spirals around the circumference of the shank  36 . The fastener bolt head  34 , on the side opposite the shank, has a torque contour that can be engaged by a torque tool such as a hex driver to place the shank in engagement with the female latch profile on the interior of the control rod guide thimble. In the preferred embodiment shown in  FIG. 2  the torque contour is shown as hex although it should be appreciated that at any number of other geometries can be employed for this purpose. The head  34  also has a raised, deformable rim or flange  42  that can be crimped into a notch or pocket  44  in the side wall of the counterbore  46  in the leg  15 . The counterbore  46  is located centered on the axis of the control rod guide thimble  18 . 
         [0026]    The counterbore  46  has a first counterbore that starts at the bottom of the bottom nozzle leg  15  having a diameter  48  at least as large as the diameter of the head of the fastener bolt and extends up into the bottom nozzle, parallel to the axis of the fuel assembly, a first distance  50 . The counterbore then extends coaxially with the first counterbore from the first distance  50  to a second distance  54 . This second counterbore has a diameter at least as large as the diameter of the fastener bolt head. The counterbore  46  then continues upward through a third counterbore that extends coaxially with the second counterbore from the second distance  54  to a third distance  58 . The third counterbore has a nominal diameter  56  at least as large as the diameter of the fastener bolt head  34 , with portions of the circumference of the third counterbore having a larger diameter than the second counterbore so that the intersection of the portions of the second counterbore and the portions of the third counterbore having the diameter larger than the diameter of the second counterbore forms a ledge  64 . Collectively the first distance, second distance and third distance are less than the height of the lower nozzle, by the thickness of the horizontal top plate  30 . A the fourth bore  66  extends coaxially with the third counterbore from the third distance  58  through the upper surface of the top plate  30  of the bottom nozzle, in line with the control rod guide thimble  18  fastened to the upper surface of the plate  30 . The diameter of the fourth bore  66  is smaller than the diameter of the head  34  of the fastener bolt  32  and extends into the third counterbore. Small holes  70  are drilled through the top surface of the top plate  30  of the bottom nozzle approximately on the center line of the circumferential wall of the third bore through a distance equal to the third and fourth distances  62  and  58  to form pockets, recesses or notches  44  in the side wall of the third counerbore. When the fastener bolt  32  is secured inside the control rod guide thimble  18  to fasten the control rod guide thimble to the bottom nozzle  14 , the bolt head rim  42  is crimped outward in four places to secure the rim  42  into the recesses  44  in the side wall of the third counterbore. This action prevents the fastener bolt  32  from loosening during reactor operation or becoming loose should the bolt fail in shear, in as much as the bolt head  34 , once crimped, is captured between the ledge  64  and the underside of the top plate  30 . A drain hole  72  is advantageously formed along the center line of the fastener bolt  32  in the preferred embodiment. 
         [0027]      FIG. 3  is a top view of the upper plate  30  of the bottom nozzle  14 . The dashed eccentric circles represent the counterbores having a first diameter  48 , and second diameter  52  and a third diameter  56 . The hole  60  through the top plate  30  of the bottom nozzle  14  is shown as a solid circle with the through fastener bolt shank  36  shown protruding through the hole. In addition, small holes are drilled through the top plate  30  around the peripheral of the top hole  60  with the center of the hole  70  approximate the outer circumference of the bore having the third diameter  56 . To form the recesses, notches or crimp pockets into which the bolted rim  42  may be crimped. Though four such recesses  70 , spaced at ninety degree angles, are shown in  FIG. 3  it should be appreciated that the invention will satisfy its intended purpose with only two such diametrically opposed crimp pockets  44 . 
         [0028]    In the embodiment illustrated in  FIGS. 2 and 3  the counterbore having the second diameter  52  and the counterbore having the third diameter  56  may have the same size diameter. The bottom nozzle leg  15  may be formed by first drilling from the underside of the leg a bore having the diameter  52  up through the leg a distance of  50  plus  54  plus  58 . Then the fourth bore  66  may be drilled from the top having a fourth diameter  60  and extending down a distance  62 . The blind holes  70  may then be drilled from the top around the periphery of the fourth bore  66  extending down a distance of  62  plus  58 . Optionally, an enlarged bore  58  may be drilled from the bottom of the leg  15  a distance of  50 . To form the clean ledge  64 . 
         [0029]      FIG. 4  shows a tool that can be employed to crimp the bolt head rim  42  into the notches or recesses  44  in the circumference of the counterbore having the third diameter  56 . The tool  76  has a forming surface  80  at its tip that spreads under pneumatic pressure to spread the rim  42  on the bolt head  34 . The fork  78  is used to align the tool on a bottom nozzle feature. 
         [0030]    The function of the thimble screw fastener bolt  32  is to secure the guide tubes  18  within a PWR fuel assembly to the bottom nozzle  14 , thus maintaining the structural integrity of the fuel assembly  10 . Given that the guide thimbles  18  serve as the structural backbone of the fuel assembly and house in-core instrumentation and control element assemblies, it is important that the thimble screw fastener bolt  32  is captured and secured, after torquing, by a secondary, positive means. The new crimped thimble screw fastener bolt design features a raised flanged  42  that is crimped into two or four blind pocket  70  which are machined into the bottom nozzle legs  15  by drilling holes through the top surface of the bottom nozzle. After the thimble screw fastener bolt  32  is crimped by the pneumatic tool  76 , the screw is positively captured and restrained from rotational movement and from falling out if the screw breaks. Should the fuel assembly, with loaded fuel rods, require re-work, the thimble crimp screw fastener bolt  32  can be easily removed and replaced during the course of rework activities. 
         [0031]    While specific embodiments of the invention have been described in detailed, it would be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breath of the claims and any and all equivalence thereof.