Patent Number: 047160187
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. 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 is the type used in a pressurized water reactor (PWR) and 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. As seen in FIG. 1 and in greater detail in FIG. 2, each fuel rod 18 includes a plurality of nuclear fuel pellets 24 disposed in a stack in an elongated hollow cladding tube 25 having its opposite ends closed by upper and lower end plugs 26,28 so as to hermetically seal the rod. The lower end plug 28 has the improved configuration of the present invention to be described later. Commonly, a plenum spring 30 is disposed within the cladding tube 25 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 PWR. 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 some of the 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 fisson process in the fuel assembly 10, all in a well-known manner. Improved End Plug Configuration Turning now to FIG. 3, there is seen the fuel rod storage and loading equipment, generally designated 40, which is currently used for loading fuel rods 18 into the fuel assembly 10 by pulling them from the bottom of the assembly through the top thereof. The equipment 40 includes a fuel rod loader 42 and a fuel rod magazine 44. The loader 42 has an extendible and retractible gripper 46. When the fuel assembly 10 under construction is located between the loader 42 and magazine 44, with the loader at the bottom of the assembly and the magazine at the top thereof, the gripper 46 is extended outwardly from the loader, through the fuel assembly grids 16, and to the fuel rod magazine. There, an arrangement of radially expandible and collapsible end projections 48 on the end of the gripper 46, as seen in FIG. 4, is brought into engagement with the lower end plug 28 of one of the fuel rods 18 stored in the magazine 44 and is then retracted back through the fuel assembly 10, pulling the fuel rod 18 from the magazine into and through the grids of the assembly. Referring to FIG. 5, there is illustrated the improved truncated tapered configuration of the end plug 28 adapted for mounting on the lower end of the fuel rod tube 25. The end plug 28 facilitates insertion of the fuel rod 18 into the fuel assembly 10 by pulling it through from the bottom or pushing it through from the top thereof without hanging up on mixing vanes (not shown) on the fuel assembly grids 16. Basically, the improved end plug 28 includes an inner portion 50 adapted to be inserted into the end of the fuel rod tube 25, and an outer portion 52 adapted to extend from the end of the tube. The outer end plug portion 52, in turn, includes a body part 54 disposed adjacent the tube end and a leading part 56 disposed remote from the tube end. The improved truncated tapered configuration of the end plug 28 is defined on its leading part 56. The leading part 56 has a hollow interior cavity 58 defined therein, an exterior annular truncated surface 60 defined on a terminal end thereof and a continuous exterior annular tapered surface 62 defined about a lateral side thereof. The exterior tapered side surface 62 extends between and merges with the body part 54 and the exterior truncated end surface 60. The tapered side surface also provides sufficient angular inclination so as to facilitate insertion of the end plug 28 on the fuel rod tube 25 into the fuel assembly 10 without hanging up on the mixing vanes (not shown) on the grids 16 of the assembly. The body part 54 of the outer plug portion 52 has a continuous exterior annular cylindrical side surface 65 which merges with the exterior tapered side surface 62 of the leading part 56 and is of a diametrical size substantially equal to that of the fuel rod tube 25. The end plug 28 is adapted to be attached to the tube end by a weld (not shown) which would be formed about the inwardly-facing shoulder 66 defined at the transition between the inner and outer portions 50,52 of the end plug 28. The cylindrical side surface 64 has an axial length sufficient to provide a cylindrical region on the plug 28 about which the plug can be rigidly held during its fabrication, such as during machining of the interior cavity 58 and the tapered surface 62. The interior cavity 58 in the leading end plug part 56 has an inner end 68, an outer opening 70 defined at and surrounded by the exterior annular truncated surface 60 and a continuous interior annular wall surface 72 interconnecting the inner end 68 and the outer opening 70. The interior wall surface 72 of the cavity 58 has the same constant diameter from its inner end 68 to its outer opening 70. The inner wall surface 72 further has a continuous undercut annular groove 74 defined therein which has a larger diametrical size than that of inner and outer annular portions of the interior wall surface 72 on opposite sides of the groove. Also, the groove 74 is spaced axially inwardly from the cavity opening 70. The undercut groove 74 is engageable by the radial projections 48 on the gripper 46 when fitted through the cavity opening 70 for loading the fuel rod 18 into the nuclear fuel assembly 10. Specifically, when the projections 48 are in their collapsed, inwardly-biased position, as seen in FIG. 4, they can be inserted through the cavity opening 70 and into alignment with the groove 74 therein. Then, upon movement of a central pin 76, the projections 48 are forced outward into the groove 74 so as to grip the end plug 28 about the interior cavity 58. Furthermore, the inner end 68 of the interior cavity 58 is located at an axial distance from the cavity opening 70 which is substantially the same as the axial distance through which the exterior tapered surface 62 of the leading part extends from the truncated surface 60 to the body part 54. Accordingly, the cavity 58 at its inner end 68 extends outwardly away from the body part 54 of the outer plug portion 52. To provide sufficient wall structure laterally surrounding the interior cavity 58 against which to react the forces created by engagement of the gripper projections 48 within the cavity groove 74, two relationships of the features contained in the leading part 56 are important. First, the leading part 56 must have a thickness through its region which extends between its exterior tapered side surface 62 and its interior cavity undercut groove 74 which is less than the radius of the groove and greater than the width of the exterior truncated surface 60. Second, the interior wall surface 72 of the cavity 58 must have an axial length between the exterior truncated end surface 60 and the undercut groove 74 which is greater than the axial width of the groove. In an exemplary embodiment of the end plug 28, the dimensions (in inch) of its above-described features are as follows: Axial length of end plug 28: 511+/-0.010 PA1 Axial length of inner portion 50: 0.130+/-0.005 PA1 Axial length of outer portion 52: 0.381+/-0.010 PA1 Diameter of inner portion 50: 0.317+/-0-1 PA1 Diameter of body part 54: 0.361+/-0.003 PA1 Axial length of body part 54: 0.150+/-0.010 PA1 Diameter of truncated end surface 60: 0.125+/-0.003 PA1 Diameter of cavity opening 70: 0.113+/-0.003 PA1 Diameter of cavity groove 74: 0.142+/-0.003 PA1 Width of cavity groove 74: 0.027+/-0.002 PA1 Angle of tapered surface 62 from axis A: 27 deg. +/-1 deg. The extent of the inclination of the tapered surface 62 on the leading part 56 is limited to the point at which an operable interior diameter of the cavity 58 can be maintained. Through experimentation, it has been determined that the minimum operable diameter for the cavity 58 which is maintainable in production is 0.113 inch. The maximum diameter for the truncated surface 60, or the most "blunt" the leading part can be, while still permitting satisfactory mixing vane avoidance is 0.150 inch. With such dimensions for the cavity opening 70 and truncated surface 60, sufficient lateral wall structure is incorporated by the leading part 56 to absorb the reaction forces of the gripper 46 on the end plug 28 without fracture of its leading part 56. 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.