Patent Number: 047160187
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 improved fuel rod end plug configuration which facilitates both pulling fuel rods into the top of a fuel assembly from the bottom thereof during initial loading at the manufacturing site and pushing fuel rods into the top of the fuel assembly from the top thereof during subsequent servicing at the reactor facility. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core includes a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends to the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a multiplicity of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. In assembling a fuel assembly, it has been conventional practice, first, to attach the transverse grids to the longitudinally extending guide thimbles at predetermined axially spaced locations therealong. Next, the fuel rods are loaded through the grids and the bottom nozzle is then attached to the lower ends of the guide thimbles. Last, the top nozzle is attached to the upper ends of the guide thimbles. Typically, the fuel rods are loaded by pulling them through the grids from the bottom of the fuel assembly. The fuel assembly under construction is located between a fuel rod loader and a fuel rod magazine. At the bottom of the fuel assembly, a gripper is extended outwardly from the fuel rod loader, through the fuel assembly, and to the fuel rod magazine located at the top of the assembly. The gripper is brought into engagement with the lower end plug of a fuel rod stored in the magazine and is then retracted back through the fuel assembly, pulling the fuel rod from the magazine into and through the grids of the assembly. Loading fuel rods into the top of the fuel assembly by pulling from the bottom of the assembly works satisfactorily in the manufacturing plant; however, once the fuel assembly is in service in a reactor facility, replacement loading of a fuel rod by pulling from the bottom of the assembly becomes more difficult. This is because servicing of a fuel assembly normally takes place while the assembly is submerged in water at a work station and also due to the unavailability of loading equipment at reactor facilities which pulls fuel rods from the bottom of the fuel assembly. Therefore, when a fuel rod needs to be replaced in the reactor facility, it will usually be loaded into the assembly from the top by pushing the rod, instead of pulling it, into the fuel assembly. However, a problem frequently arises when a fuel rod is being pushed into the fuel assembly. The lower end plug of the fuel rod, which has a relatively blunt leading surface, often gets hung up on the flow mixing vanes carried by the support grids of the fuel assembly into which the fuel rod must pass. Several competing interests make a solution to this problem difficult. On the one hand, the blunt configuration of the leading end plug surface accommodates the presence of a large open interior cavity having sufficient lateral wall structure for the gripper to enter and grab the end plug for bottom loading of the fuel rod. On the other hand, a fully tapered or "pencil point" end plug configuration would help prevent the fuel rod end plug from becoming hung up on the grid mixing vanes during top loading of the fuel rod. However, if the end plug was fully tapered, there would be insufficient lateral wall structure to define an interior cavity for the gripper to engage for effecting bottom loading of the fuel rod. Consequently, a need exists for a fuel rod end plug design which will strike a workable compromise between these competing interests and thereby facilitate loading of the fuel rod from both the bottom and top of the fuel assembly by accommodating both pulling and pushing of the fuel rod into the fuel assembly. SUMMARY OF THE INVENTION The present invention provides a fuel rod end plug with an improved truncated tapered leading end configuration designed to satisfy the aforementioned needs. The improved end plug configuration provides sufficient lateral wall structure on the leading end of the plug to allow the formation of an interior cavity large enough to be engaged by the gripper. Therefore, the present fuel rod loading equipment and methods can continue to be used. Further, the truncated tapered configuration on the end plug leading end provides sufficient angular inclination to prevent the fuel rod from hanging up on the mixing vanes of the fuel assembly grids. Thus, replacement top loading of fuel rods at the reactor facility is facilitated by the improved end plug configuration. Accordingly, the present invention is directed to an improved end plug for attachment on an end of a cladding tube of a nuclear fuel rod which facilitates using a gripper tool for loading the fuel rod into a nuclear fuel assembly. The improved end plug comprises: (a) an inner portion adapted to be inserted into the end of the tube; and (b) an outer portion adapted to extend from the end of the tube when the inner portion is inserted therein, the outer portion including a body part disposed adjacent the tube end and a leading part disposed remote from the tube end. The leading part of the outer end plug portion has a hollow interior cavity defined therein, an exterior annular truncated surface defined on a terminal end thereof and an exterior annular tapered surface defined on a lateral side of the leading part. The exterior tapered surface extends between and merges with the body part and the exterior truncated surface and provides sufficient angular inclination so as to facilitate insertion of the end plug when mounted on the fuel rod tube end into the fuel assembly. More particularly, the interior cavity in the leading part has an inner end, an outer opening defined at and surrounded by the exterior annular truncated surface and an interior annular wall surface interconnecting the inner end and the outer opening. Also, the interior wall surface of the cavity has an undercut annular groove defined therein having a larger diametrical size than the interior wall surface and being spaced from the cavity opening. The groove is engageable by the gripper tool when fitted through the cavity opening for loading the fuel rod into the nuclear fuel assembly. Still further, the inner end of the cavity is located within the leading part at an axial distance from the outer opening which is substantially the same as the axial distance through which the exterior tapered surface of the leading part extends from the truncated surface to the body part such that the cavity at its inner end extends outwardly away from the body part of the outer end plug portion. Also, the body part has an exterior annular cylindrical surface which merges with the exterior tapered surface of the leading part and is of a diametrical size substantially equal to that of the fuel rod tube. Additionally, the leading part has a thickness between the exterior tapered surface thereon and the interior cavity undercut groove therein which is less than the radius of the groove and greater than the width of the exterior truncated surface, and the interior wall surface of the cavity has an axial length between the exterior truncated surface and the undercut groove which is greater than the axial width of the groove, such that the leading part of the outer end plug portion is provided with sufficient wall structure laterally surrounding the interior cavity to react the forces created by engagement of the gripper tool within the cavity groove. 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.