Patent Number: 047388218
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Reusable Locking Tube in a Reconstitutable Fuel Assembly" by John M. Shallenberger et al., assigned U.S. Ser. No. 719,108 and filed Apr. 2, 1985 (W.E. 52,507). 2. "Improved Guide Thimble Captured Locking Tube in a Reconstitutable Fuel Assembly" by Robert K. Gjertsen et al., assigned U.S. Ser. No. 775,208 and filed Sept. 12, 1985 (W.E. 52,881). BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to reconstitutable fuel assemblies for nuclear reactors and, more particularly, is concerned with a reconstitutable nuclear fuel assembly having a top-nozzle-to-guide-thimble attachment system employing reusable locking tubes with preformed dimples. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel 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. The reactor also has control rods which can be inserted into the guide thimbles to control the fission reaction. The fission reaction releases 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. During operation in the nuclear reactor, the fuel rods may occassionally develop cracks along their lengths resulting primarily from internal stresses. These defective fuel rods must be replaced in the fuel assemblies, and this replacement must occur under water as the fuel assemblies become highly radioactive during their operation in the reactor. To gain access to a defective fuel rod, it is necessary to remove the top and/or bottom nozzle of the fuel assembly. Reconstitutable fuel assemblies exist which are designed with removable nozzles. Typical removable top (bottom) nozzles have been attached to the top (bottom) of the guide thimbles using a threaded arrangement. Typical removable top nozzles also have been attached to the top of the guide thimbles using a bulge/groove arrangement, including the use of locking tubes, such as disclosed in U.S. Pat. No. 4,631,168, hereby incorporated by reference. Commonly owned U.S. patent application Ser. No. 719,108 entitled "Reusable Locking Tube in a Reconstitutable Fuel Assembly" by John M. Shallenberger et al., filed Apr. 2, 1985 (W.E. 52,507), is hereby incorporated by reference. The invention disclosed therein is a reconstitutable nuclear fuel assembly having reusable locking tubes with preformed dimples, and it has been in use and on sale in the U.S. for more than one year. Some locking tubes in those fuel assemblies were found to be seated too low. A study revealed that this problem had two possible causes. A properly seated locking tube could become improperly seated too low due to the force exerted on it when the fuel assembly was lifted, such as at the reactor site at the time of loading the fuel assembly into the reactor core. Also, a locking tube easily could be seated too low at the time of installation. A low-seated locking tube would require special (longer-handled) tooling for locking tube removal in the event the fuel assembly required reconstitution. This would complicate the underwater reconstitution operation. What is needed in a removable locking tube design which would insure proper seating of the locking tube at the time of installation and after fuel assembly lifting, and which would avoid time-consuming measurement checks at the factory and at the reactor. SUMMARY OF THE INVENTION It is an object of the invention to provide a reconstitutable nuclear fuel assembly having a top-nozzle-to-control-rod-guide-thimble attachment system employing reusable locking tubes of a design which insures proper locking tube seating at the time of installation. It is another object of the invention to provide such a locking tube design which also maintains proper locking tube seating during handling of the fuel assembly. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects and in accordance with the purpose of the present invention as embodied and broadly described herein, the reconstitutable nuclear reactor fuel assembly top-nozzle-to-control-rod-guide-thimble attachment system includes a top nozzle, a control rod guide thimble, and a locking tube. The top nozzle's adaptor plate has a control rod passageway. The passageway's upper portion has a smaller diameter than its lower portion, and is joined thereto by a ledge portion. The lower portion includes a circumferential groove. The thimble's top portion has more than one longitudinal, open-ended slot. Between the slots are fingers having a radially outwardly projecting rim or bulge. The rim has an inside surface defining a recess including a bottom edge. The thimble top portion is coaxially disposed in the passageway with the finger ends longitudinally placed proximate the ledge portion and with the rim (bulge) portions transversely placed in the groove. The locking tube includes an annular flexible cylinder which is coaxially placed in the thimble's top portion. The cylinder has at least one upper embossed dimple with an apex placed at an elevation below and proximate the ledge portion. The cylinder also has at least two lower embossed dimples each with a tip placed at an elevation above and proximate the elevation of the bottom edge of the recess. The upper dimples project radially outward into the passageway's lower portion at a distance, from the cylinder's centerline, which is greater than half the passageway's upper portion's diameter. The lower dimples project radially outward into the recess at a distance, from the cylinder's centerline, which is greater than the difference between the thickness of a finger and half the passageway's lower portion's diameter. The lower dimples are spaced angularly apart such that when at least one of the lower dimples is angularly rotated to be placed in a slot, at least one of the other lower dimples is angularly oriented to be aligned with the inside surface of the rim of one of the fingers. Another embodiment of the invention is the locking tube itself, the locking tube having been previously described within the context of the top-nozzle-to-control-rod-guide-thimble attachment system recounted in the previous paragraph. Several benefits and advantages are derived from the invention. The upper dimple feature prevents upward movement of the locking tube. The lower dimple feature prevents downward movement of the locking tube. The upper and lower dimple features together provide for proper seating of the locking tube at installation and provide for maintaining such proper seating during fuel assembly handling.