Patent Number: 050376039
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 hand held tool for removing and replacing a locking tube in a removable top nozzle of a reconstitutable fuel assembly. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. 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 of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. At the top of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle. The guide thimbles may each include an upper sleeve for attachment to the top nozzle. During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are supported by the grids in a spaced array with the guide thimbles between the top and bottom nozzles of the fuel assembly, it is difficult to detect and remove the failed fuel rods. Until recently to gain access to these rods it was necessary to remove the affected fuel assembly from the nuclear reactor core and then break the welds securing the nozzles to the guide thimbles. In so doing, the destructive action often rendered the fuel assembly unfit for further use in the reactor because of the damage done to both the guide thimbles and the nozzle which prohibited rewelding. In view of the high costs associated with replacing fuel assemblies, reconstitutable fuel assemblies were developed to minimize operating and maintenance expenses. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. One reconstitutable fuel assembly construction is illustrated and described in U.S. Pat. No. 4,631,168 to Shallenberger et al, which is assigned to the assignee of the present invention. It incorporates an attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles. The attaching structure includes a plurality of outer sockets defined in the adapter plate of the top nozzle, a plurality of inner sockets each formed on the upper end of one of the guide thimbles, and a plurality of removable locking tubes inserted in the inner sockets to maintain them in locking engagement with the outer sockets. Each outer socket is in the form of a passageway through the adapter plate which has an annular groove. Each inner socket is in the form of a hollow upper end portion of the guide thimble having an annular bulge which seats in the annular groove when the guide thimble end portion is inserted in the adapter plate passageway. A plurality of elongated axial slots are provided in the guide thimble upper end portion to permit inward elastic collapse of the slotted portion so as to allow the larger bulge diameter to be inserted within and removed from the annular circumferential groove in the passageway of the adapter plate. In such manner, the inner socket of the guide thimble is inserted into and withdrawn from locking engagement with the outer socket. The locking tube is inserted from above the top nozzle into a locking position in the hollow upper end portion of the guide thimble forming the inner socket. When inserted in its locking position, the locking tube retains the bulge of the inner socket in its expanded locking engagement with the annular groove and prevents the inner socket from being moved to a compressed releasing position in which it could be withdrawn from the outer socket. In such manner, the locking tubes maintain the inner sockets in locking engagement with the outer sockets, and thereby the attachment of the top nozzle on the upper ends of the guide thimbles. Furthermore, to prevent inadvertent escape due to vibration forces and the like, heretofore the locking tubes have been secured in their locking positions. In the one construction of the locking tubes disclosed in the above-cited patent, after insertion of the locking tubes into their locking positions within the inner sockets of the hollow upper end portions of the guide thimbles, a pair of bulges are formed in the upper portion of each locking tube. These bulges fit into the circumferential bulge in the upper end portion of the guide thimble and provide an interference fit therewith. In another construction of the locking tubes disclosed in U.S. Pat. No. 4,699,758 to Shallenberger et al, which is also assigned to the assignee of the present invention, several small dimples are preformed on the exterior surface of the upper end portion of the locking tube circumferentially spaced from one another and projecting from the locking tube exterior surface. The use of the preformed dimples eliminates the necessity to form bulges in the locking tube after insertion into the locking position. Also the preformed dimples allow the locking tube to be reused, whereas the bulged locking tubes were discarded after each use. The reconstitutable fuel assembly construction briefly described above has proven to be an improvement by which domestic and foreign utilities can minimize both operating and maintenance expenses. A fixture developed for removing and replacing the top nozzle of the reconstitutable fuel assembly is disclosed in U.S. Pat. No. 4,664,874 to Shallenberger et al, also assigned to the assignee of the present invention. A locking tube removal and replacement tool provided for use in conjunction with the fixture is disclosed in U.S. Pat. No. 4,639,998 to Shallenberger et al, also assigned to the assignee of the present invention. The tool used both for removal and replacement of locking tubes one at a time basically has an inner tubular member, an actuatable shaft assembly, and an outer tubular member. The inner tubular member of the tool is attached at its upper end to a bail assembly for gripping by a user. At its lower end, the inner tubular member has an extension with an expandable and collapsible lower end in the form of a outwardly-turned annular segmented rim. The actuatable shaft assembly of the tool is mounted for axial movement within the inner tubular member and the extension thereof. The shaft assembly has an actuating knob coupled to its upper end and a conical nose disposed on its lower end. The lower conical nose extends beyond the segmented rim on the lower end of the tubular member extension. To use the tool, the inner tubular member and the actuatable shaft assembly are inserted from above the top nozzle downwardly through one passageway of the adapter plate and the hollow interior of the locking tube and guide thimble until the segmented rim and the conical nose are disposed below the lower edge of the locking tube. Then, the knob on the shaft assembly is rotated in one direction to force insertion of the conical nose into the lower end of the tubular extension and thereby expand the segmented rim such that it will underlie and engage the lower edge of the locking tube when the inner tubular member and shaft assembly of the tool are pulled in an upward direction. The outer tubular member of the tool is disposed about the upper end of the inner tubular member and mounted for slidable movement therealong. The outer tubular member can be manually moved in a reciprocating manner to deliver a series of forceful impacts to the bail assembly of the tool which impacts are, in turn, transmitted via the inner tubular member to the lower annular segment rim of its tubular extension. With the segmented rim expanded to underlie and engage the lower edge of the locking tube, the force of each impact will be transmitted to the lower edge of the locking tube causing displacement of the locking tube in an upward axial direction and removal of the locking tube from its locking position such that it can then be easily carried from the top nozzle by withdrawal of the inner tubular member and shaft assembly of the tool from the top nozzle adapter plate. One major problem with the design of the above-described tool is the difficulty of initially aligning and inserting the lower end of inner tubular member and actuatable shaft assembly of the tool into the hollow interior of the locking tube. The annular segmented rim on the inner tubular member extension, by surrounding and projecting outwardly from the conical nose on the shaft assembly, provides a feature which tends to catch on the upper edge of the locking tube and obstruct insertion therein. Consequently, a need still remains for an alternative design of a tool to use to effectively and efficiently carry out removal and replacement of the locking tubes. SUMMARY OF THE INVENTION The present invention provides an improved hand held tool designed to satisfy the aforementioned needs. The tool has an external configuration with smooth transitions between exterior surfaces of assembled parts for ease of alignment and insertion into the hollow locking tube. The tool is manually held and actuated by a user and capable of precise operation in removing the locking tube from and replacing it in the locking position in the removable top nozzle of the reconstitutable fuel assembly. Accordingly, the present invention sets forth a tool for removing and replacing a hollow locking tube from its locking position in reconstituting a fuel assembly. The fuel assembly includes a removable top nozzle with an adapter plate having at least one passageway, at least one guide thimble with an upper end portion, and an attaching structure having the hollow locking tube for releasably locking the upper end portion of the guide thimble within the passageway of the top nozzle adapter plate. The tool of the present invention comprises: (a) an elongated hollow tubular assembly having upper and lower opposite end portions with the lower end portion insertable in a hollow locking tube, the lower end portion including an outer tubular element having a circumferential guide wall with a plurality of circumferentially spaced apertures, the lower end portion also including a plurality of expandable and contractible lifting members disposed within the tubular element and having catch elements extendable through and retractable from the apertures of the guide wall of the tubular element for engagement with and disengagement from a lower edge of the locking tube; and (b) an actuator assembly mounted through the tubular assembly for axial movement therealong and having upper and lower end portions with the lower end portion for actuating the lifting members of the tubular assembly lower end portion between expanded and contracted conditions to extend and retract their catch elements through and from the apertures in the guide wall of the tubular assembly lower end portion for engaging with and disengaging from the lower edge of the locking tube. More particularly, the tubular assembly lower end portion includes a guide element interfitting the tubular element at an open lower end thereof and having a body portion projecting therefrom. The end of the tubular element and the body portion of the guide element have substantially the same outside diameter so as to provide a continuous smooth transition from the guide element body to the tubular element end for facilitating insertion of the guide and tubular elements of the tubular assembly lower end portion into the hollow locking tube without catching on an upper edge of the locking tube at the transition. Also, the lower end portion of the actuator assembly includes an elongated shaft member extending between the lifting members of the tubular assembly and having upper and lower tandemly-arranged segments, the upper segment being larger in outside diameter than the lower segment such that downward movement of the shaft member removes the lower segment from between the lifting members and inserts the upper segment between the lifting members causing engagement therewith and expansion of the finger elements from contracted to expanded condition, whereas upward movement of the shaft member removes the upper segment from between the lifting members and inserts the lower segment therebetween permitting contraction of the finger elements from the expanded to contracted condition. Further, the lifting members of the tubular assembly have tapered tips. The elongated shaft member extending between and past the lifting members mounts a retractor member at a lower end of the shaft member. The retractor member has a tapered portion for engaging the tapered tips of the lifting members and ensuring that the lifting members move from the expanded to contracted condition as the shaft member is moved upwardly. These and other features and advantages 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.