Patent Number: 046997504
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 apparatus for storage, retrieval and deployment of vertically suspended drag gages used in a semi-automated system for inspecting a fuel assembly for potential control rod guide thimble misalignment. 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 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. Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels or guides for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles. In addition to accommodating normal stepped insertion of the control rods, the guide thimbles must allow their rapid insertion should a reactor trip arise. Therefore, one procedure typically carried out to determine the quality of a fuel assembly is an inspection for potential control rod hang up or malfunction by performance of a drag check utilizing tooling rods. The slender tooling rods, referred to collectively as a drag gage, are lowered into the hollow control rod guide thimbles in a vertical attitude. The descending gage must not encounter a drag which results in a weight reduction of fifteen pounds or more to qualify the fuel assembly as acceptable. Heretofore, this inspection task has required use of the main bridge crane and three persons to carry it out. Particularly, in addition to a person to operate the inspection equipment, the inspection procedure has utilized the main bridge crane with the assistance of a crane operator and a hook-up man stationed on an elevating lift platform. Thus, the current inspection task is labor intensive and occupies the crane for prolonged periods of time. Consequently, a need has emerged to improve and automate the manner in which the inspection is carried out. SUMMARY OF THE INVENTION The present invention provides an apparatus designed to satisfy the aforementioned needs by facilitating automated storage, retrieval and deployment of the vertically suspended drag gages used in inspection of the fuel assembly guide thimbles. The general virtues of the apparatus include the modernization of the current drag check facility to achieve productivity and quality gains by the mechanization and automation of most test procedures. Specifically, the employment of a dedicated system eliminates the necessity to use the main bridge crane and thus the requirement for the services of a craneman and hook-up man. Other benefits derived are reduced cycle time, improved safety of personnel and product, and automated sensing of a specified drag effect. The apparatus employs a gripper mechanism on overhead handling and lifting equipment which has associated with it a cylindrical safety device which prevents inadvertent release of the gripper mechanism. The safety device being, passive and automatic in function, does not complicate the normal operation of the gripper mechanism. The apparatus also incorporates an indexable dispenser for the drag gages of the system. Uniquely, the dispenser is mounted in an unorthodox place, such being about the vertical support column of the overhead handling and lifting equipment. Orthodox practice would dictate that a more complicated and costly stand-alone mounting structure be provided to store the drag gages. Thus, the apparatus of the present invention advantageously utilizes space efficiently by incorporating the dispenser about the support column, such as the column of a Jib crane, is less costly than a stand-alone structure, simplifies operation of the apparatus by limiting the distance that must be traveled to retrieve and return the gage, and enhances safety by reducing the area vulnerable to handling/transport problems. Accordingly, the present invention is set forth in a system for inspecting fuel assembly guide thimbles and is directed to an apparatus for handling at least one drag gage being insertable within the guide thimbles. The apparatus comprises: (a) storing means for holding at least one drag gage in a storage position; (b) indexing means for moving the drag gage along a path to dispose the gage at a retrieval-and-return station; (c) loading means operable for respectively gripping and releasing the gage at the retrieval-and-return station; (d) transporting means movable between a work station and a storage station; and (e) hoisting means supporting the loading means and being supported by the transporting means. The hoisting means is operable for respectively raising ad lowering the loading means and the drag gage therewith away from and toward the retrieval-and-return station when the transporting means is at the storage station and operable for respectively raising and lowering the loading means and the gage therewith away from and toward the fuel assembly guide thimbles when the transporting means is at the work station. More particularly, the storing means holds a plurality of drag gages in a series of positions being angularly displaced for one another about a common axis. The indexing means is operable to move the drag gages about an endless path to dispose a selected one of the gages at the retrieval-and-return station. Also, the present invention provides safety means for restraining the loading means in gripping relation with the drag gage. In addition, means is provided for releasing the safety means from restraining the loading means in gripping relation with the drag gage only when disposed at the retrieval-and-return station.