Patent Number: 046876308
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Nuclear Reactor Fuel Assembly With Improved Top Nozzle And Hold-Down Means" by Robert K. Gjertsen et al, assigned U.S. Ser. No. 542,625 and filed Oct. 17, 1983, now U.S. Pat. No. 4,534,933, issued Aug. 13, 1985. 2. "Reconstitutable Nuclear Reactor Fuel Assembly With Unitary Removable Top Nozzle Subassembly" by John M. Shallenberger, assigned U.S. Ser. No. 673,681 and filed Nov. 20, 1984, a continuation-in-part of U.S. application Ser. No. 457,790, filed Jan. 13, 1983. 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 a top nozzle incorporating improvements which limit the handling loads that can be imposed on the fuel assembly especially when it is being loaded into or removed from the reactor core. 2. Description of the Prior Art Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 (No. Re. 31,583) and 3,814,667 to Klumb et al and U.S. Pat. No. 4,269,661 to Kmonk et al. Due to occasional failure of some fuel rods during normal reactor operation and in view of the high costs associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod uide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its useful life. One type of such reconstitutable fuel assembly can be seen in the aforementioned Klumb et al patents. The fuel assembly of Klumb et al includes a top nozzle which incorporates a hold-down plate and also coil springs coaxially disposed about upwardly extending alignment posts. The alignment posts extend through an upper end or adapter plate, spaced below the hold-down plate, and are joined thereto and to the upper ends of the guide thimbles with fastener nuts located on the underside of the adapter plate. The upper hold-down plate is slidably mounted on the alignment posts and the coil springs are interposed, in compression, between the hold-down plate and the adapter plate. A radially enlarged shoulder on the upper end of each of the alignment posts reacts with a shoulder on the hold-down plate to retain the hold-down plate on the posts. When the fuel assembly is free standing after being removed from the reactor core, the hold-down plate is held at its uppermost position along the alignment posts by the coil springs. Further upward sliding movement of the hold-down plate is prevented by contact of the plate with the enlarged shoulders on the upper ends of the alignment posts. On the other hand, when the fuel assembly is positioned in the reactor core, the hold-down plate is pressed downward by the upper core plate of the reactor core. Thus, during reactor service, the hold-down plate slidably moves downward away from its freestanding position. Transfer of the fuel assembly between its service position in the reactor core and a location outside of the core, such as a work station for reconstitution of the fuel assembly, is accomplished by use of a conventional fuel assembly handling machine. For handling the fuel assembly, a gripper of the machine is brought into engagement with the hold-down plate and then moved in an upward direction so as to lift the fuel assembly via its top nozzle. While the gripper so supports the fuel assembly, the load passes from the gripper to the hold-down plate and therefrom to the guide thimbles via the alignment posts in view that the connection between the hold-down plate and the guide thimbles is, in effect, substantially unyielding or rigid. The above-described type of connection of the hold-down plate with the guide thimble alignment posts in the reconstitutable fuel assembly of the Klumb et al patents imposes on the design of the fuel assembly structure the requirement that it be capable of withstanding large lifting loads, typically on the order of 6 g. These high loads are impulse type loads which are of very short duration. (The fuel assembly handling machine has a load limiting system to prevent sustained high loads on the fuel assembly.) The postulated 6 g axial load can occur when the fuel assembly is being lowered adjacent to another assembly, and it momentarily hangs up on the stationary assembly. For example, the grids interlock or the bottom nozzle of the fuel assembly being lowered catches on the top nozzle of the stationary assembly. The fuel assembly being lowered then breaks loose from its hangup and drops downwardly until it is stopped by the fuel assembly handling machine which has continued downward. The impact energy caused by this sudden drop is now absorbed by the fuel assembly structure. (The fuel handling machine is assumed to be rigid.) Although the above-described event occurs very infrequently, the fuel assembly structure must be designed to withstand these high loads. Unfortunately, the occurrence of these high loads, however infrequent, reduces the overall reliability of the fuel assembly structure and increases the complexity of the design of the top nozzle and guide thimble connections in the fuel assembly. Consequently, a need exists for a fresh approach to fuel assembly top nozzle design with the objective of reducing the loads on the top nozzle and guide thimble joints and thereby increasing fuel assembly reliability. SUMMARY OF THE INVENTIO The present invention provides an improved top nozzle and guide thimble joint structure designed to satisfy the aforementioned needs. Underlying the present invention is a recognition that the problem with the prior art fuel assembly is the rigid connection between the top nozzle hold-down plate and the fuel assembly guide thimble when the fuel assembly is in its freestanding position, such as when it is supported by the fuel assembly handling machine. If an energy absorbing means could be interposed between the fuel assembly handling machine gripper and the fuel assembly structure, the design loads for the fuel assembly could be reduced. The improved joint structure of the present invention provides a flexible connection of the hold-down plate to the guide thimble alignment posts which serves as an energy absorber. Provision of the flexible, or yieldable, joint structure reduces the loads on the top nozzle and guide thimble joints and, as a result, increases fuel assembly reliability. More importantly, it simplifies the design of the fuel assembly top nozzle and guide thimble connections. Accordingly, the present invention is provided in a nuclear fuel assembly having at least one control rod guide thimble and a top nozzle, wherein the guide thimble includes an upper extension member and the top nozzle includes an upper hold-down plate having a passageway slidably receiving an upper end portion of the extension member. The present invention is directed to an improved joint structure flexibly interconnecting the hold-down plate with the upper end portion of the guide thimble upper extension member. The improved joint structure basically comprises: (a) first overlapping means on the upper hold-down plate at the passageway thereof; and (b) second overlapping means on the upper end portion of the guide thimble extension member. The first and second overlapping means are respectively disposed to interfere with one another so as to limit upward movement of the hold-down plate along the guide thimble extension member. At least one of the first and second overlapping means is resiliently yieldable for absorbing the energy of an impulse load applied to the hold-down plate so as to thereby limit transfer of the load to the guide thimble extension member. More particularly, the first overlapping means is an internal ledge defined on the hold-down plate within its passageway, being preferably located in a lower portion of the passageway. The second overlapping means includes a recess defined on the upper end portion of the guide thimble extension member, and a spring member fitted on the upper end portions within the recess thereon and extending outwardly therefrom so as to overlie the internal ledge in the hold-down plate passageway. Still further, the recess is defined between a lower upwardly-facing shoulder on the upper end portion of the guide thimble extension member and an upper detachable member releasably applied to the upper end portion of the extension member. The spring member is at least one belleville spring which deflects axially upon application of a large impulse load thereto via the internal ledge of the hold-down plate. 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.