Patent Number: 046702130
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending application dealing with related subject matter and assigned to the assignee of the present invention: "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 copending U.S. patent 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 eliminate relative sliding engagement between the upper support plate of the reactor core and the hold-down structure of the top nozzle while providing removable mounting of the top nozzle as a unitary subassembly on the guide thimbles of a reconstitutable fuel assembly as well as desired alignment of the fuel assembly with the upper core support plate. 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 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 guide 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 usefuel 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 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 upper 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 end plate. A radially enlarged shoulder on the upper end of each of the alignment posts retains the hold-down plate on the posts. However, the Klumb et al reconstitutable fuel assembly involves a top nozzle arrangement which is difficult to remove and reattach both due to the location of the fasteners and because removal appears to cause the hold-down device of the top nozzle to come apart, requiring added steps and apparatus to prevent this or to later reassembly the hold-down device. The reconstitutable fuel assembly described and illustrated in the patent application cross-referenced above includes design improvements which overcome the aforementioned problems and shortcomings of the Klumb et al top nozzle design. Particularly, the top nozzle of the cross-referenced application is adapted to be removed and then replaced as a unitary subassembly on the guide thimbles. Notwithstanding these improvements, several additional problems are inherent in the Klumb et al design. Only recognized recently, these problems are not addressed in the Kmonk et al patent nor in the improved top nozzle design of the cross-referenced application and so likewise are present in them. These problems arise from the impingement of coolant cross-flow from adjacent fuel assemblies on the hold-down springs of a given fuel assembly and the relative sliding engagement allowed between the upper ends of the alignment posts (or guide tube extensions) and the upper core support plate at the region of the holes defined therein which receive the upper ends of the alignment posts. With regard to the first problem, cross-flow from adjacent fuel assemblies occurs because of the radial flow maldistribution across pressurized water reactor cores which is caused by core inlet flow maldistribution and by temperature differences across the core. Thus, there is a radial pressure gradient at the fuel assembly outlet which induces cross-flow above the fuel rods of the assembly. The hold-down springs in the Klumb et al type top nozzle are exposed to the cross-flow which has led to spring failure due to fatigue caused by flow induced vibration. With regard to the second problem, relative motion occurs between the upper ends of the alignment posts and the upper core support plate because fuel assembly alignment with the upper core plate is accomplished by projecting the guide thimble mounted posts into the holes in the core plate and such alignment must accommodate axially-directed differential growth of the fuel assembly due to differential thermal expansion and irradiation growth. Core plate vibration also produces relative motion between the core plate hole and the thimble alignment post. Such relative motion accompanied by contact between the upper ends of the alignment posts and the hole region of the core plate results in wear of the core plate. Small amplitude vibration, even at low frequencies, can lead to appreciable wear when considered over long periods of time. Since the core plate has a forty year life, wear at the hole region therein can lead to expensive core plate rework to resize the holes. Consequently, a need exists for a fresh approach to reconstitutable fuel assembly top nozzle design with the objective of eliminating the aforementioned problems of core plate wear and hold-down spring fatigue while retaining the capability of removal and reattachment of the top nozzle without the possibility of its hold-down device coming apart. SUMMARY OF THE INVENTION The present invention provides an improved top nozzle construction designed to satisfy the aforementioned needs. Specifically, the top nozzle of the present invention includes improved structures which eliminate relative moving contact or sliding engagement between the upper support plate of the reactor core and the hold-down structure of the top nozzle while providing removable mounting of the top nozzle as a unitary subassembly on the guide thimbles of a reconstitutable fuel assembly as well as desired alignment of the fuel assembly with the upper core support plate. Relative motion between the upper core plate and the alignment sleeves of the top nozzle still takes place but without damaging contact with one another. Consequently, there is no wear on the upper core plate. Any wear which might occur takes place between readily replaceable parts of the top nozzle. Deleterious affects on the hold-down springs of coolant cross-flow between fuel assemblies is substantially eliminated in the preferred form of the present invention by the incorporation of an enclosure wall about the perimeter of the top nozzle which protects the springs from flow induced vibration. Accordingly, the present invention is directed to an improved top nozzle on a fuel assembly for aligning the fuel assembly with an upper core plate of a nuclear reactor core. The fuel assembly has a plurality of guide thimbles with respective upper end portions and the upper core plate has a lower side and a plurality of holes defined therein which open at its lower side. The improved top nozzle comprises: (a) lower means being stationarily supported on the upper end portions of the guide thimbles; (b) upper means having a plurality of passageways defined therethrough in a pattern which matches that of the guide thimbles and being adapted to abut the lower side of the upper core plate; (c) a plurality of upstanding means having respective central bores defined therethrough, each of the upstanding means being disposed above the upper means and attached thereto such that its central bore is aligned with a respective one of the passageways of the upper means, with each upstanding means also being of a cross-sectional size adapted to interfit within one of the holes in the upper core plate when the upper means abuts the lower side of the upper core plate; (d) a plurality of elongated tubular members having lower and upper ends and being releasably connected at their respective lower ends to the upper end portions of the guide thimbles and inserted at their respective upper ends into the passageways of the upper means for slidable movement within the passageways of the upper means and the corresponding aligned bores of the upstanding means; (e) a plurality of yieldable members disposed between the lower and upper means and supporting the upper means in a spaced relation above the lower means at a stationary position in which the upper means abuts the upper core plate with the upstanding means interfitted within the holes of the upper core plate; and (f) means interconnecting the spaced lower and upper means so as to accommodate movement of the lower means toward and away from the upper means upon axial movement of the guide thimbles of the fuel assembly toward and away from the upper core plate, the interconnecting means also being effective to limit movement of the lower means away from the upper means to maintain the yieldable members in a state of compression therebetween. Therefore, concurrently as alignment of the fuel assembly with the upper core plate is achieved through abutting of the upper means with the upper core plate and interfitting of the upstanding means within the upper core plate holes, axial movement of the fuel assembly relative to the upper core plate is accommodated through movement of the lower means and the plurality of tubular members relative to the upper means. No wear is incurred by the upper core plate since there is no relative sliding engagement of the upper means, the plurality of upstanding means nor the plurality of tubular members of the improved top nozzle with the upper core plate. More particularly, the lower means includes a lower adapter plate with a plurality of openings defined therethrough in a pattern which matches that of the guide thimbles such that the upper end portions of the guide thimbles are received therethrough and extend above the adapter plate. The elongated tubular members are releasably connected at their respective lower ends to the upper end portions of the guide thimbles extending above the lower adapter plate. Specifically, each of the lower ends of the respective tubular members is internally threaded for releasable threaded connection to an externally threaded section on each of the upper end portions of the respective guide thimbles. Furthermore, each of the tubular members has a lower portion of a cross-sectional size greater than an upper portion thereof and greater than the size of the passageway in the upper means such that the tubular member remains captured between the upper means and lower means when released from its connection with the respective one guide thimble. Still further, the upper means is a hold-down plate composed of an array of hubs and ligaments extending between and interconnecting the hubs, with each of the hubs having one of the passageways defined therethrough. The interconnecting means includes at least one lug connected to each of at least several of the hubs and extending downwardly therefrom. Alternatively, the lug can be connected to each of at least several of the ligaments and extend downwardly therefrom. Finally, each upstanding means is a boss disposed above and connected to each of at least several of the hubs of the upper hold-down plate with the bore of the boss aligned with the passageway of the hub. 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 in illustrative embodiment of the invention.