Patent Number: 049903047
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with instrumentation tube features for reducing coolant flow-induced vibration of a flux thimble tube within the instrumentation tube. 2. Description of the Prior Art In a typical pressurized water nuclear reactor (PWR), 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 between the nozzles and a plurality of transverse grids axially spaced along 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 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. Further, in each fuel assembly, a provision is made to enable actual flux measurements to be taken under reactor operating conditions. For this purpose a hollow instrumentation tube is located in the approximate center of each fuel assembly and extends between the bottom and top nozzles. The instrumentation tube is open at its bottom end through the adapter plate of the bottom nozzle for insertion of a flux thimble tube into the instrumentation tube. The thimble tube is adapted to take the flux measurements. Coolant flow passes upward through an annulus formed between the outside diameter of the flux thimble tube and the inside diameter of the instrumentation tube, the coolant entering this annulus from the underside of the bottom nozzle adapter plate. The coolant exits through a bleed orifice formed by the top end of the instrumentation tube and the adapter plate of the top nozzle. The coolant flow induces vibrations in the thimble tube due to the existence of radial clearance and lack of mechanical connection between the instrumentation and thimble tubes. Vibration of the thimble tube frequently results in wall degradation in both the instrumentation tube and the thimble tubes. Instrumentation tube wall degradation, in turn, increases annular coolant flow rate, thereby increasing the jetting of coolant onto surrounding fuel rods. This jetting is known to cause erosion and breakdown of the fuel rod pressure boundary Wear-through of the tubes is presently minimized by placing a coolant flow-limiting seal between the underside of the bottom nozzle adapter plate and a nozzle attached to the core support plate. Although this has been generally successful in preventing wear-through of the tubes, it does not work in all cases. Other proposed solutions are use of small spring devices located at the entrance to the instrumentation tube. However, these devices are subject to coming loose and falling off and thus creating debris in the coolant system. Consequently, a need exists for an alternative approach to reducing coolant flow-induced vibrations and resultant wear to the instrumentation and thimble tubes. SUMMARY OF THE INVENTION The present invention provides vibration reducing features for the instrumentation tube designed to satisfy the aforementioned needs. The present invention is set forth in a fuel assembly having a hollow instrumentation tube and a flux thimble tube inserted in the instrumentation tube for taking flux measurements. The flux thimble tube is radially spaced inwardly at its exterior surface from an interior surface of the instrumentation tube so as to define a coolant flow annulus therebetween. The present invention relates to means for reducing coolant flow-inducing vibration of the thimble tube. The vibration-reducing means preferably is mechanical elements on the instrumentation tube which constrain the flux thimble tube within the instrumentation tube to maintain physical contact of the exterior of the thimble tube with the interior of the instrumentation tube at a plurality of points. The points are staggered on a single-diametral plane and spaced substantially throughout the length of the instrumentation tube. The staggered mechanical elements induce a controlled elastic sinuous deflection of the inner thimble tube. The mechanical elements of the present invention can take several different forms, for example, dimples or cantilevered spring fingers formed in the wall of the instrumentation tube and projecting radially inwardly therefrom. 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.