Patent Number: 043483534
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to nuclear reactor fuel assemblies and more particularly to a reusable system for removably attaching a nuclear reactor fuel assembly duct tube to a nuclear reactor fuel assembly inlet nozzle. To produce power from the nuclear reactor, it is necessary to assemble a concentration of fissionable uranium, thorium, and/or plutonium in a quantity and in a physical configuration capable of sustaining a continuous sequence of fission reactions. This concentration is frequently referred to as the reactor core. The heat that the fission reactions generate is transferred to a fluid such as, for example, liquid sodium. The sodium, in turn, transfers its heat to a secondary coolant, or ultimate coolant, which can be used to drive turbines which power electrical generation equipment. Because the radiation, pressure, temperature, flow velocity, and other environmental conditions within the reactor core are quite hostile, the reactor core must be of sturdy construction. These conditions produce a number of phenomena with which it is very difficult to cope. Thus, for example, thermal, pressure, and irradiation effects tend to produce considerable creep in materials which can expand dimensions, cause bow and other effects which shorten fuel assembly life. Also, due to varying material requirements throughout a reactor core, it is often difficult to achieve ideal fuel behavior due to fabrication constraints. These effects have been known to produce a type of deterioration in that dimensional constraints of a fuel assembly in a reactor core can no longer be met over a period of time. These conflicting requirements have been reconciled to a great extent by loading pellets of uranium or plutonium dioxide into long slender tubes called fuel rods. With the tubes loaded with pellets and the ends of each tube sealed, these fuel rods are arranged longitudinally parallel with each other and are arranged in generally hexagonal arrays of about 200 fuel rods. Each array is called a fuel assembly. These fuel assemblies all are mounted side-by-side in a larger, generally right circular cylindrical configuration that characterizes the reactor core. Each fuel assembly often includes an upper handling socket and a lower nozzle which are both attached to a duct tube containing the fuel rods. The fuel rods receive radial support from the duct tube and longitudinal support from the lower nozzle. Coolant usually flows in through the lower or inlet nozzle of the fuel assembly, up through the spaces between fuel rods and out through the upper handling socket of the fuel assembly. Often, duct material incompatibility with the environment can degrade the operating life expectancy of the fuel assembly. This gives a need, with a significant cost incentive, for being able to remove duct tubes from their inlet nozzles for various reasons such as inspection, testing, maintenance or replacement, without the destruction of the duct tube and/or inlet nozzle. The prior art attaches the duct tube to the inlet nozzle by welding, and uses destructive techniques to separate them. SUMMARY OF THE INVENTION It is an object of the invention to quickly and inexpensively attach a nuclear reactor fuel assembly duct tube to a nuclear reactor fuel assembly inlet nozzle. It is another object of the invention to quickly, inexpensively and nondestructively remove an attached nuclear reactor fuel assembly duct tube from a nuclear reactor fuel assembly inlet nozzle. It is a further object of the invention to allow repeated attachment and removal of a nuclear reactor fuel assembly duct tube and a nuclear reactor fuel assembly inlet nozzle. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the nuclear reactor fuel assembly duct tube and inlet nozzle attachment and removal system may comprise a nuclear reactor fuel assembly inlet nozzle, a nuclear reactor fuel assembly duct tube, a retaining collar and a locking nut. The inlet nozzle's upper end has a top section with the shape of a generally equilateral polygon and a bottom section with the general shape of a cylinder. The two sections are coaxially joined. Each top section's side contains an outside recess while the bottom section contains outside threads. The duct tube's lower end has a similar shape to that of the previously mentioned polygon, and its sides have outwardly-extending protrusions followed by terminal deflectable locking tabs which end in inwardly-extending flanges. The locking tabs can be made to slide over the inlet nozzle's top section, and the flanges can be made to engage the top section's recesses. The retaining collar's top segment has a shape similar to that of the previously mentioned polygon and its bottom segment has a generally cylindrical shape. The two segments are coaxially joined. The top segment, during attachment, surrounds and restrains the flanges and engages the protrusions. The bottom segment, during attachment, is positioned above only part of the outside threaded portion. The locking nut's inside threaded portion connects with the outside threaded portion of the inlet nozzle to lock the retaining collar against the protrusions during attachment. Several benefits and advantages are derived from the invention. The invention's rapid attachment and removal feature allows easy removal of a nuclear reactor fuel assembly duct tube from its inlet nozzle for inspection, testing, maintenance and the like. The invention's reusability feature allows the removed duct tube and/or inlet nozzle to be reconnected or replaced with a different one. This is to be contrasted with the prior art which provides for a destructive, one-time removal system. The invention's reusability feature and rapid attachment and removal feature provide an economic benefit for test and/or commercial reactors. For example, various duct tube material configurations can be tested without degrading fuel assembly life and the limited life of assemblies could be extended.