Patent Number: 052727424
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a typical nuclear fuel assembly 10. Fuel assembly 10 is generally comprised of a plurality of fuel rods 12, grid assemblies 14, guide tubes 16, lower end fitting 18, and upper end fitting 20. Fuel rods 12 are maintained in an array spaced apart by grid assemblies 14. Guide tubes 16 extend through grid assemblies 14 and are attached to lower end fitting 18 and upper end fitting 20 and, in addition to providing structural integrity to the entire assembly, also serve as guides for control rods not shown. Lower end fitting 18 and upper end fitting 20 provide structural and load bearing support to fuel assembly 10 and are also provided with openings therethrough to allow coolant to flow vertically through fuel assembly 10. FIG. 3 is a side sectional view of a prior art upper end fitting 20. Upper end fitting 20 is comprised of main body portion 22 that may be square or rectangular in section. The lower portion of main body portion 22 is rigidly attached to or may be integral with grillage or base 24. As seen more clearly in FIG. 1, the underside of base 24 is adapted for attachment to guide tubes 16. Pedestal 26 is attached to base 24 and extends upward therefrom to support control assembly 28. Spring retainer 30 is positioned inside main body portion 22 for movement therein and is provided with tabs 32 that are received in slots in main body portion 22. Spring retainer 30 thus serves to maintain helical spring 34 in its installed position in main body portion 22. FIG. 2 illustrates upper end fitting 36 of the present invention. Upper end fitting 36 is similar in basic construction to that of the prior art in FIG. 3 in that it is formed from main body portion 22, base 24, and a spring retainer 30. However, upper end fitting 36 is provided with a combined pedestal and holddown spring assembly 38. Leaf springs 40 are positioned adjacent each other such that the exterior radius of the springs are facing each other. Each leaf spring 40 is provided with a bore for receiving a bolt 42 therethrough. Nut 44 is threadably received on bolt 42 for holding leaf springs 40 together. Bolt 42 and nut 44 are sized so as to extend upwardly through a bore 46 in spring retainer 30 to the correct height to serve as a pedestal support for control assembly 28. Spring retainer 30 is free to move vertically in main body portion 22 and extends through slots therein. FIG. 4 illustrates the prior art upper end fitting 20 and upper end fitting 36 of the present invention as they appear in their installed position inside a nuclear reactor. Reactor internals 48 bear against the portion of spring retainers 30 that extend out of main body portion 22. This places the desired spring preload pressure on nuclear fuel assembly 10 to prevent unwanted movement caused by coolant flow during normal operations. Upper end fitting 36 causes less upward displacement of control assembly 28 than the prior art end fitting 20 at the time of installation and during irradiation growth of the assembly. During irradiation growth, base 24 is caused to move upward. In the prior art end fitting 20, pedestal 26 moves upward the same distance as base 24 and affects the vertical position of control assembly 28. In upper end fitting 36, the pedestal formed by bolt 42 and nut 44 do not move in proportion to that of base 24. The nut and bolt pedestal move approximately one-half the distance of base 24 as leaf springs 40 are flattened to a greater degree. The invention thus has much less effect on the vertical position of control assembly 28 than prior art end fitting 20. The length of the longer bolt and nut used to attach the springs together and form the pedestal assembly, as opposed to a shorter bolt and nut that would be used merely to attach the leaf springs together, results in added flexibility of the nut and bolt and provides a better structural connection for the spring assembly. Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.