Patent Number: 052710530
Section: summary

FIELD OF THE INVENTION This invention relates to nuclear rector fuel assemblies and in particular those assemblies which include spaced fuel rod support grids mounted in a reactor core as a unit. The fuel rods are held between an upper end fitting or top nozzle and a lower end fitting by means of spacer grids. The reactor coolant flows upwardly from holes in the lower end fitting along the fuel rods, and upwardly through holes in the upper end fitting. When the fuel assembly is loaded in a reactor core, an upper core plate over the fuel assembly reacts against fuel assembly holddown spring assemblies attached by fasteners to the upper end fitting to provide a downward force. This force combines with the fuel assembly weight to prevent fuel assembly liftoff from hydraulic forces during operation of the reactor pumps. BACKGROUND OF THE INVENTION Debris in the circulating coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 70% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. The size and shape of the debris capable of causing severe damage is quite variable and may include broken fuel assembly fasteners or other holddown spring assembly broken parts. Accordingly, it is desirable to be able to operate safely without reconstitution even if a part of the assembly breaks. This can be accomplished by retention of any potential debris. U.S. Pat. No. 5,053,191 describes an improved fuel assembly holddown cantilevered spring and it and U.S. Pat. No. 4,792,429 show how the spring ends were designed to provide spring capture and retention in the unlikely event that a cantilever spring broke during operation. However, machining of these patents' leaves and capturing mechanisms are expensive and time consuming. SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to modify the typical holddown spring assembly to eliminate the likelihood of debris from broken parts. This is accomplished by the use of a fuel assembly holddown leaf spring assembly instead of the typical cantilever spring of some commercial reactor designs. The holddown leaf spring assembly of the invention improves or duplicates the cantilever holddown spring characteristics while capturing both ends of the spring without elaborate capturing and retention mechanisms. In addition, the number of springs may be cut in half and machining costs reduced. The novel fuel assembly holddown leaf spring assembly is constructed with similar materials as the prior art cantilever springs. The location of the top of the spring is in the opposite corner as compared to the cantilever design. This is due to the fact that the length of the leaf spring has to be longer than the cantilever and must be fit onto the top nozzle. The additional length requires the leaf spring to extend from near the top nozzle corner which accepts the upper core plate's fuel assembly aligning pins to the opposite aligning pin corner. The leaves themselves require only one hole being drilled at each end. The holes are used to secure the leaves to the upper end fitting. The use of welding plugs as described in Swedish patent application No. 9002638-6, filed Aug. 14, 1990, is more amenable to the design. However, the leaf spring could be designed to be held by spring screws, as in the prior art cantilevered spring design of U.S. Pat. Nos. 5,053,191 and 4,792,429. The structure of the holddown leaf spring assembly with spring retention means of the invention for use on an upper end fitting of a nuclear fuel assembly includes a unitary elongated metal bar having two substantially tapered width leg portions joined by an arcuate transition portion therebetween adjacent the reduced width end of each of the tapered width leg portions. The wider and opposite end of each of the tapered width leg portions from the reduced width end and adjacent transition portion are adapted to be mounted to the fuel assembly end fitting by spring retention means. The spring retention means are pins welded within openings in the wider opposite ends of the leg portions and these wider opposite ends are adapted to be received and retained by the pins in spring retaining slots in the upper end fitting. The opposite ends are straight in order to facilitate their insertion into spring retaining slots in the upper end fitting. Alternatively, the spring retention means are screws within openings in the opposite ends and caps as shown in U.S. Pat. No. 4,792,429 and U.S. Pat. No. 5,053,191. The novel structure of the instant invention prevents spring breakage and resultant debris by having an even stress distribution. This is created by the tapered width or cross-section. Moreover, with both ends of the leaf spring attached by retaining structure, in the event a spring breaks, there are no unrestrained loose parts to create debris. The attachment of the leaf ends within slots in the upper end fitting also avoids bends at the leaf spring ends and permits use of only welded or staked spring holddown pins in spring leg end openings without caps and the complicated and expensive machining they require. Accordingly, the holddown leaf spring assembly of the invention is easier to manufacture and requires fewer parts than the prior art. An added advantage is derived from the narrowing of the legs and the transition portion by the taper and elimination of retaining caps in that this structure opens the flow path of reactor coolant when coupled with a narrowing of the lower end fitting marginal lip or rim. The effect of a linearly changing spring force by the tapering of the width in the illustrated embodiment is to distribute and reduce localized stresses in the metal bars as compared to an untapered bar. It is also possible to reduce localized stresses by tapering the bar thicknesses. The word "width", as used in the claims, applies both to horizontal width and vertical width or thickness, although, it is generally more expensive to taper the vertical "width" or thickness. The bars are of an INCONEL material typically used in prior art cantilevered springs.