Patent Number: 053717686
Section: summary

This invention relates to spacers for use within a boiling water reactor. Specifically, a shortened ferrule type spacer is combined with preferably overlying swirl vanes in the upper two phase region of a fuel bundle to provide improved critical power performance with reduced overall pressure drop. BACKGROUND OF THE INVENTION Fuel bundles for boiling water reactors are known. They include a plurality of upstanding parallel fuel rods supported on a lower tie plate with at least some of the fuel rods extending to and fastened to an upper tie plate. A fuel bundle channel surrounds the fuel rods at the lower tie plate and extends to the region of the upper tie plate. In operation, water flows into the fuel bundle through the lower tie plate and flows upward of the fuel bundle in a confined flow path within the channel generating steam. Water and steam exit the fuel bundle from the upper tie plate. The fuel rods are long, slender and flexible. In the dynamics of steam generation, these rods would undergo vibration and come into abrading contact one with another. Consequently, so-called fuel bundle spacers are utilized. These spacers surround each fuel rod at spaced apart vertical locations along the length of the fuel bundles and essentially maintain the fuel rods in their designed side-by-side relation interior of the fuel bundle channel. As pertinent to this disclosure, the use of so-called ferrule spacers in the upper two phase region of the fuel bundle is utilized. The ferrules in ferrule spacers constitute discrete cylinders having typically paired stops on one side of the ferrules and fuel rod biasing springs on the other side of the ferrules. The inside diameter of the ferrules exceeds the outside diameter of the fuel rods so that the fuel rod when biased by the spring is centered with respect to the ferrule. When the ferrules are held together in a matrix, a rigid and highly satisfactory spacing of the fuel rods results. It has been discovered that ferrule spacers in combination with fuel rods in the upper two phase region of a fuel bundle have a beneficial effect on critical power performance. Such spacers tend to augment the required liquid film on the surface of the steam generating fuel rods. The maintenance of a liquid film coating prevents a condition known as "dryout" on the surfaces of the fuel rods. The onset of dryout occurs at the first location where the liquid film on any fuel rod is completely evaporated, i.e., the film thickness equals zero. This phenomenon will occur just upstream of a spacer before the liquid film thickness is augmented by passing through the spacer. (Therefore, the performance of spacers 2 and 3 thicken the liquid films that might experience dryout just upstream of spacers 1 and 2, respectively. Such a condition can lead to overheating of the cladding with resultant damage to the fuel rods. Unfortunately, all spacers to some extent create pressure drop. Pressure drop in the upper two phase region of the fuel bundle is preferably to be avoided as such pressure drop contributes to both thermal hydraulic and nuclear thermal hydraulic instabilities. Swirl vanes may be used in combination with boiling water reactors in the upper two phase region of fuel bundles in such reactors. As relevant to this disclosure, at least the following three disclosures should be considered. Swirl vanes were first proposed in combination with a boiling water reactor fuel rods not confined to fuel bundles. These swirl vanes were the same length as the fuel rods and placed interstitially of the fuel rods. As both the modern expedient of fuel bundles was omitted and pressure drop was high, these designs had no commercial significance. A spacer made entirely of swirl vanes is disclosed in Johansson U.S. patent application Ser. No. 07/702,212 filed May 17, 1991 (now issued U.S. Pat. No. 5,186,891 issued Feb. 16, 1993), entitled SWIRL VANES AND INCONEL SPACER. In this design, swirl vanes are co-extensive with the height of the spacer and occupy each and every spatial interval between fuel rods. As a consequence, high pressure drop is experience in such a spacer. PREVIOUS RELATED DISCLOSURES NOT PRIOR ART Swirl vanes overlying part length rods have been disclosed in Dix patent application Ser. No. 07/702,644 filed May 17 1991, entitled SEPARATION DEVICES WITH PART LENGTH RODS, (now abandoned in favor of continuation-in-part application of Johansson et al., Ser. No. 07/914,389 filed Jul. 15, 1992 entitled OPTIMIZED CRITICAL POWER IN A FUEL BUNDLE WITH PART LENGTH RODS). In this disclosure, the swirl vane is relied upon as both a steam separation device overlying the part length fuel rod as well as a device to improve critical power. The idea is that by separating water and steam in the volume overlying the part length fuel rods, improved venting of the fuel bundle can occur. Further, it has been discovered that swirl vanes in the upper two phase region of the fuel bundle below the first space can improve critical power. It is to be noted that with this concept, the swirl vane obstructs the normal vertical removal of the fuel rod. This being the case, this design has not seen commercial usage for at least this reason. SUMMARY OF THE INVENTION In a boiling water nuclear reactor fuel bundle, the use of a shortened ferrule spacer in combination with overlying swirl vanes is set forth. In the preferred embodiment, the shortened ferrule spacer is placed under any swirl vanes and has an individual ferrule surrounding each fuel rod at the elevation of the spacer. Each ferrule is given both minimum wall thickness in the range of 0.020 inches or less as well as reduced height in the order of 0.9 inch or less. The reduced height and thickness of the ferrule spacer is required to maintain pressure drop within acceptable limits and still tends to augment the required liquid film for steam generation over the fuel rod lengths downstream (that is immediately above) the spacer. At the same time, the swirl vane structure is placed immediately above the ferrule spacer overlying the so-called subchannel region of the ferrule spacer between the fuel rods. The swirl vane placed above the ferrule spacer tends to redistribute water entrained in the subchannel region between the fuel rods to the fuel rods. The objective of this design is to obtain equivalent critical power performance to previously proposed designs while concurrently obtaining a design with minimum possible pressure drop. This is achieved in two new ways. First, the swirl structure is placed above the ferrule structure so that the severe constriction of fluid flow area that would have occurred if the swirl vanes were placed inside the existing ferrule structure has been avoided. Second, these modified spacers with swirl vanes added have only been added at locations where they will contribute to improving critical power performance, typically at the second, third and very infrequently at lower spacer locations from the top of the bundle.