Patent Number: 056639937
Section: summary

TECHNICAL FIELD This invention relates generally to fuel bundle assemblies for boiling water nuclear reactors, and specifically to water rod configurations within such assemblies. BACKGROUND Conventional boiling water reactor fuel bundle assemblies utilize one or more water rods extending upwardly through the bundle to provide a source of coolant/moderator to maintain a more uniform distribution of power throughout the bundle. For convenience, reference will be made to water rods in the plural, recognizing that a single water rod bundle is also well known in the art. Typically, the water rods are closed at both ends by end plugs received in upper and lower tie plates of the bundle assembly. Side entry and exit holes are formed in the lower and upper portions, respectively, of each rod to allow a portion of the liquid coolant flowing upwardly through the bundle to pass through the rods. In some cases, the water rods transition at both ends to a larger diameter center section which extends for most of the axial lengths of the rods. It has been attempted to combine side entry holes and a metering device within the lower diameter transition, but this arrangement has not always permitted the accurate regulation of coolant within the water rod, and because of the location of the metering device, is relatively expensive to manufacture. In all cases, it is desirable to accurately meter the flow through the water rods to the amount required to prevent boiling within the rods. If there is too little flow, the coolant will boil within the rods, thus negating the purpose of the rods which is to distribute non-boiling water throughout the bundle length. Too much flow, on the other hand, starves the region around the fuel rods, outside the water rods. DISCLOSURE OF THE INVENTION We now have determined that incorporation of a coolant flow metering device within the water rod end plugs is not only a more cost effective solution to the problem than using entry and exit holes and a separate metering device within the tube, but also a more accurate method in terms of coolant flow regulation. In the exemplary embodiment, a lower end plug construction for water rods is provided which includes a relatively large entry bore, a reduced diameter center bore and an enlarged diameter exit bore. The end plugs are preferably welded to the lower ends of the water rods about annular shoulders formed on the end plugs. The plugs are also provided with external screw threads so that the water rods can be threadably secured to the lower tie plate, but other fastening means may be employed. Accordingly, in its broadest aspects, the present invention relates to a nuclear fuel rod assembly comprising a plurality of fuel rods and at least one water rod held together in a bundle, and wherein end plugs at the lower ends of the fuel rods and the at least one water rod engage a lower tie plate, the improvement wherein the end plug at the lower end of the water rod is hollow and includes a multi-diameter flow metering bore for regulating coolant flow into the water rod. Additional objects and advantages of the invention will become apparent from the detailed description which follows.