Patent Number: 043137970
Section: summary

TECHNICAL FIELD This invention relates to fuel assemblies for nuclear reactors and, more particularly, to a sleeve disposed in surrounding relationship to non-fuel bearing members in the fuel assembly. BACKGROUND ART In water-cooled nuclear reactors, the reactor core in which the fission chain is sustained generally contains a multiplicity of fuel assemblies. These fuel assemblies are mechanically identical and interchangeable, each being designed to maintain its own structural integrity. A fuel assembly typically contains, among other things, a plurality of longitudinally extending parallel members, some bearing fuel and some not bearing fuel. The parallel members are supported vertically by end fittings. Lateral bracing and spacing of the members are provided by spacer grids which contain a plurality of cellular voids. The spacer grids are disposed across the parallel members so that each parallel member extends through one cellular void per grid. The spaces between adjacent members create flow channels through which is circulated water, flowing from bottom to top. Most of the parallel members contain fuel and are known as fuel rods. The non-fuel bearing members are hollow and comprise an instrument tube and a plurality of guide tubes. The instrument tube accommodates various monitoring devices. The guide tubes are used to receive control rods which regulate the fissioning activity, and hence the generation of heat. Henceforth unless otherwise indicated, the term guide tube will be used to denote instrument tubes as well as guide tubes. The guide tubes have a larger diameter than the fuel rods. Because of this difference in diameter and because all the cellular voids of the spacer grid have equal center-to-center distances, the flow channel between a guide tube and an adjacent fuel rod has a smaller cross-sectional area than the flow channel between two adjacent fuel rods. This difference in flow channel space affects the flow of water, there being more flow in the wider space between two adjacent fuel rods. Water flow, in turn, affects the rate of heat transfer from the fuel to the water, a greater flow providing for a greater heat transfer. Since heat transfer rate is a factor affecting the maximum operating conditions (and hence the energy producing capability) of the reactor, the flow of water in the flow channel between a guide tube and an adjacent fuel rod is a limiting factor in the operation of the reactor. SUMMARY OF THE INVENTION The disclosed invention increases the operating capacity of a nuclear reactor by causing a modification in the flow pattern of the coolant which enhances the coolant's effectiveness. The apparatus provides a thin-walled tubular sleeve closely surrounding but not attached to the exterior surface of a guide tube in a fuel assembly. The wall of the sleeve has tabs projecting outwardly into adjacent flow channels. The sleeve is attached to the wall of a cellular void through which passes the guide tube associated with said sleeve. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .