Patent Number: 039322165
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear power systems, and more particularly to grid structures for nuclear reactor fuel elements, and the like. 2. Description of the Prior Art In order to produce useful nuclear power, a suitable concentration of fissionable material must be assembled in a reactor core under proper conditions. This material, of which the uranium isotope U.sup.235 is typical, generates power through sustained fission processes. In the usual fission events, neutrons are absorbed in the U.sup.235 nuclei. These absorptions cause the nuclei, in general, to release energy and to disintegrate into the nuclei of lighter elements. If the processes are to continue, the fissioning nuclei also must produce a new generation of neutrons in an abundance that is sufficient to initiate an equal number of new fissions. There are many formidable environmental, physical, and economic difficulties that must be overcome in order to build a commercially acceptable nuclear reactor while maintaining this essential neutron balance. For example, in a power reactor that transfers the fission process energy to a flowing stream of pressurized water, the fissionable material is loaded into hollow tubes that are referred to as "fuel rods". For ease of shipment, installation, and removal, as well as to enhance the structural integrity of the core, these rods usually are grouped together in sub-assemblies that are called "fuel elements". A typical commercial reactor fuel element, for instance, may have an array of more than two hundred of these individual fuel rods. In an assembled fuel element, the rods usually are separated from each other to provide spaces for coolant water flow in order to remove the heat that is generated in the core. This rod arrangement, moreover, when viewed in cross section, is generally square. The individual rods are held in their relative positions by means of cellular grid structures that are formed from interlocking plates. The grids, moreover, are spaced from each other at intervals along the length of the array of fuel rods. Each of these fuel rods in the array are lodged in respective cells within each of the grids. Within each of the cells, the respective fuel rods are restrained, or engaged by detents or "stops" that protrude from the plates that form the grid and press against the respective fuel rod surfaces. These grids, however, conflict with other desirable features of a well-designed reactor core. For instance, to make the most efficient use of the fissionable material charge within a reactor core, pressure losses in the core coolant should be minimized. Further in this regard, because the fuel elements ordinarily are nested closely together in a reactor pressure vessel, it is customary to provide tongues on the bands that form the peripheries of the grid structures. These tongues have portions that jut out from the bands to guide the fuel elements as they are being inserted into or being withdrawn from the reactor pressure vessel. Although these tongues perform a useful function, they nevertheless tend to obstruct the flow path and thereby increase the coolant pressure losses. These tongues also impose an adverse influence on the neutron balance within the reactor core. In this regard, it should be noted that almost all non-fissionable materials within a reactor core tend to act as "poisons" that absorb neutrons without producing a corresponding generation of neutrons to sustain the fission process. Thus, the tongues increase these parasitical neutron losses within the reactor and thereby decrease the "life" of the core. The clear desirability of reducing these losses, however, is superseded by the need to insure smooth insertion and withdrawal of the fuel elements from the reactor vessel. This need is further emphasized if it is recognized that during withdrawal, the individual fuel elements probably will have to be handled with remotely operated tools because of dangerously high radioactivity levels. If, for example, two radioactive fuel elements should lock together during withdrawal from the core, the problem of disengaging these elements from each other to complete the withdrawal through some sort of remote manipulation can be time consuming, expensive and possibly dangerous. One suggested technique proposes welding the grid plates to the fuel rods and eliminating the bands that circumscribe the grid structure. On one side, the ends of the individual plates protrude beyond the fuel element framework and the corners of these protruding plates are tapered. The welded structure, however, can lead to manufacturing, processing and quality control difficulties. Accordingly, there is a need for some fuel element technique that will aid reactor core assembly and reduce coolant pressure losses and parasitical neutron absorption. SUMMARY OF THE INVENTION These inefficiencies and difficulties that have characterized the prior art have been overcome by bevelling or chamfering the edges of the band that encircle the grid structure. In this way, parasitical material that obstructs the coolant flow is removed from the core. This reduction in core material, in accordance with the terms of the invention, increases neutron efficiency within the reactor and reduces reactor coolant pressure losses without imposing the difficulties of a welded fuel rod structure. More specifically, a typical embodiment of the invention comprises a cellular structure that is formed from an array of generally planar grid plates that interlock with each other at right angles. These interlocking plates form a group of cells, each of which have essentially square cross sections. The grid plates, moreover, have stops or detents that protrude inwardly toward the center of each of the respective cells. These stops press against corresponding portions of the fuel rods that are lodged within the cells in order to secure the rods in their proper respective positions. The ends of the grid plates protrude slightly beyond the fuel rods that form the border or margin of the assembly. The protruding edges of these plates abut and are welded to a band structure that circumscribes the periphery of the interlocked plates. The band structure enhances the physical integrity of the grid and, being provided with stops that protrude inwardly toward the cell structure, also serves to engage and restrain the fuel rods in the marginal rows. The corner edges of the protruding plates are trimmed or bevelled to provide a sloping transition that matches the greater width of the grid plates to the lesser width of the peripheral band. These sloping transitions preferably commence within the volume that is defined by the assembled fuel rods, or at the margin that the border rows of these rods establish. To encircle the interlocked grid plates, the band structure usually has four generally flat sides that meet, forming four corners. In accordance with another feature of the invention, the corner edges of the band structure are chamfered, bevelled, ground down, or otherwise trimmed away to produce vee shaped slots. These novel structural characteristics of the invention eliminate sharp edges that tend to cause adjacent fuel elements to lock together. The sloping portions and vee-shaped slots provide bearing surfaces that have a camlike action which causes the fuel elements to slide relative to each other without introducing parasitical flow-obstructing materials. Viewed in a somewhat different context, the invention further includes a member for a fuel element grid that is formed from a generally planar strip. This strip is, of course, one of the plates from which the grid structure is built. More specifically, opposite sides of the member preferably have longitudinally extending faces that terminate in longitudinal and perpendicular edges. One of the longitudinal edges is intersected by equidistantly spaced slits that connect with individual paddle-shaped cutouts in the middle portion of the strip. To form the stops or detents, indentations that protrude beyond the plane of the strip also are formed in the strip's longitudinal edges and middle portion, approximately midway between the cutouts. A particularly novel feature of this member is characterized by the manner in which the edges are joined together at the corners of the plate. Sloping surfaces connect each adjacent pair of longitudinal and perpendicular edges, the sloping surfaces intersecting to form obtuse angles with the individual edges. Thus, each grid plate is provided with the cam surfaces or inclined planes that tend to keep adjacent fuel elements from jamming during relative movement within the reactor core. Clearly, the invention provides a technique that largely overcomes the disadvantages of the prior art without introducing additional material into the core or relying on troublesome welds. 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 drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .