Patent Number: 061447161
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT In a typical nuclear reactor heat is generated within the core of the reactor vessel as a result of nuclear fission. The heat is employed to generate steam, which in turn drives turbine-generators to produce electricity. In a pressurized water nuclear reactor the heat in the core is transferred to a coolant moderator, commonly borated water, which is transported under pressure to a steam generator that places the coolant in heat transfer relationship with a secondary fluid. The secondary fluid is vaporized into steam which is used to drive the turbine-generators. The nuclear fuel within the core is typically encapsulated in cylindrical, elongated rods often referred to as fuel elements. The fuel elements are maintained in a polygonal array and, in one preferred embodiment, extend in a longitudinal direction to a length of approximately fourteen feet. The array is generally referred to as a fuel assembly and is bounded by an upper and lower nozzle and maintained in position and appropriately spaced by fuel element support grids that are secured at spaced locations along the longitudinal length of the assembly. Interspersed among the fuel elements within the assembly are control rod guide tubes and instrumentation thimbles that are symmetrically arranged in place of fuel element locations and are used to guide the control rods and act as conduits for in-core instrumentation. The control rods are used to control the fission process by absorbing neutrons in the core that would otherwise react with the nuclear fuel. The control rods are movable into and out of the core through the guide tubes to control the level of reactivity. The coolant within the core that flows from a region below the fuel, up through each fuel assembly and out its nozzle. The coolant is a moderator that slows the speed of the neutrons to increase the efficiency of the fission process. When the control rods are removed from the core the corresponding thimble tubes are filled with the coolant moderator which increases the fission reactions in the fuel in the cells surrounding those guide tubes. A more detailed understanding of the operation of a pressured water nuclear reactor can be had by referring to U.S. Pat. No. 5,303,276 issued Apr. 12, 1994, entitled "FUEL ASSEMBLY INCLUDING DEFLECTIVE VANES FOR DEFLECTING A COMPONENT OF THE FLUID STREAM FLOWING PAST SUCH A FUEL ASSEMBLY." FIG. 1 is a top plan view of a fuel assembly support grid 10 incorporating features of this invention and having a perimeter 12 formed in the shape of a square. It should be appreciated, however, that the concepts of this invention can be applied to fuel element support grids employing different shaped perimeters, such as the hexagonal fuel assembly illustrated in the previously referenced U.S. Pat. No. 5,303,276. The grid assembly illustrated in FIG. 1 is constructed from an evenly spaced, parallel array of lattice grid straps 14, which intersect with a similar, orthogonally positioned, evenly spaced, parallel array of lattice grid straps 16. The lattice array is welded to a peripheral strap 20 which forms the perimeter of the grid. The walls of the straps, intermediate the intersections with the corresponding orthogonal straps, define cells through which the fuel assemblies, guide tubes and instrumentation thimbles pass. FIG. 1 illustrates a 17 by 17 array of cells, though it should be appreciated that the application of the principles of this invention are not affected by the number of fuel elements in an assembly. The lattice straps which form the orthogonal members 14 and 16 shown in FIG. 1, are substantially identical in design. While the lattice straps 14 and 16 are substantially identical, it should be appreciated that the design of some lattice straps 16 will vary from other lattice straps 16, as well as some straps 14 vary from other straps 14, to accommodate guide tube and instrument thimble locations. Reference character 42 in FIG. 1 identifies those cells which support fuel elements and reference character 34 shows the cells that are attached to the guide tubes and instrumentation thimbles. As shown in FIG. 3 most walls of the cells that accommodate fuel elements are provided with a number of stamped, protruding segments that are tooled by appropriate dies as is known and used in the industry. The upper and lower stamped segments 26 bulge out in one direction and form dimples for supporting the fuel elements against juxtaposed diagonal springs which protrude from the opposite cell wall. The remaining centrally located, stamped section 28, in the same wall as the previously described dimples, bulges in the opposite direction into the adjacent cell and forms a diagonal spring for pressuring the fuel element against dimples 26 which protrude into the that adjacent cell from its opposite wall. FIG. 3 illustrates a portion of the lattice strap that forms the wall to a single cell and extends just over the position where it would intersect with the corresponding, adjacent, orthogonal lattice straps to which it would be attached. In accordance with this invention, as shown in FIG. 3, the diagonal springs 28 are formed from two narrow parallel cuts in the cell wall that extend at a diagonal substantially over the width of the wall. The narrow slits which form the spring terminate at either end so that a phantom line 27 drawn between the ends of the adjacent slits runs parallel to the line of intersection of the cell walls. In this way the only impediment to the coolant flow at any point along the spring is limited to the thickness of the spring material. Diagonal springs constructed in this fashion maximize the contact area with the fuel element while minimizing any impediment to coolant flow. Preferably the spring is chamfered at its edges 29 where it contacts the fuel element, to reduce the potential for damaging the surface of the fuel rods as they are inserted into the grid. To increase the flexure of the spring and soften its impact, the slits which form the spring are extended at each end in a direction parallel to the line of intersection with the adjacent wall and away from the spring as illustrated by reference character 31. Mixing vanes 32 extend from the upper edges of the lattice straps at some of the segments which form the walls of the cells 42 through which the fuel assemblies pass. In accordance with this invention, the mixing vanes are arranged in a predetermined pattern that can be better appreciated by referring to patent application Ser. No. 08/887,017 (docket ARF96-003), filed concurrently herewith. As shown in FIG. 1 the cells 34 support the guide tubes and instrumentation thimbles through which the control rods and the in-core instrumentation pass. The cells 34 differ from the fuel element support cells 42 in that they have none of the support members 26 or 28 protruding into their interior, or mixing vanes 32 extending from their walls. The mixing vanes result in a pressure drop across the cells that support the fuel elements, which in turn causes a pressure differential between those adjacent cells that support the guides tube or the instrument thimbles. By reducing the opening between cells, this invention minimizes the affect of that pressure differential. By reducing the opening in the cell walls this invention increases the mass of material in the walls supporting the springs, over that of the prior art configuration shown in FIG. 1, which adds to the stiffness of the wall and improves the grids overall strength. The increased mass also adds to the stiffness of the spring. Accordingly, it is preferable to add further flexure to the spring to lessen the likelihood that the fuel elements will be scored during assembly. In a traditional fuel assembly lattice grid structure, as shown in FIG. 2, the straps are provided with slits 118 which extend from the bottom of the strap to half way up its height, at the intersection where it meets with the straps running in the orthogonal direction. The intersecting straps are provided with similar slits 118 that extend from their top surface to half way down the strap. The straps are then fitted together at their slits with one slit sliding over the other at each intersection to form an egg-crate pattern that locks the intersections and defines the cells. In accordance with this invention, As shown in FIG. 3, the slits 44 are extended more than halfway across the straps to add greater flexure to the springs when the straps are welded at their lines of intersection. Accordingly, this invention provides an improved fuel assembly incorporating a support grid spring design that optimizes reactor coolant flow during operation in a manner that improves DNB performance, reduces pressure drop and improves grid crush resistance strength.