NUCLEAR FUEL ROD SUPPORT GRID WITH IMPROVED MULTIPLE DIMPLE ARRANGEMENT

Abstract: The invention relates to an improved fuel-rod support grid for use in a nuclear fuel assembly, which grid is formed by interleaved straps arranged to form a matrix of cells of receiving one fuel rod each. Each strap portion forming a wall (44) of one of the cells (42) has two sets (50, 54) of dimples (58-62) formed respectively adjacent the upper and lower ends of the wall, each set comprising a pair of upper and lower dimples (58, 60) which project from one side of the wall and a center dimple (62) which projects from the opposite side of the wall. The dimples are arranged to resiliently and frictionally engage fuel rods, when received through the respective cells, in a manner minimizing moments in the cell walls of the grid.

Description: The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a grid having an improved arrangement of multiple dimples for supporting nuclear fuel rods. In most nuclear reactors, the reactor core is composed of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the grid straps to impart strength and rigidity to the grid. Representative of the prior art are the grids described and illustrated in U.S., Patents Nos. 28,079; 3,679,547; 4,090,918; 4,111,348; 4,124,444; 4,125,435; 4,137,125; 4,325,786; 4,364,902; 4,411,862; and, 4,474,730. For grids fabricated of zircaloy type materials, grid springs are not able to maintain their design preload after installation of the fuel assembly into the nuclear reactor core due to irradiation and temperature effects. In some circumstances it is difficult to achieve the spring rates and spring forces considered necessary to control bowing of fuel rods. It is the principal object of the invention to provide an improved dimple arrangement in fuel assembly grids which will provide the force necessary to support the fuel rods in the reactor core with minimal degradation over time. The invention accordingly resides in a nuclear-fuel-rod support grid comprising a plurality of interleaved straps arranged to form a matrix of hollow cells each for receiving a nuclear fuel rod and each of which cells is defined by pairs of oppositely disposed elongate walls shared with adjacent cells, each of said walls of each single cell forming on one side thereof a part of the perimeter of said single cell and forming on its opposite side part of the perimeter of one of several cells disposed about and adjacent to said single cell, characterized by (a) a first set of dimples formed adjacent an upper end of each wall of said pairs defining each cell, and (b) a second set of dimples formed adjacent a lower end of each wall of said pairs defining each cell, each of said first and second sets of dimples being composed of a pair of upper and lower dimples which project outwardly from the same one side of the wall on which the dimples of the respective set are formed, and a center dimple which is located between said upper and lower dimples and projects from the opposite side of the wall in which said dimples are formed, said dimples of said first and second sets being adapted to resiliently and frictionally engage fuel rods when received through adjacent ones of said cells. An arrangement of dimple sets in accordance with the invention as defined above will minimize moments in the grid cell walls. Various specific arrangements embodying the invention are possible to form a semi-rigid cell in which a fuel rod is restrained against lateral displacement and rotation. Preferably, the straps of the improved grid are joined at each intersection thereof by formation of a weld bead. A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an elevational view, partly in section, of a nuclear fuel assembly with fuel rod support grids embodying the invention; Figure 2 is a top plan view of a fuel-rod support grid embodying the invention; Figure 3 is an enlarged fragmentary top plan view of the lower right-hand corner of the grid of Fig. 2; Figures 4, 5, 6 and 7 are sectional views taken along lines 4--4, 5--5, 6--6 and 7--7, respectively, of Fig. 3 and showing the arrangements of the dimple sets and of the multiple dimples of each set in the walls of a given cell of the grid; Figure 8 is a schematic representation of two fuel rods in two adjacent cells of the improved support grid demonstrating the restraint provided by the arrangement of dimple sets in the support grid; and Figures 9a to 9e are schematic representations of different possible arrangements of the dimples in the sets thereof on two of the opposite walls of a cell in the improved support grid. In the following description, like reference characters designate like or corresponding parts throughout the several views, and terms such as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are employed as words of convenience not to be construed as limiting terms. In General Referring now to the drawings, and particularly to Fig. 1, there is shown an elevational view of a fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10. The fuel assembly 10 basically includes a lower end structure or bottom nozzle 12 for supporting the assembly on the lower core plate (not shown) in the core region of a reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 14 which project upwardly from the bottom nozzle 12. The assembly 10 further includes a plurality of transverse grids 16 which have the improved construction of the present invention, as will be described in detail later. The improved grids 16 are axially spaced along the guide thimbles 14 and an organized array of elongated fuel rods 18 transversely spaced and supported by the grids 16. Also, the assembly 10 has an instrumentation tube 20 located in the center thereof and an upper end structure or top nozzle 22 attached to the upper ends of the guide thimbles 14. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 18 in the array thereof in the assembly 10 are held in spaced relationship with one another by the improved grids 16 of the present invention, being spaced along the fuel assembly length. Each fuel rod 18 includes nuclear fuel pellets 24 and the opposite ends of the rod are closed by upper and lower end plugs 26, 28 to hermetically seal the rod. Commonly, a plenum spring 30 is disposed between the upper end plug 26 and the pellet 24 to maintain the pellets in a tight, stacked relationship within the rod 18. The fuel pellets 24 composed of fissile material are responsible for creating the reactive power of the nuclear reactor. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work. To control the fission process, a number of control rods 32 are reciprocally movable in the guide thimbles 14 located at predetermined positions in the fuel assembly 10. Specifically, the top nozzle 22 includes a rod cluster control mechanism 34 having an internally threaded cylindrical member 36 with a plurality of radially extending flukes of arms 38. Each arm 38 is interconnected to a control rod 32 such that the control mechanism 34 is operable to move the control rods 32 vertically in the guide thimbles 14 to thereby control the fission process in the fuel assembly 10, all in a well-known manner. Improved Fuel Rod Support Grid Construction For precisely maintaining the spacing between the fuel rods 18 in the reactor core, preventing both lateral and longitudinal movement thereof and minimizing the potential of the rods to bow over their useful life, the improved grids 16 are designed in accordance with the present invention to impose forces about the circumference of the individual fuel rods which project radially inwardly toward the longitudinal axis of the rod. Referring now to Figs. 2 to 7, it is seen that each of the improved grids 16 includes a multiplicity of interleaved inner straps 40 having an egg-crate configuration designed to form a matrix of hollow cells, indicated at 42, a majority of which individually accept one fuel rod 18 and a minority of which accept one control rod guide thimble 14. At each intersection of the inner grid straps 40, the straps are joined by formation of a weld bead 43. Each cell 42 for receiving one fuel rod 18 is defined by two pairs of opposing elongated walls 44 which are shared with adjacent ones of the cells. Each wall 44 of the pairs thereof which define a single cell 42, has one side 46 which forms a part of the perimeter of the single cell 42 and also has an opposite side 48 which forms part of the perimeter of one of several cells 42 disposed about and adjacent to the single cell. The improvements of the present invention incorporated by each support grid 16, as seen in detail in Figs. 3 to 7, basically include a first set of dimples, generally designated 50, formed at an upper end 52 of each wall 44 of the pairs thereof defining each cell 42 of the grid 16. Each support grid 16 also includes a second set of dimples, generally designated 54, formed at a lower end 56 of each wall 44 of the pairs thereof defining each grid cell 42. More particularly, each of the first and second sets of dimples 50, 54 is composed of a pair of upper and lower dimples 58, 60 which project outwardly from the same one side 46 of the cell wall 44 and a center dimple 62 which is located between and adjacent to the upper and lower dimples 58, 60 and projects outwardly in an opposite direction from the opposite side 48 of the wall 44. The first sets 50 of dimples on the walls 44 of the pairs thereof defining each cell 42 are formed at generally the same elevation. And, likewise, the second sets 54 of dimples on the walls 44 defining each grid cell 42 are formed at generally the same elevation. The dimples 58, 60, 62 in the first and second sets 50, 54 thereof are adapted to resiliently and frictionally engage fuel rods 18 when received through the respective cells 42. Specifically, the dimples 58, 60, 62 are fabricated from a metal material, such as zircaloy, of the inner straps 40 in any suitable manner, such as by a conventionally die punching and deforming technique. They have a generally U-shaped configuration and, because they project outwardly from the cell wall 44 in an alternating, staggered fashion, they provide relatively rigid protrusions which are capable of attaining high spring rates and of providing a controlled force on the fuel rods even though the straps 40 from which they are formed are relatively flexible in nature. Additional rigidity and strength is imparted to the grid 16 by a series of outer straps 64 which are attached together and peripherally enclose and are connected with the inner straps 40. Figure 8 shows two fuel rods 18 disposed through two adjacent cells 42 in the improved support grid 16. It represents how the arrangement of dimple sets 50, 54 on the walls 44 defining the cells 42 provide the forces which serve to resist the attempt of the two fuel rods to bow toward each other above the top of the grid 16. First, lateral rotation of the individual fuel rods 18 is restrained by the grid dimple forces at each of the dimple sets providing restraining moments. Second, the particular dimple set under direct compression, such as dimple set 50 on the wall 44 between the fuel rods 18 in Fig. 8, is very stiff because of the close proximity of the load paths across the cell wall. This stiffness limits the lateral motion of the fuel rods toward one another. finally, a third but small contribution to the restraining moment is provided at each dimple set elevation by the interaction of opposite sets of dimples on the opposing walls acting on the fuel rods. Figures 9a through 9e schematically illustrate five different possible arrangements of the dimples 58, 60, 62 in the sets 50, 54 thereof on two of the four opposite walls 44 defining a single cell 42a in the improved support grid 16. The other two of the four opposite walls 44 can contain any of these arrangements also. In Fig. 9a, the upper and lower dimples 58, 60 in each of the first and second sets 50, 54 thereof on one wall 44a and the center dimple 62 in each of the first and second sets 50, 54 thereof on the other wall 44b, of one pair of the walls 44 defining the single cell 42a project outwardly from the opposite sides 48 of the respective one and other walls 44a, 44b into the adjacent cells 42b, 42c. On the other hand, the upper and lower dimples 58, 60 in each of the first and second sets 50, 54 thereof on the other wall 44b, and the center dimple 62 in each of the first and second sets 50, 54 thereof on the one wall 44a, of the one pair of the walls 44 defining the single cell 42a project inwardly from the one sides 46 of the respective other and one walls 44b, 44a into the single cell 42. In Fig. 9b, the upper and lower dimples 58, 60 in each of the first and second sets 50, 54 thereof on both walls 44a, 44b of one pair of the walls 44 defining the single cell 42a project outwardly from the opposite sides 48 of the respective walls 44a, 44b into the adjacent cells 42b, 42c. On the other hand, the center dimple 62 in each of the first and second sets 50, 54 thereof on both walls 44a, 44b of one pair of the walls 44 defining the single cell 42a project inwardly from the one sides 46 of the respective walls 44a, 44b into the single cell 42a. In Fig. 9c, the upper and lower dimples 58, 60 in each of the first and second sets 50, 54 thereof on both walls 44a, 44b of one pair of the walls 44 defining the single cell 42a project inwardly from the one sides of the respective walls 44a, 44b into the single cell 42a. On the other hand, the center dimple 62 in each of the first and second sets 50, 54 thereof on both walls 44a, 44b of one pair of the walls 44 defining the single cell 42a project outwardly from the opposite sides 48 of the respective walls 44 into the adjacent cells 42b, 42c. In Fig. 9d, the upper and lower dimples 58, 60 in the first set 50 thereof and the center dimple 62 in the second set 54 thereof on one wall 44a, and the center dimple 62 in the first set 50 thereof and the upper and lower dimples 58, 60 in the second set 54 thereof on the other wall 44b, of one pair of the walls 44 defining the single cell 42a project outwardly from the opposite sides 48 of the respective one and other walls 44a, 44b into the adjacent cells 42b, 42c. On the other hand, the center dimple 62 in the first set 50 thereof and the upper and lower dimples 58, 60 in the second set 54 thereof on the one wall 44a, and the upper and lower dimples 58, 60 in the first set 50 thereof and the center dimple 62 in the second set 54 thereof on the other wall 44b, of one pair of the walls 44 defining the single cell 42a project inwardly from the one sides 46 of the respective one and other walls 44a, 44b into the single cell 42a. In Fig. 9e, the upper and lower dimples 58, 60 in the first set 50 thereof and the center dimple 62 in the second set 54 thereof on one wall 44a, and the upper and lower dimples 58, 60 in the first set 50 thereof and the center dimple 62 in the second set 54 thereof on the other wall 44b, of one pair of the walls 44 defining the single cell 42a project outwardly from the opposite sides 48 of the respective one and other walls 44a, 44b into the adjacent cells 42b, 44c. On the other hand, the center dimple 62 in the first set 50 thereof and the upper and lower dimples 56, 60 in the second set 54 thereof on the one wall 44a, and the center dimple 62 in the first set 50 thereof and the upper and lower dimples 58, 60 in the second set 54 thereof on the other wall 44b, of one pair of the walls 44 defining the single cell 42a project inwardly from the one sides 46 of the respective one and other walls 44a, 44b into the single cell 42a.

Claims: 1. A nuclear-fuel-rod support grid comprising a plurality of interleaved straps arranged to form a matrix of hollow cells each for receiving a nuclear fuel rod and each of which cells is defined by pairs of oppositely disposed elongate walls shared with adjacent cells, each of said walls of each single cell forming on one side thereof a part of the perimeter of said single cell and forming on its opposite side part of the perimeter of one of several cells disposed about and adjacent to said single cell, characterized by (a) a first set (50) of dimples formed adjacent an upper end (52) of each wall (44) of said pairs defining each cell (42), and (b) a second set (54) of dimples formed adjacent a lower end (56) of each wall (44) of said pairs defining each cell (42), each of said first and second sets (50, 54) of dimples being composed of a pair of upper and lower dimples (58, 60) which project outwardly from the same one side (46) of the wall (44) on which the dimples (58, 60, 62) of the respective set are formed, and a center dimple (62) which is located between said upper and lower dimples (58, 60) and projects from the opposite side (48) of the wall (44) in which said dimples (58, 60, 62) are formed, said dimples (58, 60, 62) of said first and second sets (50, 54) being adapted to resiliently and frictionally engage fuel rods when received through adjacent ones of said cells. 2. A nuclear-fuel-rod support grid according to claim 1, characterized in that the first sets (50) of dimples on the walls (44) of each cell (42) are located at generally the same elevation. 3. A nuclear-fuel-rod support grid according to claim 1 or 2, characterized in that the second sets (54) of dimples on the walls (44) of each cell (42) are located at generally the same elevation. 4. A nuclear-fuel-rod support grid according to claim 1, 2 or 3, characterized in that said upper and lower dimples (58, 60) in each of said first and second sets (50, 54) on one wall (44a), and said center dimple (60) in each of said first and second sets (50, 54) on the other wall (44b), of one pair of said walls (44) defining said single cell (42a) project outwardly from the respective walls (44a, 44b) into the respective adjacent cells (42b, 42c), whereas said upper and lower dimples (58, 60) in each of said first and second sets (50, 54) on said other wall (44b), and said center dimple (62) in each of said first and second sets (50, 54) on said one wall (44a) of said one pair of walls (44) defining said single cell (42a) project inwardly from the respective walls (44a, 44b) into said single cell (42a). 5. A nuclear-fuel-rod support grid according to claim 1, 2 or 3, characterized in that said upper and lower dimples (58, 60) in each of said first and second (50, 54) sets on the walls (44a, 44b) of one of said pairs of oppositely disposed walls (44) defining said single cell (42) project outwardly from the respective walls (44) into said adjacent cells (44b, 44c), whereas the center dimple (62) in each of said first and second sets (50, 54) on said walls (44a, 44b) of said one of said pairs of oppositely disposed walls (44) defining said single cell (42a) project inwardly from the respective walls (44) into said single cell (42a). 6. A nuclear-fuel-rod support grid according to claim 1, 2 or 3, characterized in that said upper and lower dimples (58, 60) in each of said first and second sets (50, 54) on the walls (44a, 44b) of one of said pairs of oppositely disposed walls (44) defining said single cell (42a) project inwardly from the respective walls (44) into said singel cell (42a), whereas said center dimple (62) in each of said first and second sets (50, 54) on the walls (44a, 44b) of said one of said pairs of oppositely disposed walls (44) defining said single cell (42a) project outwardly from the respective walls (44) into said adjacent cells (42b, 42c). 7. A nuclear-fuel-rod support grid according to claim 1, 2 or 3, characterized in that said upper and lower dimples (58, 60) in said first set (50) and said center dimple (62) in said second set (54) on one wall (44a), and said center dimple (62) in said first set (50) and said upper and lower dimples (58, 60) in said second set (54) on the other wall (44b), of one pair of the walls (44) defining said single cell (42a) project outwardly from the respective walls (44a, 44b) into said adjacent cells, whereas said center dimple (62) in said first set (50) and said upper and lower dimples (58, 60) in said second set (54) on said one wall (44a), and said upper and lower dimples (58, 60) in said first set (50) and said center dimple (62) in said second set (54) on said other wall (44b), of said one pair of walls (44) defining said single cell (42a) project inwardly from the respective walls (44a, 44b) into said single cell (42a). 8. A nuclear-fuel-rod support grid according to claim 1, 2 or 3, characterized in that said upper and lower dimples (58, 60) in said first set (50) and said center dimple (62) in said second set (54) on one wall (44a), and said upper and lower dimples (58, 60) in said first set (50) and said center dimple (62) in said second set (54) on the other wall (44b), one one pair of said walls (44) defining said single cell (42a) project outwardly from the respective walls (44a, 44b) into said adjacent cells (42b, 42c), whereas said center dimple (62) in said first set (50) and said upper and lower dimples (58, 60) in said second set (54) on said one wall (44a), and said center dimple (62) in said first set (50) and said upper and lower dimples (58, 60) in said second set (54) on said other wall (44b), of said one pair of said walls (44) defining said single cell (42a) project inwardly from the respective walls (44a, 44b) into said single cell (42a).

Inventors: RYLATT, JOHN ANTHONY
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G21C3
IPC codes: G21C3