Patent Number: 048030430
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS References is hereby made to the following applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Nuclear Fuel Spacer Grid With Improved Grid Straps" by Edmund E. DeMario, assigned U.S. Ser. No. 473,516 and filed March 9, 1983. 2. "Nuclear Fuel Spacer Grid With Improved Outer Straps" by Edmund E. DeMario et al, assigned U.S. Ser. No. 473,515 and filed Mar. 9, 1983. 3. "Partial Grid For A Nuclear Reactor Fuel Assembly" by Edmund E. DeMario, assigned U.S. Ser. No. 564,049 and filed Dec. 21, 1983. 4. "A Low Pressure Drop Grid For A Nuclear Reactor Fuel Assembly" by Edmund E. DeMario et al, assigned U.S. Ser. No. 567,448 and filed Dec. 30, 1983. 5. "A Coolant Flow Mixer Grid For A Nuclear Reactor Fuel Assembly" by Edmund E. DeMario et al, assigned U.S. Ser. No. 567,450 and filed Dec. 30, 1983. 6. "Nuclear Fuel Rod Support Grid With Improved Multiple Dimple Arrangement" by John A. Rylatt, assigned U.S. Ser. No. 729,387 and filed May 1, 1985. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with a nuclear fuel rod grid having unique fuel rod engaging spring and dimple structures. 2. Descriotion of the Prior Art In most nuclear reactors, the reactor core is comprised 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 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. Representative grid designs include those of some of the patent applications cross-referenced above. One popular conventional grid design, being illustrated and described in U.S. Pat. Ser. No. 4,492,844 to Kobuck et al and assigned to the assignee of the present invention, includes a multiplicity of interleaved inner and outer straps having an egg-crate configuration foming a multiplicity of 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, the outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid. However, several drawbacks arise from this particular grid design. The vertical orientation and configuration of the fuel rod engaging springs require that the springs and thus the grid be of substantial height in order for the springs to have the desired amount of resiliency to function properly. This requirement increases the amount of parasitic structural material utilized in the fuel assembly and the pressure drop through the fuel assembly. Also, the horizontal orientation of the grid dimples, although providing for coolant flow through the dimples without an accompanying pressure drop, causes catching or hang-up of the fuel rods on the dimples as the fuel rods are pulled through the grid. On the other hand, dimples of vertical orientation, although solving the fuel rod hang-up problem, don't allow for coolant flow through the dimples and thus increase pressure drop. Representative of other prior art grid designs are the ones described and illustrated in U.S. Pat. Nos. to Kooistra (3,070,534), Warberg (3,679,547), Jabsen (3,795,040), Wachter et al (3,928,131), Piepers et al (3,646,994), Creagan et al (4,061,536) and Masetti (4,090,918); French Pat. No. 1,497,083; West German Pat. No. 1,961,035; and Japanese Pat. No. 61-90085. While all of these grids may function satisfactory and generally achieve the objectives for which they were designed, none would appear to suggest an approach which will satisfactorily overcome the aforementioned drawbacks of the one conventional grid design briefly described above. Consequently, a need still exists for an improved grid spring structure which will avoid the above drawbacks without presenting new ones in their place. SUMMARY OF THE INVENTION The present invention provides a fuel rod grid designed to satisfy the aforementioned needs. The grid of the present invention includes inner grid straps containing unique fuel rod engaging spring structures and dimples on wall sections thereof defining a matrix of I0 fuel rod receiving grid cells. Each spring structure is composed of a pair of laterally spaced spring leg members each extending vertically and anchored at their respective pairs of opposite ends and a cross spring member which extends diagonally between and integrally connects to the spring leg members. Both of the leg members are curved slightly and the cross is arched, all projecting toward the longitudinal axis of the grid cell. By such arrangement, the spring structure has an effective length approximately two times greater than the actual length it occupies within the grid, allowing a substantial reduction in the height of the grid over the one having the prior vertically-oriented spring configuration. Further, the cross spring member is preferably set approximately forty-five degrees to the direction of the longitudinal axis of the grid cell and to the direction of coolant fluid flow through the grid and fuel assembly in which the grid is used. Yet the cross spring member is open with respect to the direction of flow. Forty-five degrees appears to be the optimum angle so that grid height can be reduced and spring profile exposed to coolant flow can be minimized. Angles greater than fortyfive degrees will increase pressure drop due to increased grid height, whereas lesser angles increase blockage of flow. The profile of the improved spring structure as seen by the flow is actually less than a horizontal dimple because smaller radii can be stamped due to the forty-five degree angle. The diagonal cross member also stiffens the inner grid strap and thereby the grid and increases its stability. By using vertical spring leg members as the spring levers, the design of the unique spring structure gives a soft compliant spring action. When a fuel rod is inserted, it engages the middle of the cross member and causes deflection of the vertical leg members away from the longitudinal axis of the cell so that the leg members are placed in profile (or plane) of the wall section of the inner grid strap and thereby offer no additional blockage to coolant flow. Also, as in the case of each cross spring member, each dimple is preferably set approximately forty-five degrees to the direction of the longitudinal axis of the grid cell and to the direction of coolant fluid flow through the grid. The dimple has an arched configuration which, in effect, provides a ramp which eliminates hangup of fuel rods being inserted through the grid. Accordingly, the present invention is directed to a plurality of fuel rod engaging spring structures in a nuclear fuel rod grid. The grid includes a plurality of inner and outer straps being interleaved with one another to form a matrix of hollow cells, each cell for receiving one fuel rod and being defined by pairs of opposing wall sections of the straps which wall sections are shared with adjacent cells. Each cell has a central longitudinal axis. Each fuel rod engaging spring structure made of resiliently yieldable material and integrally formed on each wall section of the inner straps, includes: (a) a pair of laterally spaced elongated leg members each having a pair of opposite ends only at which it is anchored to the wall sections; and (b) an elongated cross member having a pair of opposite ends, the cross member extending diagonally between and integrally attached at its opposite ends to the leg members such that the spring structure formed by the leg and cross members has an effective length greater than the actual length it occupies on the wall section. The cross member at one of its opposite ends is rigidly attached to one of the leg members adjacent to one end thereof and at the other of its opposite ends is rigidly attached to the other of the leg members adjacent to the opposite other end thereof. More particularly, each leg member is arcuate-shaped in its configuration along a longitudinal section therethrough so as to project from the wall section into one of the cells toward the longitudinal axis thereof. The cross member is arcuate-shaped in configuration along a longitudinal section therethrough so as to project from the wall section farther into one cell toward the longitudinal axis thereof than the leg members project into the one cell for engaging a fuel rod when received through the one cell. Still further, the cross member is capable of resiliently deflecting in a direction generally orthogonal to and away from the longitudinal axis of the cell and toward the wall section upon engagement by a fuel rod when received in the cell. Each wall section is generally planar in configuration. The leg members project from the plane of one wall section when the cell is unoccupied by a fuel rod but are capable of resiliently deflecting back within the plane of the wall section due to resilient yielding of the cross member by engagement with a fuel rod received within the cell. Also each leg member extends generally parallel to one another and in a direction generally parallel to the longitudinal axis of the cell. The cross member is disposed approximately forty-five degrees with respect to each leg member and with respect to the direction of the longitudinal axis of the cell. Furthermore, the present invention is directed to a plurality of fuel rod engaging dimples in a nuclear fuel rod grid. Each fuel rod engaging dimple is made of resiliently yieldable material and integrally formed on each wall section of the inner straps. Dimples are located both above and below the spring structures on the wall sections. Each dimple extends into the respective grid cell and is oriented diagonally with respect to the central longitudinal axis of the respective grid cell. The diagonal orientation of the dimple above the spring structure is either parallel to the orientations of the spring structures cross member and of the dimple below the spring structure, or orthogonal to one or both of them. The dimples are arcuate-shaped in configuration along a longitudinal section therethrough. Preferably, the diagonal orientation ot each dimple is approximately forty-five degrees with respect to the cell longitudinal axis. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.