Patent Number: 048030430
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are 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 is the type used in a pressurized water reactor (PWR) and 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 constructed in accordance with the principles of the present invention, as will be described in detail below. The grids 16 are axially spaced along and supported by the guide thimbles 14. The assembly 10 also includes a plurality of elongated fuel rods 18 transversely spaced and supported in an organized array 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 arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conveniently 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 grids 16 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 pellets 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 PWR. 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 has associated therewith a rod cluster control mechanism 34 having an internally threaded cylindrical member 36 with a plurality of radially extending flukes or 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. Grids With Improved Sprinq Structure Referring now to FIGS. 2-11, there is shown the preferred embodiment of the transverse fuel rod grid 16 constructed in accordance with the principles of the present invention. Basically, the grid 16 includes a plurality of inner and outer straps 40,42 having slots 44 by which the straps are interleaved with one another in an egg-crate configuration to form a matrix of hollow cells 46 and a plurality of openings 48. At the intersections of the straps 40,42, they are suitably secured together, such as by welding. The hollow cells 46 of the grid 16 receive therethrough respective ones of the fuel rods 18, whereas the openings 48 of the grid 16 have sleeves 50 inserted therein and attached to the inner straps 40 by which the grid 16 is disposed along and attached to the guide thimbles 14. Each cell 46 receiving one fuel rod 18 is defined by pairs of opposing wall sections. The wall sections 52 compose the inner straps 40, whereas the wall sections 54 compose the outer straps 42. The inner strap wall sections 54 are shared with adjacent cells. As shown in FIGS. 3 to 5, the wall sections 54 of the outer strap 42 each has a pair of horizontally extending and vertically spaced fuel rod engaging dimples 56 integrally formed thereon in association with each cell 46. Similarly, the wall sections 52 of the inner straps 40 also each has a pair of horizontally extending and vertically spaced fuel rod engaging dimples 58 integrally formed thereon in association with each cell 46. One of the dimples 58 on each wall section 52 of the inner straps 40 is located above the spring structure 60 of the present invention, whereas the other dimple 58 is located below it. Each cell 46 formed along the periphery of the grid 16 by the inner and outer straps 40,42 has associated with it four dimples 56,58 and two spring structures 60, whereas each cell 46 formed in the grid 16 by inner straps 40 only has associated with it four dimples 56 and two spring structures 60. Thus, each fuel rod in each cell 46 is contacted at six circumferentially and axially displaced locations thereon. More particularly, as seen in FIGS. 6 to 18, each fuel rod spring engaging structure 60, in accordance with the principles of the present invention, is composed of resiliently yieldable flexible material of the inner straps 40, such as stainless steel metal. The components of the spring structure 60, which will be described next, are integrally formed, such as by a conventionally stamping operation, from and on each wall section 52 of the inner straps 40 in association with each cell 46 of the grid 16. Basically, each spring structure 60 includes a pair of laterally spaced elongated spring leg members 62 and 15 W.E. 53,856-I an elongated spring cross member 64. Each leg member 62 has a pair of opposite upper and lower ends 66,688 and is anchored by being integrally and rigidly connected to the upper and lower portions 70,72 of the respective wall section 52 at only the leg member upper and lower ends 66,688. Each cross member 64 has a pair of upper and lower opposite ends 74,76 and extends diagonally between and is integrally attached at such respective ends to the leg members 62 of a pair thereof such that the spring structure 60 formed by the leg and cross members 62,64 has an effective length greater than (for instance, two times) the actual length it occupies on each wall section 52 of the inner straps 40. More specifically, the cross member 64 at its upper end 74 is rigidly attached to one of the leg members 62 adjacent to the upper end 66 thereof and at its lower end 76 is rigidly attached to the other of the leg members 62 adjacent to the lower end 68 thereof. Also, each leg member 62 extends generally parallel to one another and in a direction generally parallel to the central longitudinal axis of the respective grid cell 46. Preferably the cross member 64 is disposed approximately forty-five degrees with respect to the leg members 62 and to the direction of coolant fluid flow through the grid and to the longitudinal axis of the respective grid cell 46. Still further, as seen in FIGS. 15 and 18, each leg member 62 is slightly bowed or arcuate-shaped in its configuration along a longitudinal section through the leg member. Given such curvature, the leg member 62 projects from the plane P of the wall section 52 into its associated one of the cells 46 toward the central longitudinal axis thereof. Each cross member 64 is arched or arcuate-shaped in configuration along a longitudinal section of the cross member. Given such configuration, the cross member 64 projects from the leg members 62 away from the wall section 52 a farther distance into the associated cell 46 toward its longitudinal axis than does the leg members 62. At such position, the cross member 64 at its middle point 78 engages the fuel rod 18 received through the cell 46. The cross member 64 is capable of resiliently deflecting or yielding in a direction generally orthogonal to and away from the longitudinal axis of the associated cell 46 and toward the wall section 52 upon engagement by a fuel rod when inserted in the cell 46. The leg members 62 project from the plane P of the wall section 52 when the cell 46 is unoccupied by a fuel rod 18. However, they are capable of resiliently deflecting back within the wall section plane P due to resilient deflection of the cross member 64 by its engagement with the fuel rod 18 in the cell 46. In such deflected positions, the leg members 62 do not block coolant flow through the grid 16. As seen in FIGS. 13 14, 16 and 17, the cross member 64 like the dimples 58 project from the wall section 52 so as to define an open space therebetween which permits unimpeded flow of coolant fluid therethrough and along the fuel rod received in the cell. In summary, the configuration of the spring structure 60 allows a very low profile, a reduced grid height, and a low spring constant. The spring cross member 64 is set forty-five degrees to the direction of coolant fluid flow, but in manufacture is stamped out parallel to the direction of flow. The spring structure 60 has the pair of integral flexible spring leg members 62 which allow the spring structure to be compliant without adding to flow blockage. While not forming part of the present invention, the grid 16 can have mixing vanes 80 formed along the top edge of the inner and outer straps 40,42 thereof. Grids With Improved Dimples Referring now to FIGS. 19-48, there is shown modified versions of dimples 78 on the inner straps 40 of the transverse fuel rod grid 16 of FIG. 2. Since all of the parts of the inner straps 40 illustrated in FIGS. 6-18 are identical to the parts of the inner straps 40 illustrated in FIGS. 19-48 except for the dimples 58 which are now replaced by modified dimples 78, all of these identical parts will not be identified again in FIGS. 19-48. Each dimple 78 is made of the same resiliently yieldable material of the straps 40 and integrally formed, such as by a stamping operation, on each wall section 52 thereof of each cell 46. As before, the dimples 78 as located spaced above and below the spring structure 60 on the wall section 52. Also, each dimple 78 extends or projects into the respective grid cell 46 and is arcuate-shaped along a longitudinal section therethrough. However, each dimple 78 is modified to extend in a diagonal orientation with respect to the central longitudinal axis of the respective grid cell 46. Preferably, the diagonal orientation of each dimple 78 is approximately forty-five degrees with respect to the cell longitudinal axis, the same as the angular orientation of the cross member 64 of each spring structure 60. In FIGS. 20, 21 and 28, the upper and lower dimples 78 are oriented generally parallel to one another. However, the dimples 78 are oriented generally orthogonal or perpendicular to the orientation of the spring structure cross members 64. In FIGS. 23, 24 and 35, the upper and lower dimples 78 are oriented again generally parallel to one another. This time the dimples 78 are also oriented generally parallel to the orientations of the spring structure cross members 64. Finally, in FIGS. 26, 27 and 47, the upper dimples 78 are oriented generally perpendicular to both the lower dimples 78 and the spring structure cross members 64. It should be readily apparent that the respective orientations of the upper and lower dimples 78 could be reversed with respect to one another and to the cross members 64. It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.