Patent Number: 053316789
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1-3 show a portion of a nuclear fuel assembly grid strip 10, which represents one of a plurality of orthogonally inter-engaged strips 10, 100, 200, 300 that would make up an egg-crate type fuel assembly grid with cells as shown in FIGS. 4-7 for supporting a plurality of nuclear fuel rods. Each strip 10 is initially sized and annealed as a substantially rectangular flat plate having length, height, and width dimensions. The plate is then stamped to form a plurality of cut outs, slots, and projections. The present invention is preferably implemented with strips 10, that are made from a zirconium alloy, especially Zircaloy. Such a strip 10 has a plurality of slots 12,14 extending along the height dimension at regular intervals along the length dimension, thereby defining successive cell walls 16,18,20 between successive slots along the length dimension. A plurality of strips can thus be interengaged orthogonally at the slots to form the well-known egg-crate configuration. Each cell wall such as 16, has fuel rod support features or structure. In a preferred embodiment of the present invention, there are three fuel rod support features 22,24,26 per cell wall 16, located respectively in upper 28, central 30, and lower regions 32 of the cell wall. Each region includes a substantially flat base area 34,36,38, and cold-formed fuel rod support structure 22,24,26 projecting integrally from the base area along the width dimension of the strip. The support structure 22,26 in each of the upper 28 and lower regions 32 includes a relatively stiff, arched stop which projects in a first direction, and the support structure 24 in the central region includes a relatively soft, arched spring, which projects in a second direction opposite the first direction. In accordance with the invention, the spring 24 includes spaced apart pedestals 40,42 or similar projections formed in the base area 36 of the central region 30 and projecting in the second direction. A resilient beam 44 extends between and is rigidly supported by the pedestals 40,42, so as to project in the second direction beyond the projection of the pedestals. Preferably, each pedestal forms an arch that curves along the length dimension of the strip, and the beam forms a shallow peak or arch that bends or curves along the height dimension of the strip. The cut-outs 46,48 are formed adjacent to the locations of the pedestals 40,42 and beam 44. Moreover, additional cut-outs 50,52 and 54,56 are provided to facilitate the forming of the arches in the upper and lower regions, which project in a direction opposite to that of the spring. Preferably, each of the arch stops 22,26 is formed between a pair of longitudinal cut-outs 50,52 and 54,56 that extend along the length dimension of the strip. The beam 44 is formed between a pair of transverse cut-outs 46,48 that extend along the height dimension of the strip, and each pedestal such as 40 is formed between one longitudinal cut-out 52 and the pair of transverse cut-outs 46,48. The pedestals 40,42 project from the central region base area 36 a first distance d.sub.1, and the beam 44 has a crown 58 which projects from the central region base area a second distance d.sub.2 which is less than twice the first distance. Preferably, the beam 44 has a length 60 extending between the pedestals 40,42, that is at least about ten times greater than the distance d.sub.3 that the crown projects into the cell relative to the distance which the pedestal projects into the cell (i.e. d.sub.3 =d.sub.2 -d.sub.1). In other words, the length 60 of the beam 44 is at least about ten times greater than the difference d.sub.3 between the projection of the crown 44 relative to the base area 36 and the projection of the pedestals 40,42 relative to the base area 36. All the fuel rod support features as described in connection with FIGS. 1-7, can be formed during a single stamping operation, which cold-works the material constituting the projections. The base regions 34,36,38 are in a condition corresponding to the annealing of the strip 10, before the cutting of the slots 12,14 and cut-outs 46,48,50,52,54,56. The projecting structure 22,24,26, however, necessarily experience a certain amount of straining (cold working) during formation. The more highly strained portions of the strip, undergo greater elongation and relaxation during exposure to radiation in the reactor core. It can be appreciated from inspection of FIGS. 1 and 2, that since beam 44 has been cold worked, whereas the base area 36 has not, the relatively greater elongation of the beam would give rise to axial compression stresses, acting inwardly toward the crown 58, which thereby urge the crown further into the cell. The strips of the type shown in FIGS. 1-3, are assembled into an egg crate structure that results in the creation of grid cells with the geometry shown in FIGS. 4 and 5. Insertion of a fuel rod 500 into a grid cell produces the geometry shown in FIGS. 6 and 7. In the initial geometry of the cell as shown in FIGS. 4 and 5, the horizontal distance between each spring 24,324 and its opposing arch stops 222,226 and 122, 126 is less than the diameter of the fuel rod 500. Therefore, insertion of the fuel rod into the cell as shown in FIGS. 6 and 7, deflects each spring and thus preloads the rod against the arch stops. The preload prevents relative motion between the rod and grid during handling and shipment. During reactor operation the preload is reduced due to the short-term and long-term mechanisms described previously. Particularly with conventional Zircaloy grids, reactor operation can result in the complete loss of grid spring preload and the possible generation of gaps between the fuel rod and the rod support features. However, the inventive configuration of the spring and its supports minimizes or eliminates these gaps by using the lateral amplification of the axial compression of a nearly straight beam. As shown in FIG. 8, a more complex forming process of the spring 24' and its support projections 40'42' can further enhance the amplification effects between the beam and the base area. This is achieved by additionally straining only the spring 24 during or after the strip of FIGS. 1 and 2 has been stamped (overforming, then forcing back) and allowing a slight cant of the pedestals 40' 42' away from each other, i e , away from the crown 58' of the beam spring. The axial compression of the spring 24' can result from relaxation of the residual stresses associated with the cant of the support projections 40'42 or from the cold-worked spring being restrained from growing by the fully annealed strip. As with any virtually straight member, axial compression of the spring, however slight, results in a much larger lateral deflection. This lateral deflection of the spring is toward the rod, thus eliminating or minimizing any gap. Another desirable feature which can be implemented with the present invention, is shown in FIGS. 1 and 2. The arch stops 22,26 in the first and second regions can be formed with vertical extensions 62,64 and 66,68 for contacting the rod and preventing scratching of the rod as it is inserted into the cell. The vertical contact length increases the rod-to-arch contact area, thus decreasing rod wear by lowering the contact pressure.