Patent Number: 039322165
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a more complete appreciation of the invention, attention is invited to FIG. 1, which shows a typical grid structure 10. The grid 10 is formed from a group of equidistantly spaced parallel grid members or plates. In the plane of the drawing, however, only a grid plate 11 is shown. Almost identical plates 12, 13, 14, and 15 are arranged perpendicularly to the plane of the plate 11. All of the plates in the grid structure 10 have mutually engaging slits (not shown in FIG. 1) that enable the interlocking members to mesh with each other and thus form an array of cells. Detents or stops 16 and 17 are provided in the longitudinal edges of the plates. These detents engage the adjacent surfaces of the individual fuel rods that are lodged within the cells, as illustrated by the portion of a fuel rod 18 that is shown in broken lines. Detents or stops 19 also are formed in the middle portion of some of the grid plates. These detent-bearing middle portions, moreover, in at least one embodiment of the invention are warped or bowed slightly to add to the depth to which each of these stops protrude in toward the center of a respective cell. The plates 11 through 15 are joined at the intersections by spot welding, brazing, or the like in order to produce a sturdy rigid structure. In accordance with the invention, the corners of the grid plate 11 and the plates 12 through 15 are beveled by chamfering, stamping, grinding or the like to provide sloping surfaces 20, 21, 22, and 23. The length of the inclination that characterizes these surfaces should extend at least from the outermost edge of the fuel rod 18 to a band 25 that circumscribes the perimeter of the grid structure 10. As described subsequently in more complete detail, the sloping surfaces 20, 21, 22, and 23 act as cams or inclined planes that aid the relative movement of two adjacent grids. The band 25 that encircles the grid structure 10 has stops or detents, of which detent 24 is typical, that help to retain the outer or peripheral ranks of fuel rods, exemplified by the fuel rod 18, in the grid structure. The band 25, joined to the terminal edges of the grid plates, also tends to enhance the physical integrity of the grid structure 10. As shown in the drawing, the band 25 is not as wide as the maximum width of the plates 11 and 12 through 25. The sloping surfaces 20 through 23 thus form transition sections that match the greater width of the grid plates to the lesser width of the band 25. Because in plan view (not shown), the illustrative embodiment of the grid 10 is generally rectangular, the band 25 forms right angle corners. Further in accordance with the invention, the corner edges are finished by means of sloping portions 26 that meet at a corner crease 27. As shown in FIG. 2, these sloping portions 26 meet to provide generally vee shaped edges 30 and 31, which also function as cams or inclined planes. Generally, the beveled, vee-shaped edges 30 and 31 prevent the corners of adjacent fuel element grid structures from locking together during reactor core assembly or refueling. In operation, a typical fuel element 32 shown in FIG. 3 is moved in the direction of an arrow 37. Adjacent fuel element 33 is stationary within the reactor core (not shown). The outer surface of a band 34 on the grid structure for the fuel element 32 is in sliding engagement with a side of the fuel rod 35 that is lodged in the fuel element 33. The fuel rod 35 tends to guide the band 34 into physical contact with a corresponding portion of band 42 on the fuel element 33. Sloping surfaces 40 and 40A that characterize the plates which form the grid structure engage the adjacent portions of the opposite grid. The sloping character of the surfaces 40 and 40A respond to the movement of the fuel element 32 in the direction of the arrow 37 by forcing the element to shift away from the fuel rod 35, in the direction shown by an arrow 44. Thus, the sloping surfaces 40 and 40A act as cams or inclined planes that ease the fuel element 32 into proper relative alignment in which the bands 34 and 42 abut. Although not shown in the drawing, in a similar manner, a vee shaped edge 45, formed at the corner of the band 34, guides the fuel element 32 past the fuel element 33, because the edge 45 prevents corner edges of the grids from becoming temporarily engaged or locked together. An additional feature of the invention resides in the physical structure of the individual grid plates. Turning to FIG. 4 of the drawing, for instance, a generally planar grid plate 46 has parallel longitudinal edges 47 and 50. The edge 50, moreover, is interrupted at regular intervals by slits 51 that terminate in paddle shaped cut-outs 52, which are formed in the mid-portion of the plate 46. As described in more complete detail in F. S. Jabsen U.S. patent application Ser. No. 774,148, filed Nov. 7, 1968, now U.S. Pat. No. 3,665,586; Ser. No. 105,388, filed Jan. 11, 1971; and Ser. No. 193,383, filed Oct. 28, 1971, now U.S. Pat. No. 3,795,040, all assigned to the assignee of the instant invention, the slits 51 enable the grid plate 46 to mesh with other grid plates (not shown in FIG. 4) that are perpendicularly oriented relative to the plane of the drawing. This meshed arrangement establishes the desired cellular structure. Also as described in the foregoing applications for patent, keys (not shown) are inserted into the cellular structure in order to force protruding detents away from the center of the respective cells. This temporary deflection provides a sufficient clearance for fuel rods (not shown in FIG. 4) to pass through the cell structure without being scored, abraded, or gouged by the stops of which detents 53 are typical. After the fuel rods have been lodged within the respective cells, the keys are withdrawn from the cell structure and the detents 53 engage the rod surfaces. As shown in FIG. 4, the detents 53 are formed in bent or warped portions 54 of the plate 46. This is a characteristic of the embodiment shown in the drawing that is not essential, however, to the practice of the invention. The detents 53, for example, can be provided on a flat surface that has not been warped. The detents, moreover, in the mid-portions of at least some of the plates can be omitted, as shown in connection with the plate 14 in FIG. 1 of the drawing. The plate 46 terminates in two parallel transverse edges 55 and 56. In accordance with a feature of the invention, the longitudinal edges 47 and 50 are joined to the adjacent segments of the transverse edges by means of sloping surfaces 57, 60, 61, and 62. These sloping surfaces form obtuse angles with the respective adjacent transverse and longitudinal edges. The relationship between these surfaces and edges provides the inclined planes or cam surfaces that facilitate reactor core assembly and disassembly in the manner that was described in connection with FIG. 3. Considered from another viewpoint, the longitudinal distance that the sloping surfaces 57 and 60 can extend away from the adjacent transverse edge should be about equal to the distance to which the detents protrude into the cell structure (not shown in FIG. 4). This distance is typified by the height to which the detents 53 extend above the warped portions 54 of the plate 46. With this generalization as a guide, there is an assurance that no sharp angles or edges on the plate 46 will protrude beyond the outer rank of fuel rods in a given fuel element. Accordingly, the invention is a simple and efficient technique that enables the nuclear reactor cores to be assembled, rearranged and disassembled without incurring the penalties that have characterized the prior art.