Patent Number: 045445222
Section: description

DESCRIPTION The spacer 11 of the invention is shown generally in the plan view of FIG. 1 and the partly cutaway elevation view of FIG. 2. The spacer 11 is formed of a peripheral support band 12 to which are secured a plurality of sheet metal divider members 13(1) and 13(2) in a cross-laced or egg crate like arrangement to provide fuel rod passages 14 for fuel rods 16. (Conveniently, the peripheral support band 12 may be formed of four separate side pieces welded together at the corners after assembly with the divider members.) One or more passages 14' may be provided to accommodate coolant-moderator conducting tubes 17 which are larger in diameter than the fuel rods 16. (Such water tubes and their function are discussed in U.S. Pat. Nos. 3,802,995 and 4,314,884.) The divider members 13(1) are formed with a pair of arched portions near their upper and lower edges to provide relatively rigid projections or stops 18 for engagement with and lateral location of the fuel rods 16 in the passages 14. Preferably the stops 18 are laterally oriented to minimize spacer projected area and coolant flow resistance. Similar laterally oriented stops 18' are formed along the upper and lower edges of the peripheral support band 12 for support of the fuel rods in the peripheral ones of the fuel rod passages 14. The upper and lower edges 15 (FIG. 2) of the rigid stops 18, 18' are curved slightly outward from the passages 14 (i.e., the stops are slightly convex in the vertical direction) to facilitate entry into and removal from the passages 14 of the fuel rods 16 and to minimize scratching of the fuel rods during rod insertion. The divider members 13 (2) support separately formed, two-sided springs 19 (discussed in more detail hereinafter). The divider members 13(1), 13(2) are alternately positioned so that two intersecting divider members 13(1) and two intersecting divider members 13(2) bound each one of the fuel rod passages 14 (except where the peripheral passages are bounded by the peripheral support band 12). Thus a side of each of two of the springs 19 projects into each passage 14 from two adjacent sides thereof and urges the fuel rod therein into contact with rigid stops 18 on the other two adjacent sides of the passage. As illustrated in FIG. 1, the spacer 11 provides two diagonally adjacent passages 14' for two water tubes 17. To maximize the permissible diameter of the water tubes 17, rigid stops 18 are omitted in the passages 14'. Also, springs 19' (FIG. 2) with only one resilient side are used at spring positions adjacent the passages 14'. In lieu of springs 19 in these passages, resilient lateral support of the water tubes 17 is provided by a ring shaped spring member 21 secured in appropriate slots in the intersecting divider members 13(1)' and 13(1)" bounding two sides of the passages 14' as described hereinafter. The peripheral support band 12 of the spacer 11 is formed with a pair of inwardly curved, upward projections 22 near each corner to serve as lead-ins when a removable tubular flow channel is installed over the fuel assembly of which this spacer is a part. (See flow channel 11 of FIG. 1 of U.S. Pat. No. 3,654,077). It is a feature of the invention that, except for the lead-in projections 22, there are no springs or other spacer structure projecting above or below the planes of the upper and lower edges of the divider members 13(1), 13(2) whereby coolant flow disturbances due to velocity changes caused by changes in spacer cross-section area are minimized. The peripheral support band 12 also is formed with a pair of outwardly extending lobes 23 near each corner for providing predetermined spacing between the support band 12 and a surrounding tubular coolant flow channel (not shown herein). The divider members 13(1), 13(2) are welded together at the top and bottom of each of their intersections 24 for increased spacer strength. Details of divider members 13(1) are illustrated in FIGS. 3A, 3B. These divider members provide the rigid projections or stops 18. To form such stops, the divider member 13(1) is first formed with appropriately located vertically aligned sets of lateral slits 26. The metal between the slits 26 and the upper and lower edges of member 13(1) and between the upper and lower slits of each pair is then deformed outwardly to form the laterally arched stops 18 projecting as vertically aligned pairs from each side of the divider member 13(1). (The rigid stops 18' along the upper and lower edges of the peripheral support band 12 may be formed in a similar manner.) Each divider member 13(1) is formed with regularly spaced vertical slots 27 for interlocking with other divider members of the spacer in egg-crate fashion. Formed at the ends of each divider member 13(1) are spaced pairs of tabs 28. These tabs are welded into appropriate slots in the peripheral support band 12 for location and support of the divider member therein. If the spacer 11 is to accommodate oversized water tubes 17, as shown in FIG. 1, the intersecting divider members 13(1)', 13(1)" must be modified from that shown in FIGS. 3A, 3B to retain the ring-shaped spring 21. Details of these modifications are shown in FIGS. 4A and 4B. In alignment with the intersection of divider members 13(1)' and 13(1)", the divider member 13(1)" is formed with a cutout 29 (FIG. 4A) to receive the ring-shaped spring 21. As shown in FIG. 4B, the divider member 13(1)' is formed with a pair of spaced slots 31 for receiving the spring 21 whereby when the divider members 13(1)' and 13(1)" are interlocked the spring 21 is retained in position. Details of the divider members 13(2) are illustrated in FIGS. 5 and 6. These divider members provide support for the springs 19, 19'. As in the case of divider members 13(1), the divider members 13(2) are formed with the spaced slots 27 for interlocking with other divider members and with the pairs of tabs 28 at each end for welding into appropriate slots in the peripheral support band 12. To retain the springs 19, 19' in their proper vertically oriented positions on the divider member 13(2), each spring is captured between upper and lower pairs of projections 32 (see FIG. 6) near the upper and lower edges of the divider member 13(2). The projections preferably are in the form of quarter-spherical louvers. To form the louvers 32 the divider member 13(2) is first formed with pairs of spaced slits 33 defining the edges of the louvers 32 which face the springs 19, 19'. The springs 19, 19' are then slipped over the divider member 13(2) and each is positioned in alignment with its respective upper and lower pairs of slits 33. The metal of the divider member 13(2) is then deformed outwardly to form the louvers 32. This procedure avoids the installing of the springs over already formed louvers or projections which could overstress the spring material, the theory being that if the springs can be forced over the formed louvers during fabrication of the spacer, they might also slip over the louvers and become mislocated during operation in a reactor. With the procedure of this invention, even if one side of the spring should break during operation in a reactor, it is likely that the spring would remain in its proper position. The spring 19 is illustrated in detail in FIG. 7. The spring 19 is a double-sided, continuous-loop member. Conveniently, the stock from which such a spring may be formed is a section of seamless tubing. The spring 19 is formed with upper and lower minor end portions 36 spaced apart about the thickness of a divider member 13(2) for close fit thereon. Major middle portions 37 are outwardly bowed to extend into the fuel rod passages and are formed with a central apex portion 38 for contact with the fuel rods. Arched intermediate portions 39 extend outwardly to a lesser extent than middle portion 37 for limiting extreme movement of a fuel rod in the direction of the spring 19. The spring 19' is illustrated in FIG. 8. This spring provides a single resilient or spring member and it is used adjacent the oversized water tube passages 14' (FIG. 1). This spring has a straight side 41. The other side has a major middle portion 37' and apex portion 38' similar to that of spring 19 (FIG. 7). However, arched end portions 42 of spring 19' are different from the end portions of spring 19. Because of the straight side 41, the spring 19' as a whole cannot elongate. Therefore, the arched end portions 42 are elongated an amount sufficient to provide the middle portion 37' with a spring constant approximately that of the middle portion 37 of spring 19 while maintaining spring stresses within acceptable levels. In the embodiment of the invention illustrated in FIGS. 5 and 6, louvers 32 are provided to retain spring 19, 19' in their positions on the divider member 13(2). Illustrated in FIGS. 9A, 9B and 9C is an alternate divider member 13(2)' wherein the spring members 19, 19' are positioned in notches 43 formed in the upper end lower edges of a divider member 13(2)'. (It will be appreciated that for this embodiment the springs 19, 19' will be made somewhat shorter so as to fit down in the notches 43 or, alternatively the height of the divider members 13(1), 13(2) may be made somewhat greater.) To provide for installation of the springs 19, 19' on the notched divider member 13(2)', this divider member is first formed of separate longitudinally extending pieces including a major piece 44(1) and a minor piece 44(2). To install the springs 19, 19' on the notched divider member 13(2)', the two pieces 44(1), 44(2) are placed in overlapping relation and the springs 19, 19' are slipped thereover and placed in alignment with the notches 43. The two pieces 44(1), 44(2) are then moved toward opposite ends of the springs 19, 19' to bring the spring ends into engagement with the notches 43 and to bring the inner edges 46(1) and 46(2) of the upper and lower pieces 44(1), 44(2) into abutting relation. These abutting edges are then welded together to form the unified divider member 13(2)', devoid of cutouts, as shown in FIG. 9C. In the spacer of the invention, the peripheral support band 12 and the divider members 13(1), 13(2) are formed of a material having a low neutron absorption cross section such as a zirconium alloy, for example, Zircaloy-4. The spring members 19, 19', 21 are formed of a material having suitable strength, corrosion resistance and resiliency characteristics such as a nickel alloy, for example, Inconel. In an example of a spacer of the invention the peripheral support band 12 is about 1.25 inches (31.75 mm) high and about 0.03 inches (0.76 mm) thick. The divider members 13(1), 13(2) are about 1.125 inches (28.6 mm) high and about 0.02 inches (0.5 mm) thick. The springs 19, 19' are about 0.085 inches (2.2 mm) wide and formed of material about 0.013 inches (0.33 mm) thick. The spring 21 is about 0.175 inches (4.5 mm) high of a material about 0.006 inches (0.15 mm) thick. Thus what has been described is a nuclear fuel element or rod spacer wherein the total amount of material, the amount of high neutron cross section material, the projected area of the spacer structure, and changes in cross section area of the spacer structure are minimized to minimize neutron absorption and coolant/moderator flow resistance. As compared to prior spacers (for example, as shown in U.S. Pat. No. 3,654,077) the decreased neutron absorption of the spacer of the invention provides in the order of 0.8 percent decrease in reactivity penalty which is equivalent to a fuel enrichment benefit of about 0.0098 percent. The spacer of the invention also provides a decrease in fuel assembly coolant pressure drop of from about 10.6 psi with prior spacers to about 9.7 psi with spacers of the invention or a decrease of nearly 1 psi.