Patent Number: 050248073
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT To understand the preferred embodiment, it is helpful to first refer to prior art FIGS. 1 and 2 where the numeral 10 generally designates a conventional fuel assembly. In FIG. 1, the prior art fuel assembly 10 includes an upper end fitting 12, a lower end fitting 14, spacer grids 16 supporting fuel rods 17, and a perimeter skirt portion 18 shown partially broken away in FIG. 1. In a conventional assembly, the region enclosed by the skirt would contain only the fuel rods. The numeral 20 indicates the location for a debris catching strainer grid constructed according to principles of the prior art (U.S. Pat. No. 4,781,884). Behind the skirt 18, within the non-end-cap-contacting compartments defined by the debris catching strainer grid 20 and at the ends of fuel rods 17, are solid fuel rod end caps 22. Each fuel rod end cap 22 is in a non-end-cap-contacting compartment of the prior art strainer grid 20. In FIG. 2 the perimeter skirt portion 18 and lower grid portion 16 is in cross-section to show how spring 23 and at least one arch 21 remain in position for capture of fuel rod 17, in spite of the "rod lift" caused by abnormal coolant flow in an amount equal to the length "x" of the "shoulder gap". The novel "spring detent spacer grid" of FIG. 3 is generally designated 20a and cooperates with circumferentially grooved end caps 22a with which it is in contact for "rod lift" preventing rod capture, for debris trapping and for debris retention below the active region or cladding of the fuel even with the coolant pumps off. The fuel assembly of FIG. 3 with "spring detent spacer grid" 20a is designated 10a, for convenience. Parts in fuel assembly 10a which are substantially the same as parts in prior art assembly 10 carry the same numbers in FIGS. 3 to 18 as they have in FIGS. 1 and 2. The new parts which are used to make up new fuel assembly 10a with the novel "spring detent spacer grid" 20a of the invention are given part numbers with letter subscripts. FIGS. 4 to 16 show the details of the strips utilized in making up the embodiment of the "spring detent spacer grid" 20a. The strips 24a are the top strips of the "eggcrate" grid assembly. Strips 26a are the bottom strips and strips 28a are the perimeter strips. As seen in FIGS. 4 to 7, the top strips 24a have leaves 30, opposite arch portions or bends 32 and extending in the same direction as the springs 34 which seat in tapered sided circumferential grooves 36 of end caps 22a to provide "rod lift" preventing rod capture. The bends or arch portions 32 cooperate with the springs 34 to laterally hold the fuel rods 17a, in known manner. Slots 38 facilitate the "eggcrate" assembly. As seen in FIGS. 8 to 11 the bottom strips 26a have leaves 40a and 40b vertically displaced on opposite sides of the strip but located such that they will be substantially symmetrically located relative to the intersection of strips 24a and 26a when assembled into a grid. Bends or arched portions 42 and springs 44 cooperate with end caps 22a and grooves 36 in the same manner as do portions 32 and springs 34. Slots accommodate the "eggcrate" assembly. The perimeter strip 28a illustrated in FIGS. 12 and is one typical type, but other designs are contemplated. The leaves 52 provide the arch function and the springs 54 engage the grooves 36 in end caps 22a. The perimeter strip 28a is straight without bends of the type at 32 and 42. FIGS. 17 and 18 show the relation of symmetry and the relative elevation of the features of the new "spring detent spacer grid". From these figures it is easy to see why debris is trapped and retained in the various tapered passageways the instant invention creates in the area just above the lower end fitting 14.