Patent Number: 
Section: description

Referring now to FIGS. 1-3, there is generally shown a fuel rack 10 for storing spent fuel assemblies (depicted in chain by a fuel assembly profile 14 in FIG. 3) in spent fuel pits of commercial nuclear reactor plants (not shown) for generating electric power. The fuel rack 10 has a plurality of cells 20 defined by vertically extending walls 22 and a supporting horizontally oriented base plate 24. The rack base plate 24 is supported and leveled by a plurality of floor supports 26 that stand on the floor of a spent fuel pit. The rack walls 22 may be up to about a tenth of an inch or more in thickness and the base plate may be up to about one half inch or more in thickness. As shown in FIG. 1, the fuel rack 10 has a 12xc3x9715 array of cells 20, although other fuel racks may have fewer or more cells. Each cell 20 is generally rectangular and each cell wall 22 is roughly 6 inches in length so that the overall dimensions of the rack 10 of FIG. 1 are about six feet by about eight feet in cross section by about 15 feet high. As is shown in FIG. 3, cells 20 in commercial plants may have neutron-absorbing sheets 28 attached to the vertical walls 22, which sheets may be covered by wrappers 30 to protect sheets 28 from hydraulic forces. As may be seen in FIG. 2, each floor support 26 may include a support pad 32 and leveling screw 34 which threadedly engages a horizontal plate 36. The horizontal plate 36 may be attached to the rack base plate 24 by vertically extending plates 38 and plates 40. The vertically extending plates 38 and plates 40 preferably have flow holes 42 and 44, respectively, and the rack base plate 24 has a flow hole 46 for permitting water to flow between the interior portion of the cell 20 and the water pool. The rack base plate flow hole 46 may be up to about four inches in diameter or more. FIG. 1 shows a sleeve assembly 50 of the present invention aligned with a cell 20 of fuel rack 10 and in a position where it can be readily installed in the cell 20 with an installation tool (not shown) carried by an overhead crane to refurbish the fuel rack 10 while the rack 10 is submerged in the fuel pool. The sleeve assembly 50 generally includes at least one elongate wall 52, a sleeve base 54 and a locking or pin assembly 56. The elongate wall 52 may be flat or curved. Preferably, the elongate wall 52 and the pin assembly 56 are welded to the sleeve base 54. FIGS. 1-3 show an assembly 50 having two mutually perpendicular walls 52, although the sleeve assembly 50 may have up to four walls 52. In the preferred practice of the present invention, where it is desired to repair the deterioration of the neutron absorbing sheets 28, the walls 52 preferably contain substantial amounts of boron or other neutron absorbing materials. Most preferably, the walls 52 are an extruded composite of boron carbide and aluminum as generally disclosed by U.S. Pat. No. 5,980,602. These extruded products are available under the trademark BORALYN from Alyn Corporation of Irvine Calif. Other commercially available neutron absorbing materials are sold under the trademarks BORAL and BORAFLEX. The elongate walls 52 extend upwardly from one side 60 of the sleeve base 54. The sleeve base 54 has an opposed second surface 62 designed to rest on the rack base plate 24 when the sleeve assembly 50 is installed. Importantly, the sleeve base 54 has a flow hole 64, which preferably is no smaller than and alignable with the flow hole 46 in the rack base plate 24. The sleeve base 54 is preferably similar to the sleeve material, and may be a BORALYN extruded plate of about one half inch in thickness or more. The sleeve base 54 may have one or more undercut keyways in its corners (depicted by keyway 66 with a circular undercut) for releasably receiving the rotatable end(s) of an elongated installation tool suspended from the hooks of an overhead spent fuel pool crane (not shown). The pin assembly 56 removably attaches the sleeve assembly 50 in the cell 20 to the rack 10. As shown in FIGS. 2, 3 and 4, the pin assembly 56 has a tubular portion 70 disposed in the sleeve base flow hole 64 for engaging the rack base plate 24 while up to eight or more resilient tabs 72 extend from the tubular portion 70 into the below rack base plate flow hole. The pin assembly 56 may be extruded or otherwise fabricated of an aluminum alloy or other metal and have a thickness of about 0.05 inches. The tubular portion 70 may have a rim 74 that is designed to seat on a chamfered edge of the upper surface of the sleeve base 54 when the balance of the tubular portion 70 fits through the sleeve flow hole 64 and the resilient tabs 72 extend beyond the bottom surface 62 of the sleeve base 24 and through the rack base plate hole 46. Each resilient tab 72 has a first section 74 that extends substantially vertically from the tubular portion 70 of the pin assembly 56 for resiliently engaging the rack base plate 24 and limiting horizontal movement of the sleeve assembly 50 when the pin assembly 56 is installed. Preferably, an intermediate tab section 76 extends from the first tab section 74 and has a surface 78 facing and extending substantially parallel to the bottom surface of the sleeve base 54 and the bottom surface of the rack base plate 24. Advantageously, the intermediate tab section 76 will limit upward movement of the sleeve assembly 50 under the drag forces of an upwardly moving fuel assembly 14. Preferably, a tab end section 80 extends at an acute angle from the intermediate section 76 and inwardly of the tubular portion 70. Advantageously, the angled tab end section 80 cushions the sleeve assembly 50 (which will weigh about forty pounds or more) as it centers on the rack base plate 24 under the weight of the sleeve assembly 50. In addition, the tab end section 80 presents a self aligning mechanism for seating the sleeve assembly 50 in the cell 20 Advantageously, the pin assembly 56 will not be contacted by a fuel assembly 14 as the fuel assembly 14 is placed in or removed from a cell 20 until the fuel assembly is within about an inch of being seated. In addition, the pin assembly design can withstand substantial upward and downward drag forces of a stuck fuel assembly 14 on the sleeve walls 22. Also, the pin assembly design does not substantially obstruct the area of the flow hole 46. While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.