Patent Number: 046366458
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference first to FIG. 3, cask base element 38 has a cylindrical cavity 40 which extends from floor 42 to stepped mouth region 44. During use, cavity 40 typically contains a basket arrangement (not illustrated) which mechanically supports the spent fuel in storage slots and which transfers the heat generated thereby to wall 46 of element 38. The storage slots of the basket arrangement have axes that are parallel to the axis of element 38 and are open, in the vincinity of mouth region 44, to receive fuel assemblies 26 and/or fuel in consolidation canisters. With continuing reference to FIG. 3, cask base element 38 includes a carbon steel portion 48 which is approximately 25 cm thick and which serves to protect the environment from gamma rays. A stainless steel cladding layer 50 is applied to the interior of portion 48, for example, by placing portion 48 on a turntable and rotating it while welding a continuous spiral path around the interior using stainless steel welding rods, so that a stainless steel surface covers the interior of portion 48 entirely in order to protect it from chemical attack. Portion 48 is surrounded by a layer about 7.0 cm thick of neutron absorbing material 52, which may be a resin. A suitable neutron absorbing material is commercially available from Bisco Products, Inc., 1420 Renaissance Drive, Park Ridge, Ill. 60068, under Stock No. NS-3. Surrounding material 52 is an outer layer 54 of stainless steel to protect the cask from the environment. Carbon steel cooling finds 56 are welded to portion 48 and extend through material 52 and layer 54. Element 38 is typically about 4.8 meters high and has an outside diameter of about 2.5 meters, excluding fins 56. It has a mass of over a hundred thousand kilograms when loaded with spent fuel. Trunions (not illustrated) may be provided on element 38 to facilitate handling. Turning next to FIG. 4, stepped mouth region 44 includes a first annular step region 58 that is horizontally disposed when element 38 is positioned on cask pad 36 (FIG. 2), an annular projection 60 providing a second annular step region 62 which is also horizontally disposed when element 38 is on pad 36, and an annular groove 64 between step regions 58 and 62. Threaded bores 66 are provided around projection 60. Stainless steel layer 50 extends upward to groove 64, where it terminates in a region 68 of increased thickness. This can be accomplished by providing a recess (not numbered) in portion 48 and filling the recess with excess stainless steel when the aforesaid spiral welding with stainless steel rods is performed. Regions 58 and 62 are machined to provide smooth, flat surfaces. With continuing reference to FIG. 4, closure system 70 cooperates with stepped mouth region 44 to seal base element 38, either temporarily or permanently in order to provide a completed cask. Closure system 70 includes a generally disk-shaped primary cover 72 of stainless steel, about 10 cm thick. The bottom side of primary cover 72 has an annular groove 74 while the top side is provided with an annular recess 76. A first mechanical seal is provided by O-ring 78, which is housed in groove 74 and compressed against first region 58 by the weight of cover 72. it will be apparent that the first mechanical seal could alternately be provided by a O-ring which is housed in a groove that is cut into region 58, or by shallow grooves adjacent each other in both region 58 and cover 72, or by no grooves at all. However it is convenient to permanently install O-ring 78 in groove 74 so that primary cover 72 can be shipped and installed as a single unit. Referring next to both FIGS. 4 and 5, primary cover 72 includes an annular canopy element 80 of stainless steel. Element 80 can be fabricated, for example, by sawing away the outer portion of a hoop of stainless steel tubing. Bottom edge 82 of canopy element 80 is welded to primary cover 72 at region 84 thereof in such a manner that the weld extends around the periphery of element 72, and intermediate portion 86 of element 80 extends into recess 76. Thus canopy element 80 need not be shipped or installed independently of cover 72. Primary cover 72 is installed under water, after cask base element 38 has been lowered to cask pad 36 (FIG. 2) and loaded with spent fuel. After the loading operation primary cover 72 is lowered by remote control into mouth region 44 until its periphery rests on region 58 of element 38. The weight exerted on O-ring 78 provides a mechanical seal, but shear keys 88 and 90 (FIGS. 6A and 6B) are inserted into groove 64 by remote control, before cask base element 38 is moved, in order to prevent primary cover 72 from becoming displaced during a drop accident or other mishap. After keys 88 and 90 have been installed the water within cask base elements 78 is removed via a drain (not illustrated) and gas is injected. The gas is preferably inert, such as helium, although other gases or even air can be used instead. After primary cover 78 is applied and the water in cask base element 38 is replaced by gas, element 38 is lifted from pool 30 (FIG. 2). Primary cover 72 attenuates the radiation enough to make it safe for workers to be exposed to mouth region 44 for limited periods of time. With reference next to FIGS. 6A and 6B, shear keys 88 include insertion portions 92 and riser portions 94, which are bounded by parallel sides 96 and 98. Shear keys 90 include insertion portions 100 and riser portions 102, which are bounded by angularly disposed sides 104 and 106. Shear keys 88 and 90 can be fabricated by machining stainless steel to provide a disk which is as thick as riser portions 94 and 102, reducing the thickness at the periphery of the disk to provide insertion portions 92 and 100, and then cutting away a circular region at the center of the disk to provide an annular structure somewhat resembling in a large washer. The annular structure is thereafter cut into segments to provide individual shear keys 88 and 90. FIG. 7 illustrates base element 38 after primary cover 72 has been installed and secured by shear keys 88 and 90. It will be noted that keys 88 and 90 alternate around the periphery of cover 72. The reason why the sides of keys 88 and 90 are configured differently is to permit the keys to be inserted into and removed from groove 64; if the shear keys were fabricated by radially cutting the aforesaid annular element, so that all of the keys were identical, it will be apparent that keys could not be inserted into groove 64 in a full, 360.degree. ring. However since the sides 96 and 98 of shear keys 88 are parallel they can be readily slid into position or removed from groove 64, thereby allowing access to the adjacent shear keys 90. However it is not necessary that sides 96 be parallel to sides 98; keys 88 would still be removeable if sides 96 and 98 sloped toward an apex which is nearer to end 108 than it is to riser portion 94. In contrast, sides 104 and 106 of shear key 90 slope toward an apex that is closer to riser portion 102 than it is to end 110. Returning to FIGS. 4 and 5, primary cover 72 is installed without welding upper edge 112 of canopy element 80 to region 68 if cask base element 38 is to be temporarily sealed. That is to say, for a temporary seal edge 112 is not welded to base element 38 in the manner shown in FIG. 4, but instead is simply positioned in the upper portion of recess 76 without being permanently connected. If the ask seal is to be permanent, however, shear keys 88 and 89 are removed sequentially to expose segments of canopy element 80, and the portion of edge 112 thereby rendered accessible is welded at region 68. After a segment has been welded the shear keys are re-inserted, whereupon the shear keys are removed from the next segment and welding resumes. This process continues until edge 112 is continuously welded to cask base element 38. It will be apparent that the welding of edge 112 in this manner creates a permanent seal, since edge 82 of element 80 is welded to primary cover 72. Moreover, since there is a degree of flexibility between edges 82 and 112 of element 80, it will be aparent that primary cover 72 can expand differentially with respect to cask base element 38 in response to temperature changes. That is to say, element 80 accommodates minor movement of cover 72 with respect to mouth region 44 without unduly straining the welded seal. With continuing reference to FIG. 4, closure system 70 also includes a generally disk-shape secondary cover 114 of carbon steel about 15 cm thick. Cover 114 includes bores 115 spaced about its periphery, annular grooves 116 and 118, and central projection 120. Secondary cover 112 is affixed to base element 38, either with edge 112 of canopy element 80 being welded for a permanent seal or not, by bolts 122. Projection 120 is separated by a narrow gap 124 from primary cover 72, thereby accommodating differential expansion while nevertheless providing additional mechanical support in the event that primary cover 72 is jolted during a drop accident. Projection 120 also serves to ensure that shear keys 88 and 90 do not become dislodged. A second mechanical seal is provided by O-ring 126, which is disposed in groove 118. In the event that the cask is to be permanently sealed a canopy element 128 having lower edge 130 and upper edge 132 is disposed in groove 116 before bolts 122 are inserted, and edges 130 and 132 welded to base element 38 and secondary cover 114, respectively. As was the case with canopy element 80, the welds on canopy element 128 extend all the way around. Canopy element 128 not only permits differential expansion due to temperature changes, it also allows the position of secondary cover 114 to be adjusted slightly during installation of bolts 122 in order to align bores 115 with threaded bores 66. Cap 134 having a core 136 of neutron absorbing material enclosed by a layer 138 of stainless steel is affixed to base element 38 after the closure system is applied, either temporarily or permanently. From the foregoing discussion it will be apprent that the closure system of the present invention provides redundant covers each having a mechanical seal for a short-term use if the cask is to be reopened. Since the long-term effectiveness of mechanical seals has not been established, particularly if the cask is flooded with helium, each mechanical seal has a welded back-up seal for use during long-term storage. The welded seals employ canopy elements which permit the covers to move slightly. The primary cover is installed and secured under water, and may be weld-sealed after the cask base element is raised and before the secondary shield is installed. The mechanical seals of the closure system are sufficient during development, testing, and refinement of the cask, and the welded seals can be installed to adapt the cask to long-term storage without re-engineering either the closure system or the mouth region of the cask base element. It will be understood that the above description of the invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.