Patent Number: 
Section: description

FIG. 1 illustrates a prior art transfer cask 10 having cylindrical body 11 and a retractable door assembly 12. In order to shield the environment from the radiation emitted by a canister of spent nuclear fuel once it is placed within cavity 13, cylindrical body 11 is typically constructed of a gamma absorbing material such as lead and a suitable hydrogenous material. Retractable door assembly 12 comprises rectangular compartment 14. Rectangular compartment 14 forms space 15 in which retractable plates 16, 17 (FIG. 2) are located. Prior art transfer cask 10 further comprises cask lid 18 having lid hole 19. Referring to FIG. 2, once prior art transfer cask 10 is loaded with a canister 20 of spent nuclear fuel, transfer cask 10 is positioned above and set atop storage cask 21. This is done in order to facilitate the transfer of canister 20 from transfer cask 10 to storage cask 21. However, as discussed in detail above, prior art transfer cask 10 is not secured to storage cask 21 during this process, transfer cask 10 merely rests atop storage cask 21. Once prior art transfer cask 10 is placed atop storage cask 21, retractable plates 16, 17 are moved to an open position. Retractable plates 16, 17 comprise rollers that require lubricant in order to move properly. Moving retractable plates 16, 17 to the open position results in an unobstructed path being formed between the cavity of transfer cask 10 and the cavity of storage cask 21. As such, canister 20 can be lowered by a crane 22 from prior art transfer cask 10 into storage cask 21 for permanent storage. As discussed above, prior art transfer cask 10 has a number of deficiencies. FIG. 3 illustrates an embodiment of the present invention, transfer cask 30 having sealable bottom lid 32. Transfer cask 33 comprises a cylindrical body 31, circular bottom lid 32, and cask lid 33. Cask lid 31 comprises cavity hole 34 and a plurality of cask lid bolt holes 43 circumferentially located around cask lid 31. Cavity hole 34 facilitates access to cavity 40 (FIG. 5) which is necessary to perform certain canister transfer operations. Cylindrical body 31 comprises bottom flange 35, top flange 36, drain valve 37, fill hole 38, and crane handles 39. Bottom flange 35 comprises a plurality of bottom lid bolt holes 41 and a plurality of mating device connection holes 42, both circumferentially located around bottom flange 35. For the reasons discussed below, mating device connection holes 42 are positioned closer to the outer perimeter 55 of bottom flange 35 than bottom lid bolt holes 41. Referring to FIG. 4, circular bottom lid 32 comprises a plurality of threaded holes 51 and circular groove 52. Circular groove 52 is adapted to receive circular gasket 53 (FIG. 5). Circular bottom lid 32 is preferably constructed of carbon steel and is of a thickness that provides adequate radiation shielding. Threaded holes 51 do not extend through the entire thickness of circular bottom lid 32. Moreover, circular bottom lid 32 can be a single circular plate or can be multiple circular plates welded or otherwise fastened together. As illustrated, circular bottom lid 32 comprises two circular plates welded together (FIG. 5). Referring to FIG. 5, cylindrical body 31 of transfer cask 30 is constructed so as to provide adequate radiation shielding for a canister 50 (FIG. 6) of spent nuclear fuel placed within cavity 40. Cylindrical body 31 comprises cylindrical inner shell 43. Inner shell 43 forms cavity 40 within which canister 50 (FIG. 6) can be placed when cask lid 33 is removed. Bottom lid 32 acts as the floor of cavity 40 when secured. Cylindrical body 31 further comprises cylindrical outer shell 44 which is concentric with and surrounds inner shell 43. Both inner shell 43 and outer shell 44 are made from carbon steel. Inner shell 43 and outer shell 44 are welded to top flange 36 and bottom flange 35, forming an annulus 45 that is capable of holding radiation absorbing material such as concrete, lead, or steel. Lead is preferred because it most effectively provides gamma shielding for the radioactive spent nuclear fuel once it is placed within cavity 40. Cylindrical body 31 further comprises jacket shell 46. Jacket shell 46 is concentric with and surrounds outer shell 44. Jacket shell 46 has top surface 47. The bottom of jacket shell 46 is welded to the top of bottom flange 35 while top surface 47 is welded to outer shell 44, forming a second annulus 48, referred to herein as xe2x80x9cjacket 48.xe2x80x9d Jacket 48 is adapted for receiving a neutron absorbing liquid such as water, which provides a layer of neutron shielding for the radioactive spent nuclear fuel once it is placed in cavity 40. In order to facilitate easy filling and draining of jacket 48, jacket shell 46 comprises one or more drain valves 37 and one or more fill holes 38. Additionally, transfer cask 30 comprises a plurality of radial plates (not shown) that extend radially from outer shell 44 to jacket shell 46. The radial plates are circumferentially located around transfer cask 30. Each radial plate is welded on one side to outer shell 44 and to jacket shell 46 on the other side. The radial plates act as fins for improved heat conduction. In the illustrated embodiment, bottom flange 35 forms the bottom surface of cylindrical body 31. Circular bottom lid 32 is secured to bottom flange 35 by extending bolts 49 through bottom lid bolt holes 41 (FIG. 3) and threadily engaging corresponding threaded holes 51 (FIG. 4) located on circular bottom lid 32. As a result, cavity 40 is formed wherein circular bottom lid 32 acts as a floor. Before circular bottom 32 is secured to bottom flange 35, circular gasket 53 is fitted circular groove 52 (FIG. 4). Upon securing circular bottom lid 32 to bottom flange 35 by sufficiently tightening bolts 49, circular gasket 52 hermetically seals the bottom of cavity 40. As mentioned earlier bottom flange 35 further comprises mating device connection holes 42 located closer to outer perimeter 55 than bottom lid bolt holes 41. By positioning mating device connection holes 42 sufficiently closer to outer perimeter 55 than bottom lid bolt holes 41, transfer cask 30 can be secured to mating device 70 (FIG. 7) even when circular bottom lid 32 is secured to bottom flange 35. In the illustrated embodiment, top flange 36 forms the top surface of cylindrical body 31. Top flange 36 comprises a plurality of circumferentially located threaded holes 57. Cask lid 33 is secured to cylindrical body 31 by extending bolts 54 through cask lid holes 43 (FIG. 3) and threadily engaging, the threaded holes 57 of top flange 36. Cask lid 33 is constructed of concrete and carbon steel, so as to provide radiation shielding for the enclosed canister 50 (FIG. 6). Cask lid 33 also comprises lid handles 56 for facilitating the lifting and removing of cask lid 33. Referring to FIG. 6, cavity 40 (FIG. 5) is adapted to receive a canister 50 when cask lid 33 is removed from cylindrical body 31. In the illustration, canister 50 is already placed into cavity 40 and cask lid 33 is secured to top flange 36. When canister 50 is in cavity 40, a small annulus (not labeled) is formed between inner shell 43 and the external wall of canister 50. This small annulus is a result of the diameter of canister 50 being slightly smaller than the diameter of cavity 40. As discussed earlier, gasket 53 hermetically seals the bottom of cavity 40 when circular bottom lid 32 is secured to bottom flange 35. In order to hermetically seal the top of cavity 40 when canister 50 is placed therein, annulus seal 59 is positioned between top flange 36 and the top of the external surface of canister 50. This results in the small annulus being hermetically sealed from the top in addition to the bottom. As such, the external surface of canister 50 is not exposed to pool water when transfer cask 30 and canister 50 are lowered into the pool as described below. Finally, handles 39 facilitate crane 60 to engage, lift, and transport transfer cask 30 throughout the defueling, transfer, and storage procedures. FIG. 7 illustrates an embodiment of the apparatus of the present invention, a cask mating device 70 for use in transferring a canister of spent nuclear fuel from the transfer cask of the present invention to a storage cask. In the illustrated embodiment, cask mating device 70 comprises top plate 71, bottom plate 72, radiation absorbing shield 73, and slidable tray 74. Bottom plate 72 and top plate 71 are constructed of carbon steel and are respectively welded to radiation absorbing shield 73 which comprises substantially U-shaped steel walls filled with a radiation absorbing material such as concrete. Top plate 71 comprises a plurality of threaded holes 75, a plurality of guide extrusions 76, and opening 80. Threaded holes 75 extend into radiation absorbing shield 73 and are used to secure transfer cask 30 (FIG. 9) to mating device 70. Threaded holes 75 are positioned near and partially surround opening 80. Opening 80 is adapted so that it is large enough so that canister 50 (FIG. 6) can pass through, 80 but small enough so that bottom flange 35 (FIG. 6) can rest on top plate 71 without falling into opening 80. Top plate 71 further comprises guide extrusions 76 which help correctly position transfer cask 30 (FIG. 9) atop mating device 70 when transfer cask 30 is being lowered onto and secured thereto. Bottom plate 71 comprises a plurality of storage cask connection holes 77, low friction tracks 78, and opening 81 (FIG. 8). Opening 81 is substantially aligned with opening 80 and adapted to be large enough so that canister 50 (FIG. 6) can pass through. Cask connection holes 77 are located in recesses in radiation absorbing shield 73. Storage cask connection holes 77 are used to secure mating device 70 to the top surface of storage cask 90 (FIG. 9). In the illustrated embodiment there are three cask connection holes 77 (although only one is visible). Bottom plate 72 further comprises low friction tracks 78 for guiding the horizontal movement of slidable tray 74. Low friction tracks 78 are constructed so as to not require lubricant or grease in order for slidable tray 74 to slide thereon. Specifically, low friction tracks 78 are constructed of steel and comprise roller bearings contained within steel guides, wherein only the roller bearings contact slidable tray 74. Referring to FIG. 14, horizontal movement of slidable tray 74 is afforded by a gear drive system comprising rack 85 and pinion 84. Slidable tray 74 has rack 85 welded to the bottom of slidable tray 74 to engage the gear system controlled by pinion 84 which controls the sliding motion. Power can be supplied to pinion 84 via hydraulic pressure, electric motor, compressed air, or human power. As will be described in more detail below, slidable tray 74 comprises a plurality of pneumatic lifters 79 for controlled lowering of circular bottom lid 32 (FIG. 4). Pneumatic lifters 79 are supplied with air through pneumatic hoses 82 which are connected to a source of pressurized air. Moreover, slidable tray 74 comprises elevated ring 88 which is adapted to receive circular bottom lid 32 (FIG. 4) and stabilize circular bottom lid 32 when it is resting on slidable tray 74. Slidable tray 74 is constructed of steel and is capable of horizontal movement between a closed and an open position. In FIG. 7, slidable tray 74 is in the closed position. When slidable tray 74 is in the closed position, slidable tray 74 covers opening 81 (FIG. 8) and is positioned so as to be capable of receiving circular bottom lid 32 (FIG. 4) when transfer casks 30 is secured to mating device 70 (FIG. 9). Referring to FIG. 8, slidable tray 74 is in the open position. When slidable tray 74 is in the open position, slidable tray 74 does not obstruct opening 81. As such, canister 50 can pass from cavity 40 of transfer cask 30, through hole 83 and openings 80, 81, and into cavity 91 of storage cask 90 (FIG. 1). Radiation absorbing shield 73 partially surrounds hole 83 through which canister 50 can pass. Mating device 70 further comprises alignment ring 87 (best illustrated in FIG. 14) welded to bottom plate 72. Alignment ring 85 serves as a guide to help center mating device 70 on storage cask 90 (FIG. 9) during installation. Alignment ring 87 is preferably tapered to help guide or funnel a canister 50 (FIG. 12) from storage cask 90 and into transfer cask 30 in the event that it is necessary to withdraw canister 50 from storage cask 90. FIG. 13 is a flowchart of an embodiment of the method of the present invention. The steps of FIG. 13 will be described in detail below using mating device 70 and transfer cask 30. Specifically, the steps of FIG. 13 will be discussed in relation to FIGS. 9-12 whenever possible. In defueling a nuclear reactor and storing the spent nuclear fuel according to the method of the present invention, initially cask lid 33 is not secured to cylindrical body 31 of transfer cask 30 and canister lid 58 is not secured to canister 50. Open canister 50 is then lowered into cavity 40 of open transfer cask 30 wherein circular bottom lid 32 is secured to bottom flange 35. Transfer cask 30 (having open canister 50 within cavity 40) is then submerged into a spent nuclear fuel pool, completing step 1300 of FIG. 13. Once transfer cask 30 is fully submerged and resting at the bottom of the spent nuclear fuel pool, spent nuclear fuel is removed from the reactor as necessary and placed into open canister 50, completing step 1310 of FIG. 13. Once canister 50 is fully loaded with spent nuclear fuel, canister lid 58 is secured to canister 50, sealing both pool water and the spent nuclear fuel within canister 50. As such, step 1320 of FIG. 13 is completed. At this point, transfer cask 30 (and loaded canister 50) are ready to be removed from the pool. However, before this occurs mating device 70 is secured to storage cask 90 (FIG. 9). Referring to FIG. 9, mating device 70 is secured to top surface 92 of storage cask 90 by positioning mating device 70 on top surface 92 so that cask connection holes 77 (FIG. 7) line up with threaded holes 93 (FIG. 10) located on top surface 92. Bolts 94 are then extended through cask connection holes 77 threadily engaging threaded holes 93. Moreover, at this point, slidable tray 74 of mating device 70 is in the closed position (see FIG. 7). As such, step 1330 of FIG. 13 is completed. Once step 1330 has been performed (or possibly during or after), crane 60 (FIG. 6) completes step 1340 of FIG. 13 by lifting transfer cask 30 (having loaded canister 50 in cavity 40) from the pool. Transfer cask 30 is then set down in a staging area where the pool water is pumped out of canister 50, the spent nuclear fuel is allowed to dray, and the canister is backfilled with an inert gas such as helium and then resealed. Canister 50 is now ready for dry storage and step 1350 of FIG. 13 is completed. At this point cask lid 33 (FIG. 3) is secured to cylindrical body 31 as described above, completing step 1360. Closed transfer cask 30 is then lifted by crane 60 and positioned above mating device 70 which is secured to storage cask 90. Once transfer cask 30 is positioned above mating device 70, crane 60 lowers transfer cask 30 down onto mating device 70 (see FIG. 9). As transfer cask 30 is being lowered onto top plate 71 of mating device 70, extrusion guides 76 help guide transfer cask 30 to its proper resting position. Transfer cask 30 is positioned so that the mating device connection holes 42 (FIG. 3) on bottom flange 35 line up with threaded holes 75 (FIG. 7) of mating device 70. Once properly positioned, bolts 95 are extended through mating device connection holes 42, threadily engaging threaded holes 75. As such, step 1370 of FIG. 13 is completed. Referring to FIG. 10, once mating device 70 is properly secured to both storage cask 90 and transfer cask 30, cavity 40 (FIG. 5) of transfer cask 30, hole 83 and openings 80, 81 (FIG. 8) of mating device 70, and cavity 91 of storage cask 90 are substantially aligned. Once properly secured together, circular bottom lid 32 is unfastened from bottom flange 35 by removing bolts 49 (FIG. 5). Pneumatic lifters 79 (FIG. 7) engage circular bottom lid 32 and lower circular bottom lid 32 onto slidable tray 74 within circular elevated ring 88. Referring to FIG. 11, once slidable tray 74 receives circular bottom lid 32, slidable tray 74 is moved to the open position as defined above. As such, slidable tray 74 slides on low friction tracks 78, horizontally removing circular bottom lid 32 so that a clear path through which canister 50 can pass from transfer cask 30 into storage cask 90 is formed. Thus, step 1380 of FIG. 13 is completed. Moreover, when circular bottom lid 32 and slidable tray 74 are moved to the open position, radiation absorbing shield 73 combined with circular bottom lid 32 substantially enclose the space between transfer cask 30 and storage 90 through which canister 50 will pass. In the illustrate embodiment, this is accomplished by designing U-shaped radiation shield 73 so that the diameter of circular bottom lid 32 is substantially equal to the perpendicular distance between the legs (i.e. the straight portions of the U-shape) of radiation shield 73. This design allows canister 50 to be lowered into storage cask 90 without radiation contaminating the outside environment in unacceptable levels. Upon a clear path being formed between cavity 40 of transfer cask 30 and cavity 91 of storage cask 90, canister 50 is lowered from cavity 40 into cavity 91 until canister 50 is fully within storage cask 90 (FIG. 12). This lowering process is performed by crane 60. Crane 60 engages canister handles 59 located on canister lid 58 through cavity hole 34. In this way, crane 60, completes step 1390 of FIG. 13. The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in this art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Specifically, mating device 70 can be constructed so as not to include top and bottom plates 71, 72. In this embodiment, slidable tray 74 would slide on low friction trucks 78 which would be located on the interior of radiation shield 73. In such an embodiment, the storage cask and transfer cask are secured directly to the radiation shield. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.