Patent Number: 
Section: description

Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 1, a system for moving a canister 12 of spent nuclear fuel to a storage cask 14 according to a preferred embodiment of the invention is depicted with the storage cask 14 being mounted for transport upon a trolley 16. As may be seen in FIG. 1, system 10 utilizes a transfer cask 18 into which the canister 12 is temporarily positioned while it is being moved from a first location to a final resting space 44 that is defined within the storage cask 14. As is conventional, transfer cask 18 is preferably fabricated from steel and has a pair of opposed lifting lugs 34, 36 that are integral with an outer wall 38 thereof. Lifting lugs 34, 36 may be used to lift and reposition the transfer cask 18 during operation. Transfer cask 18 further has an internal space defined by an inner wall 40 for receiving the canister 12 and a gate mechanism 42 positioned at the bottom thereof for retaining the canister 12 until it is properly positioned to be lowered into the storage cask 14. Referring again to FIG. 1, it will be seen that system 10 further includes a first lifting mechanism 20 for engaging the transfer cask 18 and moving the transfer cask 18 from a first position to a position that is immediately adjacent to and above the storage cask 14. In the preferred embodiment, first lifting mechanism 20 is constructed as a single failure proof crane 22 having an upper block assembly 24 and a lower block assembly 26 that is suspended from the upper block assembly 24 by a reeving arrangement 28. Suspended from the lower block assembly 26 is a first lifting hook that is configured and spaced and sized so as to be able to engage the first lifting lug on the transfer cask 18 and a second lifting hook that is likewise constructed for engaging the opposed second lifting lug 36 during operation. It should be understood that the description of the lifting hooks as being suspended from the lower block assembly 26 should be construed as descriptive of any mechanical arrangement wherein the lifting hooks move substantially with the lower block assembly 26, regardless of whether they are actually mounted on the lower block assembly or one another component, such as part of the hoist mechanism, that in turn is mounted on the lower block assembly. System 10 advantageously further includes a second lifting mechanism 46 that in the preferred embodiment is attached to the lower block assembly 26 of the first lifting mechanism 20. As may be seen in the top plan view of the second lifting mechanism that is depicted in FIG. 7, the second lifting mechanism 46 is preferably embodied as a hoist 58 that is powered by a redundant pair of electric motors 60 and that is constructed and arranged to raise and lower a specialized canister grab system 64 that will engage a lid portion 65 of the canister assembly 12, as will be described in greater detail below with reference to FIG. 8. As may be seen in FIG. 7, hoist 58 includes a corresponding pair of planetary reduction gears 61 that are part of a drive train from the motors 60 to a pair of drums 63 about which are wound at least two cables or ropes, which in turn are arranged in a reeving arrangement 67 comprising a plurality of parts of rope and a plurality of sheaves 66 (shown in FIG. 5) that are mounted to a frame assembly 48 of the canister grab system 64. The reeving arrangement 67 includes an equalizer 69, best shown in FIG. 7. An electric motor 50, powered by a power supply 52, is provided to engage and disengage canister grab system 64 with the lid portion 65, as will be described in greater detail below. Referring now to FIGS. 2-4, it will be seen that electric motor 50 is coupled to a reduction gear 70 that in turn is connected to an interlock assembly that ensures that the grab system 64 will be constrained to remain engaged with the lid portion 65 when any substantial amount of weight of the canister 12 is borne by the second lifting mechanism 46. As may best be seen in FIG. 4, an interlock assembly is preferably constructed as a clutching arrangement in a bevel drive 72 that includes a first, male, conical drive member 74 and a second, female conical drive member 76. As may be seen by comparing the positions of the grab system 64 and FIGS. 2 and 3, the frame assembly 48 is divided into an upper grab frame assembly 54 and a lower grab frame assembly 56 that is mounted so as to be permitted to travel a predetermined vertical distance DT with respect to the upper grab frame assembly 54. As may best be seen in FIG. 4, a system of biasing springs 78 and pins 80 are provided to bias the upper and lower grab frame assemblies 54, 56 together so that under normal circumstances the first male conical drive member 74 remains frictionally engaged with the second female conical drive member 76, ensuring an intact drive train between the reduction gear 70 and a drive shaft 82 as is shown in FIG. 2. Drive shaft 82 is coupled to a screw drive arrangement 86 by means of a bevel gear 84. Screw drive arrangement 86 includes a shaft 87 that is provided with an external helical thread and that is interengaged with a mating internally threaded sleeve 89 that is integral with the canister ring hook 88. Accordingly, rotation of the shaft 82 in a first direction will tend to extend a canister ring hook 88 and rotation in a second, opposite direction will tend to retract the canister ring hook 88. A limit switch 90 is positioned as is shown in FIG. 2 to monitor the travel of the canister ring hook 88. When the canister ring hook 88 is in the retracted position, the canister grab assembly will be able to engage the lid member 65 of the canister 12. When the canister ring hook 88 is in the fully extended position, the canister grab system 64 will be securely locked into the lid portion 65 of the canister 12. When the canister grab system 64 bears the weight of the canister 12, the lower grab frame assembly 56 will be pulled downwardly with respect to the upper grab frame assembly 54, against the biasing of springs 78, and the first conical drive member 74 will disengage from the second conical drive member 76. This state of disengagement may be sensed by a sensor 92. Accordingly, when the second lifting mechanism 46 is effectively bearing the weight of the canister 12, the canister grab system 64 will be unable to disengage from the canister 12. Referring now to FIG. 8, it will be seen that the canister ring hook 88 includes at its circumferentially outer extreme lower end a projection 94 for engaging the ring member 98 that is bolted to the lid portion 65 of the canister 12. As may be seen in FIG. 8, ring member 98 also includes an inwardly extending projection 99 that has a lower surface 100. According to one important aspect of the invention, the lower surface 100 of the projection 99 on the ring member 98 and an upper surface 96 of the projection 94 are both angled so as to tend to retain the canister ring hook 88 to the ring member 98 that any time during operation that the weight of the canister 12 is resting to any significant extent on the canister ring hook 88. Preferably, each of these surfaces is angled within a range of about 2 degrees to about 10 degrees with respect to horizontal. More preferably, each of these surfaces is angled by about 6 degrees with respect to the horizontal. FIG. 9 depicts a grab assembly 210 that is constructed according to a second, alternative embodiment of the invention. Grab assembly 210 is identical to the grab assembly described above in reference to the first embodiment, except that it is configured to engage a canister lid assembly of slightly different construction by radially retracting the canister ring hook assembly 212. Accordingly, a screw drive arrangement 214 is provided that has a threading that is opposite from that of the screw drive arrangement 86 of the first embodiment. The canister ring hook assembly in the second embodiment has a projection 216 that faces inwardly, as may be seen in FIG. 9. In operation, the canister 12 will first be positioned and secured within the transfer cask 18 and the transfer cask 18 will then be engaged by the first lifting mechanism 20, specifically by engagement of the lifting hooks 30, 32 with the corresponding lifting lugs 34, 36 on the sides of the outer wall 38 of the transfer cask 18. At this point, the first lifting mechanism 20 and specifically the crane 22 will be used to move the transfer cask 18 and the enclosed canister 12 to a position (as is shown in FIG. 1) immediately above the storage cask 14. At any point in the process up to and including this point, the canister grab system 64 may be lowered automatically or by an operator by instructing the hoist 58 to be lowered into the proximity of the lid member 65 of the canister 12. The motor 50 will then be actuated, causing the canister ring hook 88 to extend, thereby locking the canister grab system 64 into the lid assembly 65. This is verified by the limit switch 90, which will so advise a remote human operator or an automated control system. At this point, while the first lifting system and 20 remains engaged with the transfer cask 18, the gate mechanism 42 at the lower end of the transfer cask 18 may be opened, preferably by remote control. The hoist 58 may be instructed to lower the canister 12 into the space 44 that is defined within the storage cask 14. Because the canister grab system 64 is suspended from the lower block assembly 26, which also provides the lifting engagement with the transfer cask 18, relative stability is provided during this step of lowering the canister 12. Accordingly, it is not necessary for the transfer cask 18 to be tied down using supplemental restraints which, as described above, is a time-consuming process that may also result in the exposure of humans to potentially harmful radiation. As the canister 12 is lowered, the bevel drive 72 will be pulled into the disengaged position that is shown in FIG. 4, 50 that the grab system 64 may not be accidentally disengaged. When the canister 12 is in the final storage position, the control system or remote human operator will instruct the electric motor 50 to disengage the canister ring hook 88 from the lid member 65 of the canister 12. The hoist 58 will then be instructed to retract the grab system 64, and the first lifting mechanism may then be instructed to transport the transfer cask to another location where it may be stored or used in a second transfer procedure. It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the fill extent indicated by the broad general meaning of the terms in which the appended claims are expressed.