Patent Number: 
Section: description

FIG. 1 illustrates an embodiment of the apparatus of the present invention, wherein cylindrical transfer cask 10 having a drainable jacket defined by jacket shell 28. A cask lid 11 is removable during the loading step. Cask lid 11 has lid opening 38 for facilitating access to cavity 3 (FIG. 2). Bottom lid 12 is secured to bottom flange 34 by a plurality of bolts 41 that extend through holes in bottom flange 34 and threadily engage bottom lid 12. Handles 4 are provided for attaching the crane. Transfer cask 10 and jacket shell 28 have a top and bottom. As used herein throughout, xe2x80x9ctopxe2x80x9d refers to the ends of jacket shell 28 and cask 10 that are closest in proximity to top flange 35, while xe2x80x9cbottomxe2x80x9d refers to the ends of jacket shell 28 and cask 10 that are closest in proximity to bottom flange 34. Referring to FIG. 2, transfer cask 10 comprises cylindrical inner shell 36. Along with bottom lid 12, cylindrical inner shell 36 forms cavity 3 within which a canister of spent nuclear fuel can be placed when cask lid 11 is removed. Transfer cask 10 further comprises cylindrical outer shell 37 which is concentric with and surrounds inner shell 36. Both inner shell 36 and outer shell 37 are made from carbon steel. Inner shell 36 and outer shell 37 are welded to top flange 35 and bottom flange 34, forming an annulus 19 that is capable of holding gamma 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 3. Transfer cask 10 further comprises jacket shell 28. Jacket shell 28 is concentric with and surrounds outer shell 37. Jacket shell 28 has top surface 40. The bottom of jacket shell 28 is welded to bottom flange 34 while top surface 40 is welded to outer shell 37, forming a second annulus 20, referred to herein as xe2x80x9cjacket 20.xe2x80x9d Jacket 20 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 3. In order to facilitate easy filling and draining of jacket 20, jacket shell 28 further comprises one or more drain valve 31 and one or more fill holes 30. Fill hole 30 is located on top surface 40 and extends through top surface 40, providing an opening into jacket 20. Drain hole 30 is large enough so that it can be used to fill jacket 20 with water in an amount of time that does not substantially increase the cycle time required to prepare a canister of spent nuclear fuel for dry storage. Fill hole 30 can be closed and hermetically sealed by inserting a properly sized plug therein. Drain valve 31 is located at or near the bottom of jacket shell 28 and is fluidly connected to jacket 20 (i.e., the second annulus). Drain valve 31 is adapted so as to have both an open and a closed position. When closed, drain valve 31 is hermetically sealed. When open, fluid contained in jacket 20 can freely flow through drain valve 31, thus draining any water contained in jacket 20. The size and maximum flow-rate of drain valve 31 is chosen so that when jacket 20 is filled with water, it can be completely drained without substantially increasing the cycle time required to prepare a canister of spent nuclear fuel for dry storage. Moreover, when drain valve 31 and fill hole 30 are hermetically sealed, jacket 20 is also hermetically sealed. The positioning of drain valve 31 at or near the bottom of jacket shell 28, and the positioning of fill hole 30 at or near the top of jacket shell 28, make it possible to easily fill or drain the jacket while keeping transfer cask 10 in an upright position (i.e., resting on its bottom end). As such, when transfer cask 10 is being used to transport a canister of spent nuclear fuel, jacket 20 can be filled or drained without increasing the transport time substantially. Moreover, the positioning of drain hole 30 and drain valve 31 make it possible to drain and fill jacket 20 without employing special pumping equipment. Additionally, transfer cask 10 comprises a plurality of radial plates 29 that extend radially from outer shell 37 to jacket shell 28. The radial plates are circumferentially located around transfer cask 10. Each radial plate 29 is welded on one side to outer shell 37 and to jacket shell 28 on the other side. Radial plates 29 act as fins for improved heat conduction. Referring to FIG. 3, transfer cask 10 is adapted to be capable of receiving a canister 23 filled with spent nuclear fuel in cavity 3 (FIG. 2) when cask lid 11 is removed. Canister 23 has canister lid 18. Once canister 23 is placed in transfer cask 10, cask lid 11 can be secured to top flange 35. Cask lid 11 is secured to top flange 35 by extending bolts 51 through holes in cask lid 11 and threadily engaging top flange 35. Transfer cask 10 and its payload (i.e., canister 23 and its contents) are lifted and handled by power plant crane 32. Crane 32 lifts and handles transfer cask 10 and its payload by engaging handles 4. Lid opening 38 provides access to canister 23 for performing certain handling operations of canister 23 while cask lid 11 is secured to top flange 35. FIG. 4 is a flowchart of an embodiment of the method of the present invention, providing the maximum amount of radiation shielding during all stages of transferring a canister of spent nuclear fuel from a storage pool to a storage cask for long-term dry storage, even when the weight of the transfer cask""s payload is varied during the transfer procedure. The steps of FIG. 4 will be discussed in relation to the apparatus embodiment shown in FIGS. 1-3. In defueling a nuclear reactor and storing the spent nuclear according to the method of the present invention illustrated in FIG. 4, canister 23 is first placed in cavity 3 of transfer cask 10 without its lid 18. At this point, jacket 20 of transfer cask 10 is empty and hermetically sealed by closing drain valve 31 and sealing drain hole 30 with a plug. Annulus 19, is filled with lead. Transfer cask 10 and open canister 23 are then submerged into a storage pool, completing step 400. As the nuclear reactor uses up the nuclear fuel, the spent nuclear fuel is removed from the reactor, lowered into the storage pool, and placed in canister 23, completing step 410. Once canister 23 is fully loaded, lid 18 is secured to canister 23, enclosing both the spent nuclear fuel and water from the pool therein. Crane 32 is lowered into the pool and secured to handles 4 of transfer cask 10. Once secured to handles 4, crane 32 lifts transfer cask 10 and its current payload out of the storage pool, completing step 420. Transfer cask 10 is designed so that at this stage in the transfer procedure, the combined weight of transfer cask 10 and its payload is equal to or less than the rated lifting capacity of crane 32. Once lifted out of the storage pool, crane 32 sets transfer cask 10 and its payload down in a staging area, completing step 430. At this point, canister 23 contains the pool water in addition to the spent nuclear fuel. As discussed earlier, this pool water acts as a neutron absorbing layer as long as it is in canister 23. Despite providing neutron shielding, this pool water must be removed from canister 23 in order to store the spent nuclear fuel in a dry-state, eliminating any neutron shielding provided thereby. However, according to the method of the present invention, jacket 20 is filled with water before the pool water is pumped out of canister 23. Jacket 20 is filled with water by removing the plug from fill hole 30 and supplying water therethrough, completing step 440. Upon jacket 20 being filled with water, fill hole 30 is sealed with its plug once again. At this point, the pool water contained in canister 23 is pumped out, completing step 450. The water within jacket 20 now provides the necessary neutron shielding for the remainder of the transfer procedure. The spent nuclear fuel contained therein is allowed to dry and canister 23 is then backfilled with an inert gas such as helium. Cask lid 11 is then secured to transfer cask 10. Transfer cask 10 is then lifted by crane 32, completing step 460. If the spent nuclear fuel is going to be stored long-term, transfer cask 10 is transported by crane 32 and positioned above a storage cask, completing step 470. Once properly positioned above the storage cask, bottom lid 12 is removed and canister 23 is lowered into the storage cask, completing step 480. Alternatively, transfer cask 10 can be used to transport the spent nuclear fuel a transport cask for moving spent nuclear fuel over long distances using a similar procedure. Once canister 23 is removed from transfer cask 10, transfer cask 10 can be reused to perform the above described procedure again. If transfer cask 10 is going to be reused, the water is drained from jacket 20 by opening drain valve 31. Cask lid 11 is then removed and drain valve 31 is moved to the closed position, hermetically sealing jacket 20. The procedure is then started over. The method and apparatus of the invention allow for the combined weight of the transfer cask and its load to be approximately equal to the rated lifting capacity of the crane at all stages of the transfer procedure. This is desirable because the greater the weight of the transfer cask, the greater the amount of radiation shielding. For example, suppose a canister loaded with spent nuclear fuel and pool water weighs 45 tons when lifted in step 420. If the rated lifting capacity of the crane is 125 tons, the transfer cask must weigh 80 tons in order to maximize radiation shielding at this stage. Now suppose that once the transfer cask is set down and the pool water removed from the canister, that the canister and its contents weigh 25 tons. The combined weight of the transfer cask and its payload would only weigh 105 tons at this stage. This is 20 tons less than the rated lifting capacity of the crane. This 20 tons of available crane capacity is unused for the remainder of the transfer procedure when using prior art methods, resulting in a less than maximum amount of radiation shielding. However, according to the present invention, 20 tons of water can be added to the jacket, increasing the combined weight of the transfer cask and its payload so that it is equal to the lifting capacity of the crane. Thus, keeping the possible radiation shielding at maximum capacity for that payload and that crane during the entire transfer procedure. The present invention allows for a thicker a layer of gamma absorbing material to be used in constructing a transfer cask. For example, suppose a crane capacity of 125 tons and a transfer cask payload weight of 45 tons during step 420 (the weight of the canister, spent nuclear fuel, and pool water). This leaves 80 tons for the weight of the transfer cask. A transfer cask must provide both gamma and neutron radiation shielding. While the pool water trapped in the canister provides adequate neutron shielding, this water must be drained in order to store the spent nuclear fuel in a dry-state. Prior art casks have both a layer of gamma absorbing material and a separate layer of neutron absorbing material at all times so that the weight of both of these layers is included in designing a prior art transfer cask to be under 80 tons. If a layer of neutron absorbing material weighing 20 tons is needed to provide adequate neutron shielding once the pool water is drained from the canister, a total of 60 tons is available to make the rest of the transfer cask. Supposing the carbon steel frame of the transfer cask weighs 15 tons, this leaves 45 tons to be used for creating the gamma absorbing layer for the prior art transfer cask. However, according to the present invention, at no time during a crane lifting operation does the transfer cask have both the enclosed pool water and the separate layer of neutron absorbing liquid. This allows for a greater weight of gamma absorbing material to be used in designing the transfer cask. Because the combined weight of the transfer cask and its payload is not at its heaviest during the step 420 lift (because the jacket is empty), the acceptable weight of the gamma absorbing layer is not calculated at this step. Instead, the acceptable weight of gamma absorbing layer is calculated for the lift at step 460, when the pool water has been drained from the canister and the jacket has been filled with water. Assuming that 20 tons of pool water has been drained from the canister during step 450, the weight of the transfer cask""s payload is 25 tons during step 460. Assuming a rated lifting capacity of 125 tons for the crane, the transfer cask""s permissible weight can be 100 tons. As above, assuming that 20 tons of a neutron absorbing liquid is needed to provide adequate neutron shielding, and that the carbon steel frame weighs 15 tons, 65 tons is left for constructing the gamma shielding layer when using the present invention. This is opposed to the mere 45 tons allowed in the prior art for the identical payload. Because the gamma shielding layer can be heavier, it can also be thicker, thus, providing greater gamma radiation shielding. Another benefit of the present invention is that a lower capacity crane can be used to transfer loads of spent nuclear fuel that could not previously be used. If a crane has a rated lifting capacity of 100 tons and safety standards require a transfer cask to have a 20 ton layer of neutron absorbing material and a 35 ton layer of gamma absorbing material, and if the carbon steel frame of the transfer cask weighs 15 tons, the total weight of the transfer cask must be at least 70 tons. However, if the canister and its contents (i.e., the spent nuclear fuel and pool water) at step 420 weigh 45 tons, the combined weight of the transfer cask and its payload will exceed the crane""s lifting capacity, in which case a 100 ton capacity crane can not be used to transfer the spent nuclear fuel using prior art procedures and apparatus. Using the same numbers for a power plant utilizing the present invention, and assuming that the jacket of the transfer cask is empty during step 420, the weight of the transfer cask will be 50 tons and the combined weight of the transfer cask (50 tons) and its payload (45 tons) is only 95 tons, a weight capable of being lifted by the 100 ton capacity crane. Assuming that 20 tons of water is removed from the canister during step 450, and 20 tolls of water is added to the jacket, the combined weight of the transfer cask and its payload is still below the rated lifting capacity of the 100 ton crane. Thus, using the present invention enables smaller cranes that would otherwise be unable to perform the spent nuclear fuel transfer to do so. 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 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.