Abstract:
An apparatus and method for inter-unit transfer of spent nuclear fuel. In one aspect, the invention is a method of transferring high level radioactive waste comprising: a) loading high level radioactive waste into a water-filled cavity of a canister body having an open top end at a first location; b) coupling a lid to the canister body to enclose the open top end; c) removing a volume of water from the cavity so that a water level of the water within the cavity is above a top end of the high level radioactive waste and a space exists between the water level and a bottom surface of the lid; d) hermetically sealing the cavity; and e) transferring the canister to a second location, the water level remaining above the top end of the high level radioactive waste during the transfer.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Application Ser. No. 61/286,905, filed Dec. 16, 2009, the entirety of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of transferring high level radioactive materials, and specifically to a canister apparatus and method for transferring high level radioactive materials in a submerged state. 
     BACKGROUND OF THE INVENTION 
     In the operation of nuclear reactors, the nuclear energy source is in the form of hollow zircaloy tubes filled with enriched uranium, typically referred to as fuel assemblies. When the energy in the fuel assembly has been depleted to a certain level, the assembly is removed from the nuclear reactor. At this time, fuel assemblies, also known as spent nuclear fuel, emit both considerable heat and extremely dangerous neutron and gamma photons (i.e., neutron and gamma radiation). Thus, great caution must be taken when the fuel assemblies are handled, transported, packaged and stored. 
     After the depleted fuel assemblies are removed from the reactor, they are placed in a canister. Because water is an excellent radiation absorber, the canisters are typically submerged under water in a pool. The pool water also serves to cool the spent fuel assemblies. When fully loaded with spent nuclear fuel, a canister weighs approximately 45 tons. The canisters must then be removed from the pool because it is ideal to store spent nuclear fuel in a dry state. 
     Removal from the storage pool and transport of the loaded canister to the storage cask is facilitated by a transfer cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the storage pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor and lowered into the storage pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the spent nuclear fuel for storage in the “dry state.” Once in the staging area, the water contained in the canister is pumped out of the canister. This is called dewatering. Once dewatered, the spent nuclear fuel is dried using a suitable process such as vacuum drying. Once dry, the canister is back-filled with an inert gas such as helium. The canister is then sealed and the canister and the transfer cask are once again lifted by the plant&#39;s crane and transported to an open storage cask. The transfer cask is then placed atop the storage cask and the canister is lowered into the storage cask. 
     Because a transfer cask must be lifted and handled by a plant&#39;s crane (or other equipment), transfer casks are designed to be a smaller and lighter than storage casks. A transfer cask must be small enough to fit in a storage pool and light enough so that, when it is loaded with a canister of spent nuclear fuel, its weight does not exceed the crane&#39;s rated weight limit. Additionally, a transfer cask must still perform the important function of providing adequate radiation shielding for both the neutron and gamma radiation emitted by the enclosed spent nuclear fuel. As such, transfer casks are made of a gamma absorbing material such as lead and contain a neutron absorbing material. 
     However, the allowable weight of a transfer cask is limited by the lifting capacity of the plant&#39;s crane (or other lifting equipment). The load handled by the crane includes not only the weight of the transfer cask itself, but also the weight of the transfer cask&#39;s payload (i.e., the canister and its contents). A transfer cask must be designed so that the total load handled by the crane during all handling evolutions does not exceed the crane&#39;s rated weight limit, which is typically in the range of 100-125 tons. 
     Because the weight of the transfer cask&#39;s payload varies during the different stages of the transport procedure, the permissible weight of the transfer cask is equal to the rated capacity of the plant crane less the weight of the transfer cask&#39;s maximum payload at any lifting step. The weight of the transfer cask&#39;s payload is at a maximum when the transfer cask and canister are lifted out of the storage pool, at which time the canister is full of spent nuclear fuel and water. Thus, according to prior art methods, it is at this stage that the permissible weight of a transfer cask is calculated. The transfer cask is then constructed using this permissible weight as a design limitation. 
     Additionally, many nuclear sites have more than one reactor unit and more than one storage pool. Each of the storage pools might have its own crane, and the rated capacity of one crane at one storage pool might be different from the rated capacity of the crane at other storage pools. In nuclear sites with multiple pools and multiple cranes with different rating capacities, it might be desirable to move the depleted fuel assemblies from one pool, with a crane having a lower rating capacity, to another pool having a crane with a higher rating capacity, prior to placing the depleted fuel assemblies into a canister, such as a multi-purpose canister (“MPC”) within a transfer cask. This is because the rated capacity of a crane at one pool might not be able to safely lift a fully loaded transfer cask (with depleted fuel assemblies and canister). Therefore, there is a need for a system and method of transferring the depleted fuel assemblies from one pool, having a crane that cannot safely lift a fully loaded transfer cask, to another pool, having a crane with a rating capacity that can safely lift a fully loaded transfer cask. Since the pools in some of these sites are not interconnected to permit underwater transfer of the depleted fuel assemblies from one pool to another, a transfer canister for inter-unit transfer of depleted fuel assemblies is needed. It is desirable that depleted fuel assembly transfer from one pool to another be accomplished in the minimum amount of time (and hence radiation dose), with multiple assemblies at one time, with minimized upending and downending operations that carry the risk of handling accidents, with minimized (or eliminated) reliance on forced cooling methods that may introduce operation vulnerability to the transfer process, ensuring no risk of a criticality event, and with maximized protection against events such as crane malfunctions. 
     Thus, a need exists for a method and apparatus for transferring high level radioactive materials from a first submerged environment to a second submerged environment that accomplishes the aforementioned goals. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for transferring high level radioactive materials from a first pool to a second pool that uses the pool water to minimize the thermal shock to the high level radioactive waste payload, to provide neutron radiation shielding, and to extract the decay heat from the high level radioactive waste payload to keep them cool. In one embodiment, the high level radioactive waste payload is depleted fuel assemblies. 
     The present invention is also directed to a shielded transfer canister for inter-unit transfer of spent nuclear fuel assemblies with additional pressure relief volume that is isolated from canister&#39;s fuel storage cavity through one or more pressure relief devices, and a method incorporating the canister. 
     In one embodiment, the invention can be a method of transferring high level radioactive waste comprising: a) loading high level radioactive waste into a water-filled cavity of a canister body having an open top end at a first location; b) coupling a lid to the canister body to enclose the open top end; c) removing a volume of water from the cavity so that a water level of the water within the cavity is above a top end of the high level radioactive waste and a space exists between the water level and a bottom surface of the lid; d) hermetically sealing the cavity; and e) transferring the canister to a second location, the water level remaining above the top end of the high level radioactive waste during the transfer. 
     In an alternate embodiment, the invention can be a method of transferring spent nuclear fuel from a first body of water to a second body of water comprising: a) submerging a canister into the first body of water, the canister having a cavity having an open top end and a closed bottom end, the water filling the cavity; b) loading spent nuclear fuel into the cavity of the submerged canister; c) positioning a lid atop the loaded canister to enclose the open top end of the cavity; d) removing the loaded canister from the first body of water, the spent nuclear fuel remaining submerged in the water within the cavity; e) hermetically sealing the cavity; f) transferring and submerging the loaded canister to the second body of water; g) removing the lid from the loaded canister; and h) removing the spent nuclear fuel from the submerged canister. 
     In another alternate embodiment, the invention can be a canister apparatus for transferring spent nuclear fuel comprising: a tubular body forming a cavity for receiving spent nuclear fuel, the tubular body having a longitudinal axis, a floor enclosing a bottom end of the tubular body, an open top end; and a lid detachably coupled to the tubular body that encloses the open top end of the tubular body and hermetically seals the cavity, the lid comprising a chamber and a pressure relief device hermetically sealing an opening into the chamber, the pressure relief device automatically opening upon the pressure within the cavity exceeding a predetermined threshold so as to form a passageway from the cavity into the chamber. 
     These and various other advantages and features of novelty that characterize the invention are pointed out with particularity below. For a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a transfer cask according to one embodiment of the present invention. 
         FIG. 2  is a longitudinal cross-sectional view of the transfer cask of  FIG. 1  in partial cut-away along the A-A axis. 
         FIG. 3  is a perspective view of a lid according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2  concurrently, the exterior of a shielded canister  100  is illustrated according to one embodiment of the present invention. The shielded canister  100  is a pressure vessel designed for use in a substantially vertical orientation, as depicted in  FIG. 1 . The shielded canister  100  is preferably a substantially cylindrical containment unit with a longitudinal axis A-A having a horizontal cross-sectional profile that is substantially circular in shape. It should be noted, however, that the invention is not limited to cylinders having circular horizontal cross sections but may also include containers having cross-sectional profiles that are, for example, rectangular, ovoid or other polygon forms. While the shielded canister  100  is particularly useful for use in conjunction with transporting spent nuclear fuel (SNF) assemblies, the invention is in no way limited by the type of high level radioactive materials to be transported in certain embodiments, unless specifically recited in the claims. The shielded canister  100  can be used to transport almost any type of high level radioactive materials (HLW). However, the shielded canister  100  is particularly suited for the transport and/or cooling of high level radioactive materials that have a residual heat load and produce neutron and gamma radiation, such as SNF. 
     As discussed in more detail below, the shielded canister  100  generally comprises a tubular body  60  and a removable lid  20 . The tubular body  60  comprises a body portion  13 , an upper structural ring  11 , and a floor/base plate  12 . The tubular body  60  is preferably tubular in shape and forms an internal storage cavity  31  for storing spent nuclear fuel assemblies. When fully assembled, the tubular body  60  forms a hermetically sealed fluid containment boundary about the storage cavity  31 . In the exemplified embodiment, the body portion  13  of the tubular body  60  comprises three concentrically arranged tubular shells, namely an inner shell  14 , an intermediate shell  15 , and an outer shell  16 . The body portion  13  of the tubular body  60  provides a desired level of gamma radiation shielding. If desired, the body portion  13  could further include layers to provide a level of neutron radiation shielding. Thus, the tubular body  60  may provide both gamma and neutron radiation shielding properties while at the same time facilitating improved cooling of the HLW stored inside the cavity by efficiently conducting heat away from the HLW. In an alternate embodiment, the body portion  13  of the tubular body  60  is formed with two concentrically arranged and spaced apart shells that comprise an annular gap in-between. The annular gap is then filled with a gamma radiation absorbing material such as lead. It is desired that the body portion  13  of the tubular body  60  be constructed so as to be efficient conductive path for thermal energy. 
     As noted above, the shielded canister  100  comprises the tubular body  60  and a removable lid  20 . The tubular body  60  comprising a body portion  13 , an upper structural ring  11 , and a floor/base plate  12 . Further, the tubular body  60  is preferably tubular in shape and forms an internal storage cavity  31  for storing spent nuclear fuel assemblies. 
     The floor/base plate  12  is hermetically sealed to a bottom end  99  of the body portion  13  of the tubular body  60 . The floor/base plate  12  fully encloses and seals the bottom of the tubular body  60 . Preferably, the floor/base plate  12  is welded to the bottom of the body portion  13  of the tubular body  60 , thereby hermetically sealing the bottom end of the cavity  31 . The floor/base plate  12  functions as the floor of the cavity  31  of the shielded canister  100  and preferably has a flat bottom for stability. 
     The upper structural ring  11  is connected to the top of the body portion  13  of the tubular body  60  and forms an open top end  98  of the tubular body  60 . The upper structural ring  11  comprises an opening and is concentric with the body portion  13  of the tubular body  60 , thereby forming a passageway into an open top end of the cavity  31 . Preferably, the upper structural ring  11  is welded to the top edge of the body portion  13  of the tubular body  60 . It is also preferred that the opening in the upper structural ring  11  be the same diameter as the internal storage cavity  31  of the shielded canister  100 . In the preferred embodiment the floor/base plate  12  and top structural ring  11  are hermetically sealed to the inner and outer shells  14 ,  16  of the body portion  13  of the tubular body  60 . 
     As discussed in more detail below, the removable lid  20  is configured so that it may be detachably coupled to the top end  98  of the tubular body  60  in a manner that hermetically seals the open top end  98  of the tubular body  60 . One or more annular gaskets may be used at the interface between the lid  20  and the tubular body  60 . In the exemplified embodiment, the removable lid  20  is sealed to the structural ring  11  using bolts  50 . The removable lid  20  is designed to rest atop and be removable/detachable from the top structural ring  11  of the tubular body  60 . When the removable lid  20  is bolted to the top structural ring  11  of the tubular body  60 , the removable lid forms a hermetic seal with the tubular body  60  of the shielded canister  100 . In the preferred embodiment, both the floor/base plate  12  and the structural ring  11  are thick steel forgings. 
     The tubular body  60  forms an internal storage cavity  31  for receiving and storing the SNF assemblies, which can still give off considerable amounts of heat. The cavity  31  is a cylindrical cavity having an axis that is in a substantially vertical orientation. The invention is not so limited however, and the axis could be in a substantially horizontal orientation or another orientation. The horizontal cross-sectional profile of the cavity  31  is generally circular in shape, but is dependent on the shape of the inner shell  14  of the tubular body  60 , which is not limited to circular. The top end of the cavity  31  is open, providing access to the cavity  31  from outside of the shielded canister  100  (the removable lid  20  provides closure to the top end of the cavity  31  when secured to the shielded canister  100 ). The bottom end of the cavity  31  is closed by the floor/base plate  12 . More specifically, the top surface of the floor/base plate  12  acts as a floor for the cavity  31 . 
     The shielded canister  100  forms a containment boundary about the storage cavity  31  (and thus the stored SNF assemblies). The containment boundary can be literalized in many ways, including without limitation a gas-tight containment boundary, a pressure vessel, a hermetic containment boundary, a radiological containment boundary, and a containment boundary for fluidic and particulate matter. These terms are used synonymously throughout this application. In one instance, these terms generally refer to a type of boundary that surrounds a space and prohibits all fluidic and particulate matter from escaping from and/or entering into the space when subjected to the required operating conditions, such as pressures, temperatures, etc. 
     Referring to  FIG. 2 , the internal components making up the body portion  13  of the tubular body  60  of the shielded canister  100  according to one embodiment of the present invention are illustrated. As noted above, in the exemplified embodiment, the body portion  13  of the tubular body  60  comprises the inner shell  14 , the intermediate shell  15  and the outer shell  16 . In the preferred embodiment, the tubular body  60  is made as thick as possible within the constraints of the lifting equipment&#39;s capacity. The maximized weight enhances shielding protection and imparts a greater thermal inertia to the shielded canister  100 , making the temperature rise more gradual as the shielded canister  100  is lifted out of a pool and carried in open air. 
     The inner shell  14  comprises an inner surface  97  and an outer surface  96 , and is the innermost shell of the body portion of the tubular body  60 . As a result, the inner surface  97  of the inner shell  14  forms the walls of the cavity  31  in which the spent nuclear fuel assemblies are placed and held for storage and/or transport. The inner shell  14  forms the initial boundary separating the spent nuclear fuel from the external environment. Accordingly, the inner shell  14  is preferably made of a high strength steel such as, for example, SA 203 E and is preferably sufficiently thick to account for the known degradations in molecular structure from long-term exposure to neutron and gamma rays. Steel is also a preferred material to use for the inner shell  14  due to its good thermal conductivity, which is important for providing a path for the decay heat generated by the contained radioactive material to pass through (and ultimately be dissipated into the environment). Finally, steel is also preferred due to its high melting point, which ensures that the integrity of the inner shell  14  is not compromised even at high temperatures. 
     The intermediate shell  15  comprises an inner surface  95  and an outer surface  94 , and is concentrically arranged to circumferentially surround an outer surface  96  of the inner shell  14 . In the preferred embodiment, the inner surface  95  of the intermediate shell  15  is concentric to and in contact with the outer surface  96  of the inner shell  14 . Therefore, the intermediate shell  15  is both concentric to and coaxial with the inner shell  14 . In the preferred embodiment, the intermediate shell  15  is formed of lead, however in alternate embodiments the intermediate shell  15  may be formed of steel or another good conductor of heat that also acts a gamma radiation absorber. 
     The outer shell  16  comprises an inner surface  93  and an outer surface  92 , and circumferentially surrounds an outer surface  94  of the intermediate shell  15 . The outer shell  16  is both concentric to and coaxial with the inner shell  14  and the intermediate shell  15 . The outer surface  92  of the outer shell  16  comprises the outer surface of the tubular body  60  of the shielded canister  100 . In the exemplified embodiment, the outer shell  16  is formed of steel, however in alternate embodiments the outer shell  16  may be formed of lead, another metal or a metal alloy. 
     In the exemplified embodiment, the outside surface  92  of the outer shell  16  of the tubular body  60  comprises extended surfaces  19  that extend radially from the tubular body  60  to enhance the heat dissipation to the shielded canister  100 . Preferably, the extended surfaces  19  are fins or dimples. The extended surfaces  19  minimize the heat-up rate of water within the shielded canister  100  through the use of convection. 
     The term “concentric” as used herein is not limited to an arrangement wherein the shells  14 ,  15 ,  16  are coaxial, but includes arrangements wherein the shells  14 ,  15 ,  16  may be offset. Furthermore, the term “annular,” as used herein, is not limited to a circular shape and does not require that the object or space have a constant width. For example, the inner shell  14  may have a circular transverse cross-section while the intermediate shell  15  may have a rectangular transverse cross-section. 
     Any of the shells may be formed by bending a rectangular plate into a cylinder or other shape and welding together the two meeting ends, welding a series of elongated rectangular plates together end-to-end, or by any other method known to those skilled in the art to produce the desired shape. A machining process may also be used. 
     Referring still to  FIG. 2 , a longitudinal cross-sectional view of the shielded canister  100  in partial cut-away along line A-A of  FIG. 1  is illustrated according to one embodiment of the present invention. From this perspective, the outer shell  16 , the intermediate shell  15  and the inner shell  14  are seen oriented along axis A-A and extending from the floor/base plate  12  to the upper structural ring  11  of the shielded canister  100 . It is preferred that the upper structural ring  11  and the floor/base plate  12  are made of carbon steel and are each welded to the respective ends of the inner shell  14  and outer shell  16 . As discussed in more detail below, once the cavity  31  of the inner shell  14  is loaded from the top, the removable lid  20  may be installed over to seal the opening of the structural ring  11 . 
     In the preferred embodiment, inside the cavity  31  is an upright fuel basket  30  with multiple fuel storage cavities  36  for receiving spent nuclear fuel assemblies (not shown). An example of a basket assembly is disclosed in U.S. Pat. No. 5,898,747 (Singh), issued Apr. 27, 1999, the entirety of which is hereby incorporated by reference. The invention, however, is not limited to the use of any specific canister structure. 
     The basket  30  is formed from a honeycomb gridwork  32  of plates  33   a - 33   c  and  34   a - 34   c  having neutron absorber material  35  positioned in areas which form walls of storage cells formed by the honeycomb structure. The honeycomb structure of fuel basket  30  results in vertical cells  36  (also called “fuel cavities” or “storage cells”), each one of which is designed to hold one spent nuclear fuel assembly. The storage cells  36  are preferably created by arranging a gridwork of plates in a rectilinear arrangement. The basket  30  is formed from an array of plates  33   a - 33   c  and  34   a - 34   c  welded to each other, such as to form a honeycomb structure. Of course, slotted connection can be sued. In the exemplified embodiment, the height of the fuel basket  30  is less than the height of the cavity  31  of the shielded canister  100  so as to allow room for a top plenum of water and/or vapor as discussed below. In alternate embodiments, the number of plates or storage cells may differ, and/or the basket  30  may employ sleeves or boxes within the storage cells. 
     The fuel basket  30  is configured to facilitate a natural thermosiphon flow of fluid within the hermetically sealed cavity  31  when the spent nuclear fuel assemblies are loaded within the cavity  31  and giving off a heat load. When SNF is loaded into storage cells  36  of the shielded canister  100 , the heat emanating from the SNF conducts into the fluid of the cavity  31  that is contact with the SNF. The warmed fluid (which as discussed below is preferably pool water) rises within the cells  36  and into a top plenum of fluid. As the heated fluid comes into contact with the walls of the cavity  31 , the heat is conducted radially outward through the tubular body  60  and the lid  20 . As a result of this cooling, the fluid adjacent the walls flows downward through downcomer passageways (which can be empty fuel cells). This downardly flowing cooled fluid flows to the bottom of the cavity  31 . A plurality of openings  185  are provided at the bottom end of the basket  30 . The openings  185  form passageways between the all of the cells  36  and the downcomer passageways, thereby creating a bottom plenum. Once the cooled fluid flows into the bottom plenum, it is redistributed back in the cells  36  loaded with the SNF where it is heated and rises, thereby completing a thermosiphon cycle. 
     In one embodiment, the top plenum may be formed by that volume of fluid located above a top edge of the basket  30  and below a fluid level. The existence of the top plenum allows for radially outward fluid flow. The bottom plenum allows radially inward fluid flow. 
     Referring now to  FIG. 3 , an embodiment of the removable lid  20  of the shielded canister  100  of the present invention is illustrated. In the preferred embodiment, the removable lid  20  is a non-unitary structure relative to the tubular body  60 . The lid  20 , in the exemplified embodiment, is detachably coupled by bolts  50  to the upper structural ring  11  of the tubular body  60 . The removable lid  20  rests atop and is supported by the upper structural ring  11 , which rests atop and is secured to the top edges of the inner, intermediate, and outer shells  14 ,  15 ,  16  of the tubular body  60 . The removable lid  20  encloses the top of the cavity  31  and provides the necessary radiation shielding so that radiation can not escape from the top of the cavity  31  when the canister is loaded with HLW stored therein. The removable lid  20  is specially designed to hermetically seal the open top end of the shielded canister  100  when properly installed. In one embodiment, the lid  20  is formed as a multi-layered construct of lead and steel. It should be noted that in alternate embodiments, the removable lid  20  may be detachably coupled to the shielded canister  100  through other means. 
     The components of the removable lid  20  according to one embodiment of the present invention will be discussed. In the exemplified embodiment, the removable lid  20  comprises a body portion  21  and a dome portion  24 . The body portion  21  of the removable lid  20  comprises a flange portion  22  and a plug portion  23 . Further, the body portion  21  has a top surface  91  and a bottom surface  90 . When the removable lid  20  is positioned atop the tubular body  60 , the bottom surface  90  of the body portion  21  of the lid  20  forms a roof of the cavity  31 . The plug portion  22  extends downward from the bottom of the flange portion  22 . The flange portion  22  surrounds the plug portion  23 , extending therefrom in a radial direction. The dome portion  24  is attached to the top surface  91  of the body portion  21 . The dome portion  24  extends upward from the top surface  91  of the body portion  21  in the shape of a dome and forms an internal chamber  25  therein. In alternate embodiments, the dome portion  24  can be any shape or size that is desired. In one embodiment, the body portion  21  of the removable lid  20  can be formed of both neutron and/or gamma radiation absorbing materials, including neutron absorbing plates such as lead. 
     The cooperational relationship of the elements of the removable lid  20  and the elements of the tubular body  60  will now be described. When the removable lid  20  is properly positioned atop the tubular body  60  of the shielded canister  100 , the plug portion  23  of the removable lid  20  extends into the cavity  31  until the flange portion  22  of the removable lid  20  contacts and rests atop the upper structural ring  11 . The flange portion  22  eliminates the danger of the removable lid  20  falling into the cavity  31 . 
     When the removable lid  20  is positioned atop the upper structural ring  11 , one or more gasket seals  18  are compressed between the flange portion  22  of the removable lid  20  and the top end  98  of the tubular body  60 , thereby forming a hermetically sealed interface. The gasket seal  18  provides a positive seal at the lid/body interface, hermetically sealing the shielded canister  100 . Once the removable lid  20  is positioned atop the upper structural ring  11  of the shielded canister  100 , the removable lid  20  is secured to the upper structural ring  11  with bolts  50 . In alternate embodiments, the removable lid  20  may be secured to the upper structural ring  11  through other connecting means. 
     The dome portion  24  comprises an inner surface  26 , an outer surface  27 , and a resulting dome chamber  25 . The bottom portion of the outer surface  27  of the dome portion  24  is secured to the top surface  91  of the body portion  21  of the removable lid  20 . The dome chamber  25  is formed by a cavity created by the inner surface  26  of the dome portion  24 . As discussed in more detail below, the dome chamber  25  may be formed having a vacuum pressure therein to provide additional volume to relieve excess pressure that may accumulate in the cavity  31  of the shielded canister  100  when the removable lid  20  is hermetically sealed atop the shielded canister  100 . 
     The removable lid  20  further comprises a first passageway  70  that extends from the dome chamber  25  through the flange portion  22  and plug portion  23 , and into the cavity  31  of the shielded canister  100 . The first passageway  70  extends from an opening  71  in the bottom of the dome chamber  25  to an opening  72  in the bottom surface  90  of the body portion  21  of the lid  20 , thereby forming a passageway from the cavity  31  to the chamber  25 . A pressure relief device  73  is operably coupled to the opening  72  in the bottom surface  90  of the body portion  21  of the removable lid  20  and hermetically seals the first passageway  70 . The pressure relief device  73  extends from at least part way in the first passageway  70  into the cavity  31  of the shielded canister  100  in the exemplified embodiment. In the preferred embodiment, the pressure relief device  73  is made of a ductile and thermally conductive material, such as steel or lead, or a combination thereof. As discussed in more detail below, the pressure relief device  73  is configure to open the first passageway  70  when the pressure inside the cavity  31  exceeds a threshold value, and thereby reduce the pressure inside the cavity  31  of the shielded canister  100  by opening the first passageway  70  between the cavity  31  and the dome chamber  25  of the removable lid  20 . The pressure relief device  73  may be a pressure relief valve, a rupture disk, or other devices as are know in the art. In alternate embodiments, the first passageway  70  may be a tortuous passageway so no direct line of sight exists between the cavity  31  of the shielded canister  100  and the chamber of the dome portion  23  of the removable lid  20 . 
     The removable lid  20  further comprises a valve port  75  for adjusting the water level within the cavity  31  of the shielded canister  100 . The valve port  75  comprises a port that extends through a second passageway (not shown) and is operably coupled to a valve on the outside of the shielded canister  100 . In the preferred embodiment, a second passageway extends from a second opening (not shown) in the bottom of the plug portion  23  of the removable lid  20  to an opening (not shown) in the top surface  91  of the body portion  21  of the removable lid  20 . The valve port  75  preferably extends from inside the cavity  31  of the shielded canister  100 , through the second passageway, and out of the opening in the top surface  91  of the body portion  21 . Preferably, the valve port can be adjusted between a closed position where the cavity  31  remains hermetically sealed, and an open position where the hermetic seal is alleviated so that the valve port can be sued to introduce or expel a fluid into or out of the cavity  31 . In the preferred embodiment, the valve port  75  extends into the cavity  31  to a point above the top of the basket assembly  30  located within the cavity  31 . Thus, the valve port is capable of reducing the water level within the cavity  31  of the shielded canister  100  to a level slightly above the top of the basket assembly by pumping out the excess water. Keeping the water level above the basket assembly  30  allows for proper thermosiphon flow of the water to aid in cooling the spent nuclear fuel assemblies residing within the basket assembly  30 . Further, in alternate embodiments, the valve port,  75  is configured to backfill the cavity  31  of the shielded canister  100  with a gas, preferably steam, to alter the internal pressure of the shielded canister  100  and to maintain a space as the volume of liquid water may expand due to the thermal heating. 
     Next, the preferred method of the present invention will be described in detail. Many nuclear plant sites have more than one reactor unit and more than one fuel pool. At such plants, it may be necessary to have the means to transfer the spent nuclear fuel assemblies from one pool to another by moving them out of the fuel building of one unit and into another through the recipient building&#39;s truck bay. One method of carrying out such a transfer in accordance with the present invention will be described below. 
     At an initial step, the tubular body  60  (with the lid  20  removed) is lifted with a first crane in the first building and is lowered and submerged in a first storage body of water (pool) at a nuclear site. Specifically, the first storage pool may have a crane with a limited rated lilting capacity, and particularly with a lifting capacity that is not rated for the removal of a fully loaded transfer cask from a submerged state within the first pool. Once the tubular body  60  is submerged in the first pool, the pool water automatically fills the cavity  31 . Spent nuclear fuel assemblies are then loaded into the basket  30  of the tubular body  60  while the tubular body  60  and spent nuclear fuel assemblies remain submerged in the first fuel pool. Specifically, the spent nuclear fuel assemblies are loaded into the cells  36  of the cavity  31  via the open top end  98  of the tubular body  60 . Thereafter, the removable lid  20  is coupled atop the tubular body  60  to enclose the open top end  98  while the shielded canister  100  remains submerged underwater in the pool. Next, the first crane lifts the shielded canister  100  from the pool and into a staging area. Once at the staging area, the removable lid  20  is secured to the tubular body  60 , thereby forming a pressure vessel in which the spent nuclear fuel assemblies and the water are contained assuming the valve port  75  is in the closed position). 
     In alternate embodiments, an intermediate lid may be placed atop the shielded canister  100  prior to removing it from the pool. In such embodiments, once outside of the pool and in the staging area, the intermediate lid is removed and the removable lid  20  is hermetically sealed to the shielded canister  100 . After the shielded canister  100 , with removable lid  20  hermetically attached, is removed from the pool, the shielded canister  100  is transferred to a second pool. 
     In one embodiment, once removed from the first pool, a portion of the water from the cavity  31  of the shielded canister  100  is removed using the valve port  75  so that the water level within the cavity  31  is above the top of the fuel basket  30  (and HLW) and a space exists between the water level and the bottom surface  90  of the lid. Controlling the water level so that it is above the top of the baskets ensures proper thermosiphon flow to aid in cooling the spent nuclear fuel assemblies while they are inside the hermitically sealed shielded canister  100 . It should be noted that the removal of the water from the cavity  31  may be done before or after the removable lid  20  is hermetically sealed atop the shielded canister  100 . After the water level within the cavity  31  is adjusted to be at the desired level, the valve of the valve port  75  is closed, thereby hermetically sealing the shielded canister  100 . 
     In one embodiment, the space formed above the water level and the bottom surface of the lid  20  may be backfilled with a gas via the valve port  75 , preferably steam, to alter the internal pressure of the shielded canister  100 . After backfilling the cavity with a gas, the valve of the valve port  75  is closed and the cavity  31  is hermetically sealed. 
     During transfer from the first pool to the second pool, the shielded canister  100 , and specifically the removable lid  20  provide for the safe transfer of the spent nuclear fuel assemblies. Preferably, the water level remains above the top of the fuel baskets  30  during transfer to the second pool. Due to the limited lifting capacity of the first crane, a transfer cask (weighing in the upwards of 100-125 tons) could not be used. To counteract the reduced material and weight of the shielded canister  100 , and yet still provide sufficient neutron and gamma radiation protection, the shielded canister  100  contains water within cavity  31  during the transfer from the first pool to the second pool. In order to ensure that the water within the cavity  31  does not reach a boiling point, the shielded canister  100  is hermetically sealed with the removable lid  20  that comprises the pressure relief device  73 . The removable lid  20 , the pressure relief device  73  and the dome chamber  25  aid in preventing the water within cavity  31  from reaching a boiling point. To do so, the pressure relief device  73  is configured to automatically open the passageway  70  between the cavity  31  and the dome chamber  25  upon the equilibrium pressure within the cavity  31  reaching a threshold potential. If the equilibrium pressure within the cavity  31  does reach and/or exceed the threshold potential, the pressure relief device  73  automatically opens the passageway  70  thereby increasing the overall volume and reducing the resulting pressure, while maintaining the hermetic seal within the shielded canister  100 . 
     Upon arriving at the second pool, the shielded canister  100  is lowered into the pool through the use of a second crane. Preferably, the second crane has a rated lifting capacity that exceeds the weight of a fully loaded transfer cask. Once in the second pool, the spent nuclear fuel assemblies located within the basket of the shielded canister  100  are removed and preferably placed into a second canister that is located within a transfer cask. Thereafter, the shielded canister  100  is removed from the pool. 
     While the invention has been described and illustrated in sufficient detail that those skilled in this art can readily make and use it, various alternatives, modifications, and improvements should become readily apparent without departing from the spirit and scope of the invention.