Abstract:
A system, method and apparatus for providing radiation shielding to a ventilated cask for holding high level radioactive materials. In one aspect, the tubular shell is positioned to circumferentially surround the cask so that an annular gap exists between the tubular shell and a sidewall of the cask. The tubular shell includes a first air flow inlet and a second air flow inlet. An air flow barrier is placed within the annular gap, separating the annular gap into a first chamber and a second chamber. A first air flow into the first air flow inlet passes through the first chamber and into the inlet vent of the cask, a second air flow into the second air flow inlet passes through the second chamber and to an opening at the top end of the tubular shell, and the air flow barrier prohibits cross-flow of air between the first and second chambers.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     Priority is claimed as a continuation application to U.S. patent application Ser. No. 12/940,804, filed Nov. 5, 2010 and now U.S. Pat. No. 8,995,604, which claims priority to U.S. Provisional Application Ser. No. 61/258,240, filed Nov. 5, 2009. The disclosures of the aforementioned priority documents are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The present invention relates generally to the field of containing high level radioactive materials, and specifically to a system, apparatus and method that provides an ancillary for providing additional radiation shielding to a cask containing high level radioactive waste. 
     BACKGROUND 
     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. The canister alone, however, is not sufficient to provide adequate gamma or neutron radiation shielding. Therefore, apparatus that provide additional radiation shielding are required during transport, preparation and subsequent dry storage. 
     The additional shielding is achieved by placing the canisters within large cylindrical containers called casks. Casks are typically designed to shield the environment from the dangerous radiation in two ways. First, shielding of gamma radiation requires large amounts of mass. Gamma rays are best absorbed by materials with a high atomic number and a high density, such as concrete, lead, and steel. The greater the density and thickness of the blocking material, the better the absorption/shielding of the gamma radiation. Second, shielding of neutron radiation requires a large mass of hydrogen-rich material. One such material is water, which can be further combined with boron for a more efficient absorption of neutron radiation. 
     There are generally two types of casks, transfer casks and storage casks. Transfer casks are used to transport spent nuclear fuel within the nuclear facility. Storage casks are used for the long term dry state storage. Guided by the shielding principles discussed above, storage casks are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because storage casks are not typically moved, the primary focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of spent nuclear fuel. 
     One type of known storage cask is a ventilated vertical module (“VVM”). A VVM is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel. VVMs stand above ground and are typically cylindrical in shape and extremely heavy, weighing over 150 tons and often having a height greater than 16 feet. VVMs typically have a flat bottom, a cylindrical body having a cavity to receive a canister of spent nuclear fuel, and a removable top lid. 
     In using a VVM to store spent nuclear fuel, a container loaded with spent nuclear fuel, such as a multi-purpose canister (“MPC”), is placed in the cavity of the cylindrical body of the VVM. Because the spent nuclear fuel is still producing a considerable amount of heat when it is placed in the VVM for storage, it is necessary that this heat energy have a means to escape from the VVM cavity. This heat energy is removed from the outside surface of the MPC by ventilating the VVM cavity. In ventilating the VVM cavity, cool air enters the VVM chamber through bottom ventilation ducts, flows upward past the loaded MPC, and exits the VVM at an elevated temperature through top ventilation ducts. The bottom and top ventilation ducts of existing VVMs are located circumferentially near the bottom and top of the VVM&#39;s cylindrical body respectively. 
     While it is necessary that the VVM cavity be vented so that heat can escape from the MPC, it is also imperative that the VVM provide adequate radiation shielding and that the spent nuclear fuel not be directly exposed to the external environment. The inlet duct located near the bottom of the VVM is a particularly vulnerable source of radiation exposure to security and surveillance personnel who, in order to monitor the loaded VVMs, must place themselves in close vicinity of the ducts for short durations. 
     Existing VVMs are made of a dual metal shell structure with shielding concrete inside. The density of concrete can be increased in certain applications to the extent necessary to increase the dose attenuation. Increasing the density of concrete is an effective way to reduce dose. Calculations in specific cases show that increasing the density of concrete from 150 lb/cubic feet to 200 lb/cubic feet reduces the accreted dose from a VVM by a factor as high as 10. However, circumstances arise where it is desired to drive down the local area dose rate from one or more VVMs at an Independent Spent Fuel Storage Installation (ISFSI) to a value which is even smaller than that obtainable by using locally available high density concrete. Such a situation may arise, for example, if local or state authorities impose even more stringent dose rate limits than those specified in 10CFR72, or if there is an inhabited space (say, an office building) close to where the loaded casks are arrayed. 
     SUMMARY 
     The present invention is directed to an ancillary prismatic shell that can be positioned to circumscribe a vertical ventilated cask loaded with high level radioactive waste to reduce the radiation dose emitted to the environment, and a system incorporating the cask and the apparatus. 
     In one embodiment, the invention can be a system for containing high level radioactive materials comprising: a cask extending along a longitudinal axis and having an internal cavity for holding high level radioactive materials, the cask comprising at least one inlet vent at a bottom end of the cask for allowing cool air to enter the internal cavity and at least one outlet vent at a top end of the cask for allowing heated air to exit the internal cavity; a tubular shell extending from a bottom end to a top end, the tubular shell positioned to circumferentially surround the cask in a spaced apart manner so that an annular gap exists between the tubular shell and a sidewall of the cask, the tubular shell comprising at least one primary aperture forming a passageway through the tubular shell and at least one secondary aperture forming a passageway through the tubular shell; and an air flow barrier extending between the tubular shell and the sidewall of the cask that separates the annular gap into: (1) a first chamber that forms a passageway between the primary aperture and the inlet vent of the cask; and (2) a second chamber that forms a passageway between the secondary aperture and an opening at the top end of the tubular shell, wherein cross-flow of air between the first and second chambers of the annular gap is prohibited by the air flow barrier. 
     In another embodiment, the invention can be a system for containing high level radioactive materials comprising: a cask extending along a longitudinal axis and having an internal cavity for holding high level radioactive materials, the cask comprising a plurality of inlet vents at a bottom end of the cask for allowing cool air to enter the internal cavity and a plurality of outlet vents at a top end of the cask for allowing heated air to exit the internal cavity; a tubular shell extending from a bottom end to a top end, the tubular shell positioned to circumferentially surround the cask in a spaced apart manner so that an annular gap exists between the tubular shell and a sidewall of the cask, the tubular shell comprising a plurality of primary apertures forming passageways through the tubular shell and a plurality of secondary apertures forming passageways through the tubular shell; and a flexible annular seal coupled to the tubular shell that separates the annular gap into: (1) an upper chamber that forms a passageway between the primary aperture and the inlet vent of the cask; and (2) a second chamber that forms a passageway between the secondary aperture and an opening at the top end of the tubular shell, wherein cross-flow of air between the first and second chambers of the annular gap is prohibited by the flexible annular seal. 
     In a further embodiment, the invention can be an apparatus for providing additional radiation shielding to a cask holding high level radioactive materials comprising: a tubular shell extending from an open bottom end to an open top end, the tubular shell having an inner surface that forms a cavity about a longitudinal axis; a plurality of primary apertures forming passageways through the tubular shell and circumferentially arranged in a spaced-apart manner about the tubular shell; a plurality of secondary apertures forming passageways through the tubular shell and circumferentially arranged in a spaced-apart manner about the tubular shell; an annular seal coupled to the tubular shell and extending from the inner surface of the tubular shell; and wherein the secondary apertures are located at an axial height above the annular seal and the primary apertures are located at an axial height below the annular seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top perspective view of a system for containing high level radioactive waste according to one embodiment of the present invention. 
         FIG. 2  is a bottom perspective view of the system of  FIG. 1 . 
         FIG. 3  is a top perspective view of the system of  FIG. 1  having a section of the ancillary shield cut-away. 
         FIG. 4  is a perspective view of the system of  FIG. 1  wherein shield is being assembled by stacking a plurality of tube segments. 
         FIG. 5  is a perspective view of the system of  FIG. 1  wherein all of the tube segments have been arranged in a stacked assembly that circumscribes the cask, wherein a section of tube segments are cut-away. 
         FIG. 6  is close-up view of area VI-VI of  FIG. 5 . 
         FIG. 7  is a longitudinal cross-sectional view of the system of  FIG. 1  taken along the longitudinal axis A-A, wherein the natural convective cooling of the system is exemplified. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1-3  and  7  concurrently, a system  1000  for containing high level radioactive waste according to one embodiment of the present invention is illustrated. The exemplified embodiment of the system  1000  generally comprises three major components, a canister  100  that forms a fluidic containment boundary about the high level radioactive materials, a ventilated vertical cask  200  and an ancillary shield  300 . In certain embodiments, the invention may be directed solely to the shield  300 . In other embodiments, the invention may be directed to the combination of the shield  300  and the ventilated vertical cask  200 . In still other embodiments, the invention may be directed to the combination of the canister  100 , the ventilated vertical cask  200  and the shield  300 . 
     The canister  100  can be any type of container that forms a fluidic containment boundary about the high level radioactive materials disposed therein and can conduct heat emanating from the high level radioactive materials outwardly through the canister  100 . In one embodiment, the canister  100  is engineered for the dry processing of spent nuclear fuel. Suitable canisters can include multi-purpose canisters (“MPCs”) and thermally conductive casks that are hermetically sealed for the dry storage of high level wastes, such as spent nuclear fuel. Typically, such canisters comprise a honeycomb grid-work/basket, or other structure, built directly therein to accommodate a plurality of spent fuel rods in spaced relation. An example of an MPC that is particularly suitable for use in the present invention is disclosed in U.S. Pat. No. 5,898,747 to Krishna Singh, issued Apr. 27, 1999, the entirety of which is hereby incorporated by reference. Of course, the invention is not so limited in all embodiments. 
     When the canister  100  is loaded with high level radioactive materials, the canister  100  is housed within an internal cavity  201  of the cask  200 . In the exemplified embodiment, the cask  200  is vertically oriented and extends from a bottom end  202  to a top end  203  along a longitudinal axis A-A. The cask  200  generally comprises a cylindrical body  204  and a removable lid  205 . An inner surface  206  of the cylindrical body  204  forms the internal cavity  201  which has an open top end and a closed bottom end. 
     When the canister  100  is positioned within the cavity  201  of the cask  200 , the lid  205  is secured to the top end of the cylindrical body  204  to substantially close the open top end of the internal cavity  201 . The transverse cross-section of the internal cavity  201  is designed so that an annular gap  207  exists between the inner surface  206  of the cylindrical body  204  and the outer surface  101  of the canister  100 . In the exemplified embodiment, the transverse cross-section of the internal cavity  201  can accommodate no more than one canister  100 . However, in alternative embodiments, the internal cavity  201  may be designed to accommodate more than one canister in a side-by-side and/or stacked arrangement. 
     The annular gap  207  circumscribes the outer surface  101  of the canister and extends along the entire axial length of the canister  100 . The annular gap  207  forms an axially extending passageway between a bottom plenum  208  formed between a bottom surface of the canister  100  and a floor of the internal cavity  201  and a top plenum  209  formed between a top surface of the canister  100  and a bottom surface of the lid  205 . As discussed in greater detail below, the annular gap  207  allows cool that enters the bottom plenum  208  via the inlet ducts  210  to flow upward along the outer surface  101  of the canister  100  and into the top plenum  209  where it can exit the cask  200  via the outlet ducts  211  as warmed air. 
     Referring now to  FIGS. 2 ,  3 ,  6  and  7  concurrently, the cask  200  further comprises a plurality of air inlet ducts  210  at the bottom end  202  of the cask  200 . The plurality of inlet ducts  210  are circumferentially arranged in a spaced-apart manner about the cask  200 . Each of the air inlet ducts  210  extend from an inlet opening  212  in the sidewall  213  of the cask  200  to the bottom plenum  208  of the internal cavity  201 , thereby forming an air-flow passageway between a position external of the cask  200  and a bottom portion of the internal cavity  201 . As can be seen, the canister  100  is supported within the cavity  201  so that a bottom surface of the canister  100  is at an axial height above a top of the inlet vents  210  to eliminate radial shine through the inlet ducts  210 . In the exemplified embodiment, the cask  200  comprises a total of four inlet vents  210  arranged circumferentially about the cask  200  and spaced apart 90 degrees from each other. Of course, in other embodiments, more or less of the inlet vents  210  can be included in the cask  200  as desired. 
     The cask  200  further comprises a plurality of outlet ducts  211  at the top end  203  of the cask  200 . The plurality of outlet ducts  211  are circumferentially arranged in a spaced-apart manner about the cask  200 . Each of the air outlet ducts  210  extend from the top plenum  209  of the internal cavity  201  to an outlet opening  214  in the sidewall  213  of the cask  200 , thereby forming an air-flow passageway between a position external of the cask  200  and a top portion of the internal cavity  201 . In the exemplified embodiment, the outlet vents  211  are located within the lid  205  of the cask  200 . However, in other embodiments, the outlet vents  211  can be located within the cylindrical body  204  of the cask  200 . In the exemplified embodiment, the cask  200  comprises a total of four outlet vents  211  arranged circumferentially about the cask  200  and spaced apart 90 degrees from each other. Of course, in other embodiments, more or less of the outlet vents  211  can be included in the cask  200  as desired. 
     Both the lid  205  and the cylindrical body  204  of the cask  200  are constructed of material(s) that provide both gamma and neutron radiation shielding and are designed to provide the majority of the required radiation shielding (both gamma and neutron). In the exemplified embodiment, the lid  205  and the cylindrical body  204  of the cask  200  are constructed of a combination of carbon steel plates, carbon steel shells and concrete. The main structural function of the cask  200  is provided by its carbon steel components while the main radiation shielding function is provided by the annular plain concrete mass  215  and the disk plain concrete mass  216 . The annular plain concrete mass  215  is enclosed by concentrically arranged cylindrical steel shells  217 ,  218 , the thick steel baseplate  219 , and the top steel annular plate  220 . 
     The plain concrete masses  215 ,  216  are specified to provide the necessary shielding properties (dry density) and compressive strength for the cask  200 . The principal function of the concrete masses  215 ,  216  is to provide shielding against gamma and neutron radiation. However, the concrete masses  215 ,  216  also help enhance the performance of the cask  200  in other respects as well. For example, the massive bulk of the concrete mass  215  imparts a large thermal inertia to the cask  200 , allowing it to moderate the rise in temperature of the cask  200  under hypothetical conditions when all ventilation passages  210 ,  211  are assumed to be blocked. The case of a postulated fire accident at an ISFSI is another example where the high thermal inertia characteristics of the concrete mass  215  of the cask  200  controls the temperature of the canister  100 . Although the annular concrete mass  215  is not a structural member, it does act as an elastic/plastic filler of the inter-shell space. 
     One example of ventilated vertical cask  200  that can be used in the system  1000  is described above. However, it is to be understood that other ventilated vertical casks can be used in conjunction with the canister  100  and/or the shield  300 . For example, an additional example of a suitable cask can be found in U.S. Pat. No. 6,718,000 issued to Krishna Singh, on Apr. 6, 2004, the entirety of which is hereby incorporated by reference. Still another example of a suitable cask can be found in U.S. patent application Ser. No. 12/774,944, filed May 6, 2010, the entirety of which is hereby incorporated by reference. 
     Referring now to  FIGS. 1-3  and  5 - 7  concurrently, the exemplified embodiment of the ancillary shield  300  will be described in greater detail. The shield  300  is a sleeve-like structure that is designed to slidably fit over a ventilated vertical cask, such as the cask  200 , to provide additional radiation shielding and missile protection. The shield  300  is intended to be provided to circumscribe the cask  200  once it is at rest on a support surface, such as the ground. It is to be further understood that the shield  300 , in and of itself, is a novel device and can constitute an embodiment of the invention independent of the cask  200  and canister  100 . 
     The shield  300  is a free-standing structure that circumscribes the cask  200  and provides shielding blockage over the entire height of the cask  200 , as necessary depending on the specific applications. The shield  300  is effective in blocking radiation from the inlet and outlet ducts  210 ,  211  of the cask  200  (locations of relatively high fluence), without impeding air ventilation entering, exiting or inside the cask ( FIG. 7 ). In order for the shield  300  to get down to very, very low dose rates, the shield  300  may be formed of material(s) so as to impart both neutron and gamma blockage capability. In certain embodiments, the shield  300  may be formed of steel, lead, concrete and/or an appropriate neutron absorber resin (such as Holtite), depending on the allowable thickness and type of radiation to be blocked (steel and concrete for both gamma and neuron, resin for neurons, and lead for gamma). 
     The shield  300  generally comprises a tubular shell  301  and an annular top plate  302  coupled to a top end  303  of the tubular shell  301 . The shield  300  (and the tubular shell  301 ) extends along the longitudinal axis A-A from a bottom end  304  to a top end  303 . The bottom end  304  of the shield  300  is open, comprising a bottom opening  305  through which the cask  200  can be inserted into an internal cavity  306  of the shield  300 . The top end  303  of the shield  300  is also open, comprising a top opening  307 , which is also the central opening of the annular ring plate  302 . 
     The shield  300  has a vertical height that is greater than the vertical height of the cask  200 . More specifically, the shield  300  has a first axial height, measured from the bottom end  304  of the shield  300  to the top end  303  of the shield  300  along a line parallel to the longitudinal axis A-A. Similarly, the cask  200  has a second axial height, measured from the bottom end  202  of the cask  200  to the top end  203  of the cask  200  along a line parallel to the longitudinal axis A-A. The first height is greater than the second height. 
     The annular ring plate  302  is coupled to the top end  303  of the shield  300  and extends radially inward therefrom, terminating in an inner edge  308  that defines the central opening  307 . The annular ring plate  302  extends radially inward from the tubular shell  301  beyond the sidewall  213  of the cask  200 . As such, the central opening  307  has a transverse area that is less than the transverse cross-sectional area of the cask  200  in the exemplified embodiment. The annular ring plate  302  is axially spaced a distance from a top surface  220  of the lid  205  of the cask  200  so that an air flow passageway exists between the central opening  307  and the annular space  310  (discussed below). The annular ring plate  302  blocks off skyshine radiation emanating at an oblique angle. 
     When the shield  300  is positioned, as illustrated in  FIGS. 1-3  and  5 - 7 , the tubular shell  301  circumferentially surrounds the cask  200 . Because the inner diameter of the tubular shell  301  is greater than the outer diameter of the cask  200 , an annular gap  310  is formed between the inner surface  311  of the tubular shell  301  and the sidewall  213  of the cask. The annular gap  310  extends along the entire axial height of the cask  301  (i.e., from the bottom end  202  of the cask  200  to the top end  203  of the cask  200 ). The annular gap  310  also circumscribes the cask  200 . 
     The tubular shell  301  further comprises a plurality of the primary apertures  312  at the bottom end  304  of the shield  300 . The primary apertures  312  form radial passageways through the tubular shell  301 . The primary apertures  312  are circumferentially arranged in a spaced-apart manner about the tubular shell  301 . The circumferential location of the primary apertures  312  is selected so that the primary apertures  312  are radially offset from the inlet openings  212  of the inlet vents  210  of the cask  200 . As mentioned above, the inlet openings  212  of the inlet vents  210  present a particularly vulnerable source of radiation exposure. Thus, by radially offsetting the primary apertures  312  from the inlet openings  212  of the inlet ducts  210  of the cask  200 , portions  301 A of the structure of the tubular shell  301  are radially aligned with the inlet openings  212  of the inlet ducts  210  of the cask  200 , thereby minimizing environmental dose. 
     In the exemplified embodiment, the primary apertures  312  are notches formed in the bottom edge of the tubular shell  301 . However, the invention is not so limited and in other embodiments, the primary apertures  312  may be formed as prismatic openings. Furthermore, in the exemplified embodiment, the shield  300  comprises a total of four primary apertures  312  arranged circumferentially about the tubular shell  301  and spaced apart 90 degrees from each other. Of course, in other embodiments, more or less of the primary apertures  312  can be included in the shield  300  as desired. 
     The tubular shell  301  also comprises a plurality of the secondary apertures  313  at or near the bottom end  304  of the shield  300 . The secondary apertures  313  form radial passageways through the tubular shell  301 . The secondary apertures  313  are circumferentially arranged in a spaced-apart manner about the tubular shell  301 . In the exemplified embodiment, the secondary apertures  313  are narrow elongated slits. However, the invention is not so limited and in other embodiments the secondary apertures  313  may take on other shapes. 
     In the exemplified embodiment, the secondary apertures  313  are located at first axial height from the bottom edge of the tubular shell  301  while the primary apertures  312  are located at a second height from the bottom edge of the tubular shell  301 , wherein the second height is different than the first height. In the specific embodiment exemplified, the first axial height is greater than the second axial height. Of course, the invention will not be so limited in all embodiments. 
     The system  1000  further comprises an air flow barrier  314  extending between the tubular shell  301  and the sidewall  213  of the cask  200 . The air flow barrier  314  separates the annular gap  310  into: (1) a first chamber  310 A that forms a passageway between the primary apertures  312  of the tubular shell  301  and the inlet vents  310  of the cask; and (2) a second chamber  310 B that forms a passageway between the secondary apertures  313  of the tubular sell  301  and the opening  307  at the top end of the shield  300 . The air flow barrier  314  prohibits cross-flow of air between the first and second chambers  310 A,  310 B of the annular gap  310  so that two distinct cool air inlet flow pathways are formed in the system  1000 . The air flow barrier  314  can prohibit cross-flow of air between the first and second chambers  310 A,  310 B of the annular gap  310  by itself or in conjunction with a flange on the cask and/or tubular shell. 
     In the exemplified embodiment, the air flow barrier  314  is coupled to and extends radially inward from the inner surface  311  of the tubular shell  301  and comes into surface contact with the sidewall  213  of the cask  200 . More specifically, in the exemplified embodiment, the air flow barrier  314  is an annular plate. In such an embodiment, the first chamber  310 A is a lower chamber while the second chamber  310 B is an upper chamber. In this embodiment, the secondary apertures  313  are located at an axial height above the air flow barrier  314  and the primary apertures  312  are located at an axial height below the air flow barrier  314 . 
     In order to ensure a proper seal and/or reduce interference during installation onto a cask  200 , the air flow barrier  314  may be formed so as to be flexible in certain embodiments of the invention. For example, in some embodiments, the air flow barrier  314  may be formed of an elastomeric material, such as rubber or the like. In other embodiments, the flexibility of the air flow barrier  314  may be achieved by designing its thickness suitably thin so as to bend easily. Of course, the invention is not so limited and in other embodiments of the invention the air flow barrier  314  may be a rigid structure. 
     Referring now to  FIGS. 4-6  concurrently, it can be seen that the tubular shell  301  of the shield  300 , in the exemplified embodiment, is formed by a plurality of tube segments  317  arranged in a stacked-assembly so that a surface contact interface  320  is formed between a top edge  321  and a bottom edge  322  of adjacent tube segments  317 . 
     When the tubular shell  301  is formed by tube segments  317 , it may be preferred in certain instances to provide a collar  319  at each surface contact interface  320  that extends above and below the surface contact interfaces  320 . In certain embodiments, the collars  319  may be integrally formed with the tube segments  317  and protrude from the top and/or bottom edges  321 ,  322 . In other embodiments the collars  319  may be separate structures. The collars  319  prevent radiation escape through the surface contact interfaces  320 . The collars  319  also prohibits the adjacent tube segments  317 ,  318  from becoming axial misaligned while allowing the adjacent tube segments  317 ,  318  to be separated from one another through relative movement between the adjacent tube segments  317 ,  318  in the axial direction. However, all tube segments  317  may be mechanically interconnected in the axial direction, if required (not shown in the figure). 
     In the exemplified embodiment, the primary apertures  312  and the secondary apertures  313  are located in a bottom-most tube segment  318  of the stacked assembly. Further, the air flow barrier  314  is also coupled to the bottom-most tube segment  318  of the stacked assembly in the exemplified embodiment. Of course, the invention is not so limited in all embodiments. Moreover, in certain embodiments, the tubular shell  301  could be a single unitary structure. However, by forming the shield  300  from a plurality of short tube segments  317 , the shield  300  is installable without raising the cask  200  or the shield  300  to excessive heights (to protect against heavy load drop scenarios). 
     Further, each of the tube segments  317  comprise a plurality of spacers  315  circumferentially arranged in a spaced-apart manner about the tube segment  317  and protruding from an inner surface  311  of the tube segment  317 . The spacers  315  maintain the annular gap  310  by ensuring proper relative positioning between the cask  200  and the shield  300 . Each of the spacers  315  further comprise a means for facilitating engagement and lifting of the tube segment  317 . In the exemplified embodiment, the lifting means is a hole  316 . However, in other embodiment, the lifting mean can be a hook, a tang, a protuberance, a latch, a bracket, a clamp, a threaded surface, and/or combinations thereof Thus, the spacers  315  can also be though of as lifting lugs. 
     In addition to the shield  300  serving as a radiation mitigation device, the shield  300  also largely eliminates the insulation heat flux on the cask  200 , thus giving the system  1000  a heat load dividend of about 3 kilowatts. The shield  300 , if properly sized, can boost the heat rejection rate from the system  1000  even more. It is recognized that the secondary openings  313  are provided to allow air to enter the upper chamber  310 B of the annular gap  310 . The ventilation air will help cool the external surface of the cask  200 , thereby improving the heat rejection rate from the system  1000 . Thus, if the annular gap  310  is properly sized then the overall heat rejection from the system  1000  will actually be enhanced. The size (width) of the annular gap  310  must be set in the narrow range that maximizes the rate of air up flow. Maximizing the air ventilation rate will allow maximum thermal-hydraulic advantage to be derived from the shield  300 . The optimal gap size will depend on a number of parameters including the system heat load and cask height. Therefore it can not be set down herein a priori. However, calculations show that the optimal gap in a typical situation will lie in the range of 1 to 4 inches. The shield  300  also acts to provide a barrier against blockage of inlet vents  210  of the cask  200  by snow accumulation. Furthermore, because most of the environmental radiation dose emitted by a vertical ventilated cask, such as cask  200 , comes from the casks located at the periphery, the shield  300  may be used selectively on those casks  200  where dose emission needs to be blocked to meet a specified target dose limit in the vicinity of the ISFSI (such as the §72.104 &amp; 72.106 dose limits at the site boundary in the U.S.). 
     A method of containing high level radioactive materials according to one embodiment of the present invention using the system  1000  will be described. In an initial sequence, the canister  100  is transferred from a transfer cask (not illustrated) into the vertical ventilated cask  200 . An example of this transfer procedure is set forth in U.S. Pat. No. 6,625,246 to Krishna Singh, issued Sep. 23, 2003, the entirety of which is hereby incorporated by reference. 
     Once the canister  100  is in the cask  200  and the lid  205  is secured to the cylindrical body  204 , natural convective cooling (via the chimney-effect) of the canister  100  is achieved. Specifically, heat emanating form the canister  100  warms the air within the annular gap  207 . The warmed air within the annular gap  207  rises as result of being warmed, thereby gathering in the top plenum  209  and exiting the cask  200  via the outlet vents  211 . The outflow of the warmed air through the outlet vents  211  causes a siphon effect at the inlet openings  212  of the inlet vents  210 , thereby drawing cool air that is external to the cask  200  into the bottom plenum  208  via the inlet vents  210  where the cycle is repeated. 
     At this stage, the cask  200  is free standing and supported on a support surface, which can be the ground or engineered surface outside or within a building. The cask  200  is vertically oriented so that the longitudinal axis A-A extends substantially vertically. 
     Once the cask  200  is in position, the shield  300  is installed to circumscribe the cask  200  as described below. The bottom-most tube segment  318  is first positioned above the cask  200  using a crane connected to the spacers  315 . The bottom most tube segment  318  is then lowered so that the cask  200  extends through the bottom opening  305  of the shield  300 . The bottom-most tube segment  318  continues to be lowered until it rests atop the support surface as illustrated in  FIGS. 4 and 7 . The bottom-most tube segment  318  is rotationally arranged so that the primary apertures  312  are radially offset from the inlet openings  212  of the inlet vents  210  of the cask  200 . The additional tube segments  317  are then lowered in the same manner as described above for the bottom-most tube segment  318  and are stacked atop the bottom-most segment  318  (and previously positioned tube segments  317 ) to form a stacked assembly that extends the entire height of the cask  200 , thereby forming the tubular shell  301 . 
     Once the tubular shell  301  is complete, it circumscribed the cask  200  as described above. The annular ring plate  302  is then positioned atop the tubular shell  301  and couple thereto. If necessary the adjacent tube segments  317  and the annular ring plate  302  can be secured together via additional mechanical means if necessary to prohibit separation in the axial direction. For example, welding, fasteners, interference fits, or the like can incorporated as necessary. 
     At this point, the shield  300  is free standing structure supported on the support surface. The annular gap  310  between the shield  300  and the cask  200  is maintained as discussed above. When fully assembled, cool air enters the system  1000  as two separate and distinct fluid flow paths. The first flow path of cool air is siphoned into the system  1000  via the primary apertures  312 . After entering the primary apertures  312 , this cool air enters the first chamber  310 A where it is drawn into the bottom plenum  208  of the internal cavity  201  of the cask  200  via the inlet ducts  210 . This cool air then undergoes the flow discussed above for the cask  200 . The second flow path of cool air is siphoned into the system  1000  via the secondary apertures  313 . After entering the secondary apertures  313 , this cool air enters the second chamber  310 B where it is heated by heat emanating from the sidewall  213  of the cask  200 . As this cool air is warmed, it rises within the second chamber  310 B. 
     The warmed air of the first flow path that exits the outlet vents  311  of the cask converges with the warmed air of the second air flow path that rising within the second chamber  310 B. The converged warm air then exist the system  1000  via the top opening  307 . By converging the two air flow paths in the system  1000 , the volume of outgoing warmed air flow is increased, thereby contributing a greater siphon effect at the primary and secondary apertures  312 ,  313 . 
     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.