Abstract:
The shipping container includes an outer container body, an inner containment vessel housing a pair of superposed product pails and a self-extinguishing fire-retardant foam insulation layer between the outer container and inner vessel. The inner vessel includes a gusseted upper flange and a lid bolted to the flange with a sealing gasket therebetween. Upper and lower dunnages are provided at the upper and lower ends of the outer container. The upper dunnage includes ceramic fiberboard panels and foam material straddled by steel sheets and additional ceramic fiberboard panel to separate the lid of the vessel and the top of the container. The top is secured to the outer container body by bolts passing through the top and internally into tapped bolt brackets along an interior wall of the outer container body. A retaining ring secures the arcuate overlying rolled edge of the top about the beaded rim of the outer container body. A reinforced plate covers the seam of the outer container underneath the retaining ring bolt for additional container integrity. Vent holes with plastic plugs which melt in response to a predetermined temperature vent the container body to preclude pressure buildup within the container body by expanding gases.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a shipping container for shipping radioactive materials such as low enriched uranium powder, pellets and scrap material and particularly relates to a shipping container for radioactive materials having improved protection for the radioactive materials, as well as affording shipping economy. 
     Over the years, various types of shipping containers have been designed specifically for shipping radioactive materials, for example, low enrichment uranium oxide powder, pellets or radioactive scrap. One form of prior shipping containers essentially comprised 55-gallon drum-type vessels with inner steel compartments for two 5-gallon pails of radioactive material. Experience with these containers, and over time, have brought to the fore certain problems associated with their use. For example, such shipping containers were typically fabricated from standard 18-gauge carbon steel and the product pails were standard 24-gauge carbon steel. Not only were the containers and pails susceptible to rust and corrosion, but were also susceptible to denting and deformation due to routine industrial handling. Further, those early designs were not sized for optimal loading into currently commercial sea vans. This not only reduced the amount of floor space that could be efficiently used in sea vans but it also required additional bracing and supports to keep the containers from shifting during transport. 
     Additionally, new regulations, both in the United States and abroad, relating to the shipment of radioactive materials have required a higher degree of structural integrity, resistance to fire and watertightness for the containers than previously applied to older container designs. From the regulatory perspective, neither the inner container drum nor the radioactive material product pails should lose their integrity. That is, the sealed inner containment drum inside the outer drum should not allow contents to leak out or allow water to leak in. Similarly, the product pails should not allow the radioactive material contents to spill. A high degree of resistance to fire is also an important requirement. As a result of the restrictions on structural integrity, fire resistance and watertightness, the radioactive material-carrying capacity per drum of older-style containers has been significantly downgraded. 
     More particularly, in certain instances, older containers have been found to have significant amounts of rust, including rust on the internal surfaces, which are not capable of inspection without destroying the container. Further, the insulating material has proven to be difficult to fabricate and install, especially in a manner to ensure that the insulation is homogeneous without voids or holes in the region between the inner vessel and outer drum. Further, many of the fixturing devices such as bolts and other securing devices of the older-style containers have been fabricated from typical industrial-grade materials rather than nuclear-grade materials, as consistent with current regulatory requirements. The size and geometry of the older-style containers also is not optimal for loading into standardized sea vans. Such older-style containers achieve a utilization space of only about 38%, while the factor for the present invention is 57%. 
     The overall regulatory objective of safety for this type of container is principally to ensure avoidance of any possibility of a criticality accident during transportation of special nuclear materials. Both the foreign and domestic regulatory requirements specify that shipping containers for special nuclear materials must undergo a number of tests, such as drop, burn and water intrusion tests, the results of which must be taken into consideration in the safety analysis submitted in support of licensing. 
     A recent effort by the assignee hereof has resulted in a container for high-density shipment of uranium oxide powder and pellets. Such newer container design employs stainless steel materials for fabrication with silicon rubber gaskets and heavy-duty locking rings for positive leak-tight seals. Fire retardant foam and ceramic fiberboard panel are also employed in such newer-style container to protect the contents against the effect of accident and fire. Moreover, the size and geometry is cubical rather than cylindrical and its inner containers are nine in number, arranged in a 3×3 array. This newer container is the subject of U.S. patent application Ser. No. 09/315,729, titled “Uranium Oxide Shipping Container,” filed May 21, 1999. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, there is provided a container for shipping radioactive material comprised of an outer container and an inner containment vessel for receiving and containing product pails. Particularly, the outer container comprises a generally cylindrical container open at its upper end. Bolt brackets are arranged about the inner periphery of the container for receiving bolts to bolt a top onto the container. The bolt brackets are arranged, preferably in 90° sectors spaced about the circumference of the container with a pair of brackets straddling the container&#39;s weld seam to reinforce the container when the top is installed. In addition to bolting the top to the container, a retaining ring clamps the top to the container, with a bolt securing end lugs of the retainer ring to one another. The bolt and lugs in the retaining ring are located as close to the outside wall of the outer drum as possible to avoid breakage of containment should the container be dropped or impacted. 
     The inner containment vessel is generally cylindrical and has a radially outwardly directed flange at its open upper end supported by a plurality of circumferentially spaced gussets to provide strength to the lid-sealing region of the vessel. A lid is bolted to the flange with a heat-resistant gasket therebetween to effect a water seal. The vessel also includes a plurality of spiders or rods which project outwardly, preferably radially, to maintain the vessel centered within the outer container. Between the inner vessel and the outer container is a heat retardant polyurethane foam. The foam limits the maximum temperature the inner containment vessel and its gasket are subjected to, for example, during a fire. The foam also protects the inner containment drum from impact forces resulting from drop and impact tests. Panels formed of neutron-absorbing poisons may also be optionally applied about the exterior surface of the inner vessel. 
     Upper and lower dunnages are provided at opposite ends of the outer container. The dunnages comprise foam and ceramic fiberboard panels for fire resistance. The upper dunnage includes foam disposed between a pair of circular ceramic fiberboard panels with each having a stainless steel overlay. A reduced combined stainless steel and ceramic fiberboard panel underlies the upper dunnage for reception within the lid of the inner vessel to maintain stability. The lower dunnage is likewise a combination of foam and ceramic fiberboard panels. 
     The inner containment vessel is permanently fixed within the outer container. For shipping, product pails are placed inside the inner containment vessel. When the lid is bolted to the inner vessel, the upper dunnage is disposed between the lid and the top of the outer container. The upper dunnage is also provided with circumferential slots to enable the upper dunnage to be lowered into the container and pass the bolt connections for disposition on top of the inner vessel container. 
     Further, to improve resistance to fire, a plurality of plastic-filled vent holes are provided about the outer container and the upper and lower dunnages. The plastic plugs prevent water intrusion during normal conditions but will melt away in a fire to vent the container thereby preventing buildup of gases within the container in the event sufficient heat is supplied to ignite and burn the foam. Consequently, the vent plugs enhance the structural integrity of the shipping container in the event of a fire. 
     From the foregoing, it will be appreciated that there are a number of significant aspects according to a preferred embodiment of the present invention. For example, the locking retainer ring provides structural integrity for the top upon impact, but is backed up by the retaining bolts securing the top. The spiders maintain the inner vessel and, consequently, the pails containing the radioactive material centered within the outer container, affording stabilized geometry following the impact and fire tests. The outer container is provided with closely spaced annular reinforcing ribs adjacent its top which affords increased shock-absorbing capability and resistance to impact from above the container. The upper and lower dunnages provide impact and fire resistance at the opposite ends of the container. Additionally, all of the materials and fittings are formed of a stainless steel to preclude rust and corrosion. Dimensionally, and as set forth below, the shipping container “fits” into sea van containers in a manner to minimize unused sea van capacity. 
     Further, the shipping container hereof is readily and easily fabricated. To accomplish this, the lower dunnage is placed in the bottom of the outer container. A disk assembly is bolted to the flange of the inner vessel with the outermost disk having a margin extending beyond the flange of the inner vessel and an outer diameter corresponding to the inner diameter of the outer container. This margin contains a pair of diametrically opposite openings enabling injection of foam into the annular space between the inner vessel and outer container. With the disk assembly applied, the inner vessel is lifted, located and centered within the outer container resting on the lower dunnage. U-shaped channels are provided on both the top and bottom of the outer container and interconnected by tie rods to maintain the inner vessel centered within the outer container during foaming. The foam may then be applied through the openings of the margin of the outer disk. After curing, the disk assembly is removed, the foam maintaining the inner vessel within the outer container. Vent holes are drilled into the sides, bottom and top of the outer container, completing the fabrication. 
     In a preferred embodiment according to the present invention, there is provided a shipping container for radioactive materials comprising an outer, generally cylindrically-shaped container body having a closed lower end and an open, upper end, a top for releasable securement to the container body and closing the open upper end thereof, a generally cylindrical inner containment vessel generally concentrically disposed in the outer container body for receiving at least one radioactive material containing pail, the inner containment vessel having a lid for closing an open upper end thereof, a foam material between the outer container body and the inner vessel, the inner vessel having an outwardly directed flange about the open end thereof, a plurality of circumferentially spaced reinforcing gussets between an outer surface of the vessel and an underside of the flange for reinforcing the flange, the lid and the flange having cooperating fastening elements for fastening the lid and the flange to one another. 
     In a further preferred embodiment according to the present invention, there is provided a shipping container for radioactive materials comprising an outer, generally cylindrically-shaped container body having a closed lower end and an open, upper end, a top for releasable securement to the container body and closing the upper end thereof, a generally cylindrical inner containment vessel, generally concentrically disposed in the outer container for receiving at least one radioactive material containing pail, the vessel having a lid for closing an open upper end thereof, a foam material between the outer container and the inner vessel, the inner containment vessel including a plurality of rods projecting outwardly of the vessel toward the outer container body and extending into the foam material for maintaining the inner vessel substantially concentric within the outer container body. 
     In a still further preferred embodiment according to the present invention, there is provided a shipping container for radioactive materials comprising an outer, generally cylindrically-shaped container body having a closed lower end and an open, upper end, a top for releasable securement to the container body and closing the open upper end thereof, a generally cylindrical inner containment vessel, generally concentrically disposed in the outer container body for receiving at least one radioactive material containing pail, the vessel having a lid for closing an open upper end thereof and a closed lower end, a foam material between the outer container body and the inner vessel, an interior dunnage for the outer container body and overlying the inner containment vessel between the lid thereof and the top for the outer container body, the interior dunnage including a foam material disposed between upper and lower metal sheets and ceramic fiberboard panels and an interior dunnage underlying the inner vessel within the container body, the lower dunnage including foam material disposed between the closed lower end of the vessel and the closed lower end of the container body. 
     In a still further preferred embodiment according to the present invention, there is provided a shipping container for radioactive materials comprising an outer, generally cylindrically-shaped container body having a closed lower end and an open, upper end, a top for releasable securement to the container body and closing the upper end thereof, a generally cylindrical inner containment vessel, generally concentrically disposed in the outer container for receiving at least one radioactive material container pail and having a lid and neutron absorbing material disposed about the inner vessel and within the outer container body. 
     In a still further preferred embodiment according to the present invention, there is provided a shipping container for radioactive materials comprising an outer, generally cylindrically-shaped container body having a closed lower end and an open, upper end, a top for releasable securement to the container body and closing the open upper end thereof, a generally cylindrical inner containment vessel, generally concentrically disposed in the outer container body for receiving at least one radioactive material containing pail, the vessel having a lid for closing an open upper end thereof, a heat-resistant fire-retardant foam material between the outer container body and the inner vessel and a plurality of vent holes in the outer container body and plugs sealing the vent holes responsive to a predetermined temperature for opening the vent holes. 
     In a still further preferred embodiment according to the present invention, there is provided a method of fabricating a container for shipping radioactive materials, including an outer, generally cylindrically-shaped container body having a closed lower end and an open upper end and an inner container for receiving pails of the radioactive materials, comprising the steps of lining the lower end of the outer container body with an insulating material, closing the top of the inner container with a closure member having a peripheral margin laterally outwardly of the periphery of the inner container, locating the inner container within the outer container body forming a generally annular space between the exterior side walls of the inner vessel and interior walls of the outer container body and injecting a self-extinguishing fire-retardant foam material through at least one opening in the closure member and into the annular space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view through a central axis of a shipping container constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is an enlarged fragmentary cross-sectional view of the part circled in FIG. 1; 
     FIG. 3 is a side elevational view of an inner containment vessel; 
     FIG. 4 is a top plan view of the inner containment vessel; 
     FIG. 5 is a fragmentary perspective view of an upper end portion of the inner containment vessel showing the optional poison panels in place; 
     FIGS. 6 and 7 are top and side elevational views of an upper dunnage between the outer container and inner containment vessel; 
     FIG. 8 is a perspective view of the upper dunnage; 
     FIGS. 9 and 10 are bottom and side elevational views of a bottom dunnage for the shipping container; 
     FIG. 11 is a top plan view of the shipping container; 
     FIG. 12 is a perspective view of an upper end portion of the shipping container; 
     FIGS. 13 and 14 are top and side elevational views, respectively, of an inner lid for the inner containment vessel; 
     FIG. 15 is a plan view of a gasket for use between the inner lid and inner containment vessel; 
     FIG. 16 is a fragmentary enlarged view of an upper end portion of the outer container illustrating the bolt brackets adjacent the seam of the outer container; 
     FIG. 17 is a fragmentary enlarged perspective view of a clamping ring for sealing the top of the container to the outer container; 
     FIG. 18 is a fragmentary cross-sectional view illustrating initial fabrication steps for the shipping container hereof; 
     FIG. 19 is a fragmentary perspective view with parts spaced from one another for clarity illustrating further steps in the fabrication of the shipping container hereof; and 
     FIG. 20 is a vertical cross-sectional view through the shipping container illustrating the fixtures of the jig for fabricating the shipping container. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, particularly to FIG. 1, there is illustrated a shipping container constructed in accordance with a preferred embodiment of the present invention and generally designated  10 . Shipping container  10  includes an outer container  12 , an inner containment vessel  14  and a pair of product pails  16  stacked one on top of the other and disposed within the inner containment vessel  14 . Shipping container  10  also includes upper and lower dunnages  18  and  20 , respectively, and a top  22  for the outer container  12 . Outer container  12  is generally cylindrical and preferably fabricated from stainless steel. Reinforcing ribs  17  are formed at axially spaced locations along the container  10  and preferably two such ribs are closely spaced to one another and to the top of the container to reinforce the container, particularly adjacent the top  22 . Also, between the outer container  12  and the inner containment vessel  14  is provided a heat-retardant-foam, preferably a polyurethane foam  19 . 
     Referring now to FIGS. 3,  4  and  5 , the inner containment vessel  14  is preferably cylindrical, having a bottom  24  and an open top closed by a lid  26 . A plurality of spiders or rods  28  project outwardly, preferably radially, from the cylindrical containment vessel  14  and into the region between the vessel  14  and outer container.  12  to ensure that the vessel remains centered. Preferably, four rods  28  are equally spaced about the periphery of the vessel  14  adjacent its upper end and a similar number and spacing of the rods are provided adjacent the lower end of the vessel  14 . The rods  28  extend into the foam which is adhered to the outer container. Consequently, the vessel  14  remains centered within the outer container and is prevented from rotation relative to the outer container. 
     Additionally, neutron-absorbing material such as cadmium may be provided about the external surface of the inner vessel  14  in the form of poison panels  29 . The panels  29  preferably extend between the top and bottom of the inner vessel and may be provided in an arcuate length of 90°. The panels overlie the external peripheral surface of the inner vessel  14  and are provided with openings to receive the spiders  28 , as well as the gussets described below. The panels  29  as illustrated in FIG. 1 are overlaid by the foam  19 . 
     An annular flange  30  extends about the periphery of the vessel  14  adjacent its open upper end and projects radially outwardly therefrom. A plurality of gussets  32  are disposed between the upper end of the vessel  14  and the underside of the flange  30  to reinforce the lid sealing region about the open end of the vessel  14 . Lid  26  comprises a circular disk overlying the flange  30  and a gasket  21  formed of a fire retardant material is disposed between the lid and flange. The lid has a plurality of predrilled holes for registration with tapped holes in the flange  30  whereby bolts  36  passing through the holes and threaded into the tapped openings secure the lid and gasket to the vessel  14 , closing its upper end. As illustrated in FIG. 5, the flange  30  may also mount two or more dowel pins  38  to assist in orienting the lid  26  onto the vessel  14  during installation. 
     Referring back to FIG. 1, the product pails  16  are preferably formed of 18-20-gauge stainless steel. The product pails are closed containers having a lid with a retaining ring and bolt about the lid securing the lid to the pail. The radioactive material is, of course, located in the product pails. 
     Referring now to FIGS. 6 and 7, the upper dunnage  18  is illustrated. The upper dunnage comprises a foam core and ceramic fiberboard panels  40  and  41 , respectively, sandwiched between a pair of plates  42  and  44 , preferably formed of 24-guage stainless steel. The plates, as well as the foam and ceramic fiberboard panels, have cutouts  48  along their margins for receiving portions of the bolt lugs used to secure the top  22  to the outer container  12  during assembly as described below. Additionally, a circular ceramic fiberboard panel  46  having an underlayer  47  of stainless steel is secured to the bottom of the upper dunnage  18  to bear against the lid  26  of the inner containment vessel  14  in assembly. The lower dunnage  20  illustrated in FIG. 1 is constructed of a similar upper layer of foam  50  underlaid by a ceramic fiberboard panel  52 . The bottom of container  12  is closed by steel plate  54 . 
     Referring now to FIGS. 11,  12  and  16 , the top  22  for the outer container  12  is circular and formed from stainless steel. From a review of FIGS. 11,  12  and  16 , it will be appreciated that top  22  includes a plurality of bolt holes extending through lugs  60  for threaded engagement with inserts  62  threaded into bolt brackets  63  secured to the inside surface of the outer container  12 . The bolts  64  are threaded into the inserts  62  to secure the top  22  with a watertight O-ring seal  61  to the container  12 . As seen in FIGS. 11 and 12, three of the bolts  64  and associated lugs, plugs and brackets are spaced 90° from one another about the margin of the top  22 . The remaining two bolts are placed approximately 30° from one another and centered on opposite sides of a weld seam  68  extending down the side of the outer container  12 . Thus, the bolted connections between the top and the container in the region of the seam  68  provide added reinforcement for the lid. 
     To supplement the securement of the top  22  to the outer container  12  and as illustrated in FIGS. 2 and 17, a heavy-duty retaining ring  70  is applied about the arcuate rolled edge  72  of the top  22  and a beaded rim  74  formed along the upper edge of the outer container  12 . The ring  70  terminates at opposite ends in lugs  76  formed to lie close along the outer drum wall rather than projecting radially so that the extent of the projection of the lugs is minimized to avoid shearing of the lugs. As illustrated in FIG. 17, the wall of the outer container immediate the area about the lugs is further supported by a stainless steel plate  81  welded to the outside of the outer drum  12 . The steel plate prevents the bolt lugs from cracking the outer drum weld seam  68  due to accidental impact. Additionally, a bolt  83  threadedly secures the lugs  76  to one another. Lock nut  87  keeps the threaded bolt  83  from coming loose while securing the retaining ring  70  about the margin of top  22  and outer container  12  to reinforce the securement of the top and outer container one to the other. 
     A plurality of vent holes  80  (FIG. 1) are provided at vertically and circumferentially spaced positions about the outer container  12 . For example, three vent holes are provided through the container  12  in vertically spaced relation to one another at 90° intervals about the container  12 . Each vent hole is sealed by a plastic plug  82 . Upon reaching a predetermined temperature, the plastic of the plug  82  melts, opening the vent hole, enabling the escape of expanding gases from within the container. Additionally, and referring to FIG. 11, the top  22  has a vent hole  84  filled with a plastic plug  86 . Likewise, the bottom  54  of the container  12  has a central vent hole and a plastic plug. The top and bottom vent holes operate similarly as the side vent holes  80  in FIG. 1 to preclude a buildup of pressurized gases within the container which otherwise might rupture the container. The size and geometry of the invention is such that a standard sea van can accommodate up to  72  containers. Older-style containers had sizes and geometries that would only allow a maximum  54  containers per sea van. 
     Referring now to FIG. 18, which illustrates initial fabricating steps for the shipping container hereof, the bottom  54  of container  12  is provided with a central hole  90 . Next, the ceramic fiberboard panel  52  and the layer of foam  50  of the lower dunnage  20  are placed in the bottom of the outer container  12 . A fixture assembly is then provided. The fixture assembly includes a pair of channel members  92  and  94  connected at their centers to one another by welding and/or by a bolt  96  and extending at right angles to one another. Each of the channels has a slot at its distal end for receiving the lower end of a threaded rod  98 . It will be appreciated that four threaded rods  98  are disposed about the outer container  12  and secured at their lower ends by nuts  100  to the channel members  92  and  94 . The outer container  12  is then centered within and on the fixture. 
     Referring to FIG. 19, a closure member or disk assembly comprised of a series of disks is disposed on top of the flange  30  of the inner container  14 . In the order placed on the flange  30  of inner vessel  14 , the disk assembly includes a first disk  104  having a plurality of circumferentially spaced bolt holes  106 , vent holes  108  and apertures  110  for receiving the dowel pins  38  formed on the flange  30 . Disk  104  is preferably formed of ⅜″ thick stainless steel and has an outer diameter corresponding to the outer diameter of flange  30 . The next disk  112  is preferably formed of 22-gauge stainless steel having bolt holes  114  and vent holes  116 . The third disk  118  is preferably formed of ½″ thick aluminum and has bolt holes  120  and vent holes  122 . From a review of FIG. 19, it will be appreciated that disks  104 ,  112  and  118  have like diameters. A final disk  124 , preferably formed of ½″ thick aluminum, includes bolt holes  126 , vent holes  128  and a pair of openings  130  at diametrically opposite locations about the disk  124 . The diameter of disk  124  is slightly smaller than the inner diameter of the outer container  12 . Additionally, a hook  140  is provided in the center of the top disk  124  for purposes of lifting the inner container  14 . In assembling these disks, the vent holes  108 ,  116 ,  122  and  128  are aligned with one another and bolts  132  (FIG. 20) extend through the four disks and thread into correspondingly located threaded bolt openings  136  (FIG. 19) in flange  30 . It will be appreciated that the dowel pins  38  in this assembly are received in the apertures  110  and  111  of the lower disk  104  and disk  112 . 
     Additionally, a quick-release material is provided along the underside of the margin about the disk  124  which projects beyond the outer diameters of the disks  104 ,  112  and  118  to facilitate release of the disk assembly from the foam, i.e., prevents the foam from sticking to the fixture during the foaming operation. The inner container  14  with the four disks attached is then lifted, using hook  140 , and located and centered in the outer container  12 . In placing the inner vessel  14  within the outer container  12 , it is aligned with the seam weld  68  along the outer container  12 . 
     Levelers, not shown, are placed on top of the disk  124  to ensure that the inner container is set within the outer container as level as possible. A similar fixturing assembly like  92 ,  94 ,  95  and  100  in FIG. 18 is then applied to the top of the inner and outer containers as illustrated in FIG.  20 . Particularly, a pair of channel-shaped elements  150  and  152  are located at right angles to one another and secured to one another, extending across the open top of the outer container. The ends of the members  150  and  152  have slots for receiving the upper ends of the threaded rods  98 . The rods are secured in place by nuts  154 . Elongated bolts  156  extend through the members  150  and  152  and their lower ends engage the upper surface of the upper disk  124 , ensuring that the inner container remains level within the outer container  12 . The container is now ready for the foaming operation. Foam is injected through the two openings  130  in the upper disk  124  to fill the annular space between the inner vessel  14  and outer container  12 . The foam is injected simultaneously through holes  130  and fills the annular space to a level corresponding to an elevation above flange  30  to the bottom side of disk  124 , at which time the foaming operation ceases. 
     After curing, the fixtures, both top and bottom, are removed. Additionally, the disk assembly is removed from the flange  30  of the inner vessel  14 . From a review of FIG. 20, it will be appreciated that the spiders  28  extend into the foam  19  securing the inner vessel  14  within outer container  12 . Next, the bolt brackets  63  (FIG. 16) are drilled and tapped and the inserts  62  are threaded into the brackets. The brackets  63  are then welded to the inside surface of the outer container  12 , with two of the brackets closely straddling the seam  68 . A master template gauge, not shown, may be used to locate the brackets about the inner circumference of the outer container  12 . The backing plate  80  (FIG. 17) can also be welded to the outer container at this time. Next, a template, also not shown, may be used to locate the lugs  60  (FIG. 12) and holes for drilling through the top  22  for the outer container  12 . Additionally, the template may be used to locate the center vent hole  84  in top  22 . The lugs  60  are then welded to the top  22 . The ceramic fiberboard panel  52  is pre-drilled with a central opening through the openings  84  and  90  in the top and bottom of the outer container  12 . A plug  86  is installed in these openings, the bottom one of which is inserted prior to foaming. The upper dunnage  18  is then located overlying the top of the inner vessel  14  and the foam  19 , the slots  48  being provided to enable the dunnage  18  to pass by the lugs  63  (FIG.  16 ). Next, the container&#39;s top  22  and outer ring  70  is bolted into place. An opening is drilled through part of the upper dunnage disks  41  and  42  in FIG. 7 for venting purposes. Also, a translucent silicone is used to seal around the lugs  60  on the top of the top  22 . An O-ring washer  61  seals bolt  64  to the top  22 , making the top  22  completely watertight. 
     It will be appreciated from the foregoing that there has been provided a shipping container having substantial structural integrity and resistance to fire and water intrusion as well as a quick and inexpensive method of fabricating the container. Importantly, the container provides safety from radiation and criticality while material parts of the shipping container are formed of materials resistant to rust and corrosion, such as stainless steel, whereby the integrity of the container can be maintained over long periods of time and in hundreds of shipments. The structural integrity of the container is enhanced by the retaining ring, the spiders or rods which maintain the inner vessel centered within the outer container and the engagement of the upper and lower dunnages against the top and bottom of the inner vessel, respectively, the dunnages being sandwiched between the vessel and the top and bottom of the container. The arrangement of the reinforcing ribs on the outer container, particularly adjacent the top of the container, reinforce the top of the container, enhancing its resistance to impact. Fire resistance is provided by the combination of foam and ceramic fiberboard panels. Resistance to the destructive effects of high temperatures is also provided by the provision of vent holes disposed and arranged to vent any gases generated within the container upon the container reaching a predetermined temperature. That is, the plastic plugs melt at high temperature and enable the container to be vented. Further, the use of bolt brackets with removably threaded inserts improves the life cycle of the container by permitting the inserts to be removed and replaced by fresh threaded inserts. Consequently, any damage to the bolts or female threads may be readily repaired. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.