Abstract:
An unirradiated nuclear fuel assembly component transport system that includes a clamshell-type inner liner that opens along its axial dimension to load and unload the fuel component being transported. The exterior dimensions of the liner conform to a generic overpack tubular container. The overpack is separable into an upper and lower section with the lower section having a V-shaped channel extending along its longitudinal axis. The liner is supported in the V-shaped channel by at least one row of shock mounts on either side of the channel. The lower section of the overpack is secured from rotation and the top section is latchably engaged to the lower section of the overpack.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. patent application Ser. No. 10/025,728, filed Dec. 19, 2001 and entitled “Unirradiated Nuclear Fuel Transport System”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a shipping container for nuclear fuel components and, in particular, to such a container for unirradiated nuclear fuel assemblies and nuclear fuel rods. 
     2. Related Art 
     In the shipping and storage of unirradiated nuclear fuel elements and assemblies, which contain large quantities and/or enrichments of fissile material, U 235 , it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments (i.e., weights and weight-% U 235 ) for each fuel assembly design. In order for a new shipping container design to receive licensing approval, it must be demonstrated to the satisfaction of the NRC that the new container design will meet the requirements of the NRC rules and regulations, including those defined in 10 CFR § 71. These requirements define the maximum credible accident (MCA) that the shipping container and its internal support structures must endure in order to maintain the subcriticality of the fuel assembly housed therein. 
     U.S. Pat. No. 4,780,268, which is assigned to the assignee of the present invention, discloses a shipping container for transporting two conventional nuclear fuel assemblies having a square top nozzle, a square array of fuel rods and a square bottom nozzle. The container includes a support frame having a vertically extending section between the two fuel assemblies, which sit side by side. Each fuel assembly is clamped to the support frame by clamping frames, which each have two pressure pads. This entire assembly is connected to the container by a shock mounting frame and a plurality of shock mountings. Sealed within the vertical section are at least two neutron absorber elements. A layer of rubber cork-cushioning material separates the support frame and the vertical section from the fuel assemblies. 
     The top nozzle of each of the conventional fuel assemblies is held, along the longitudinal axis thereof, by jackposts with pressure pads that are tightened down to the square top nozzle at four places. The bottom nozzle of some of these conventional fuel assemblies has a chamfered end. These fuel assemblies are held, along the longitudinal axis thereof, by a bottom nozzle spacer, which holds the chamfered end of the bottom nozzle. 
     These and other shipping containers (e.g., RCC-4 for generally square cross-sectional geometry pressurized water reactor (PWR) fuel assemblies used by the assignee of the present invention) are described in Certificate of Compliance No. 5450, Docket No. 71-5450, U.S. Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of the Nuclear Material Safety and Safeguards, Washington, D.C. 20555. 
     U.S. Pat. No. 5,490,186, assigned to the assignee of the present invention, describes a completely different nuclear fuel shipping container designed for hexagonal fuel and, more particularly, for a fuel assembly design for Soviet style VVER reactors. Still, other shipping container configurations are required for boiling water reactor fuel. 
     There is a need, therefore, for an improved shipping container for a nuclear fuel assembly that can be employed interchangeably with a number of nuclear reactor fuel assembly designs. 
     There is a need for such a fuel assembly shipping container that can accommodate a single assembly in a lightweight, durable and licensable design. 
     These and other needs have been partially resolved by co-pending application Ser. No. 10/025,728, filed Dec. 19, 2001 and assigned to the assignee of the instant invention. The shipping container described in the foresighted application includes an elongated inner tubular liner having an axial dimension at least as long as a fuel assembly. The liner is preferably split in half along its axial dimension so that it can be separated like a clamshell for placement of the two halves of the liner around the fuel assembly. The external circumference of the liner is designed to be closely received within the interior of an overpack formed from an elongated tubular container having an axial dimension at least as long as the liner. Preferably, the walls of the tubular container are constructed from relatively thin shells of stainless steel coaxially positioned with close-cell polyurethane disposed in between. Desirably, the inner shell includes boron-impregnated stainless steel. The tubular liner enclosing the fuel assembly is slidably mounted within the overpack and the overpack is sealed at each end with endcaps. The overpack preferably includes circumferential ribs that extend around the circumference of the tubular container at spaced-axial locations, that enhance the circumferential rigidity of the overpack and form an attachment point for peripheral shock absorbing members. An elongated frame, preferably of a birdcage design, is sized to receive the overpack within the external frame in spaced relationship with the frame. The frame is formed from axially spaced circumferential straps that are connected to circumferentially spaced, axially oriented support ribs that fixedly connect the straps to form the frame design. A plurality of shock absorbers are connected between certain of the straps and at least two of the circumferential ribs extending around the overpack, to isolate the tubular container from a substantial amount of any impact energy experienced by the frame, should the frame be impacted. 
     Though the shipping container described in the foresighted application is a substantial improvement in that it can accommodate different fuel assembly designs through the use of complementary liners, while employing the same overpack and birdcage frame, further improvement is desired that will achieve the same objectives while further improving the protective characteristics of the transport system and the ease of loading and unloading the nuclear fuel components transported therein. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by the individual fuel assembly containment and transport system design of this invention to safely transport unirradiated nuclear fuel assemblies and other nuclear fuel components under normal and hypothetical accident conditions. The shipping container includes an elongated tubular container or shell designed to receive and support a nuclear fuel product such as a fuel assembly therein. The interior of the tubular container preferably conforms to the external envelope of the fuel assembly. The exterior of the tubular container has at least two substantially abutting flat walls, which extend axially. In the preferred embodiment, the cross-section of the tubular member is rectangular or hexagonal to match the outer envelope of the fuel assembly and three of the corner seams are hinged so that removal of all the kingpins along a seam will enable two of the sidewalls to swing open and provide access to the interior of the tubular container. The tubular member or container is designed to seat within an overpack for transport. The overpack is a tubular package having an axial dimension and cross-section larger than the tubular container. The overpack is split into a plurality of circumferential sections, for example two sections, a lower support section and an upper cover, or three, a lower support section and two upper cover sections that are respectively hinged to either circumferential side of the lower support section and joined together when the overpack is closed. The lower support section includes an internal central V-shaped groove that extends substantially over the axial length of the overpack a distance at least equal to the axial length of the tubular container. Shock mounts extend from both radial walls of the V-shaped groove to an elevation that will support the tubular container in space relationship to the groove. The axial location, number, size and type of shock mount employed is changeable to accommodate different loadings. The tubular container is seated on the shock mounts preferably with a corner of the container aligned above the bottom of the V-shaped groove. The top cover section (sections) of the overpack has (have) a complementary inverted V-shaped channel that is sized to accommodate the remainder of the tubular container with some nominal clearance approximately equal to the spacing between the lower corner of the tubular container and the bottom of the V-shaped groove. The ends of the overpack are capped and the overpack sections are latched. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a perspective view of the shipping container system of this invention; 
     FIG. 2 is a front view of the shipping container system of this invention showing neutron moderator material lining the inner channel of the overpack; 
     FIG. 3 is front view of the shipping container system of this invention with thermal insulation lining the interior of the stainless steel shell and neutron-absorbing material lining the exterior of the tubular container surrounding a fuel assembly; 
     FIG. 4 is a perspective view of the latch mechanism used to anchor the overpack segments together; 
     FIG. 5 is a perspective view of the tubular container housing a nuclear fuel assembly with two sides of the tubular container swung open; 
     FIG. 6 is a top view of the tubular container with all four side walls closed; 
     FIG. 7 is a simplified top view of a hexagonal tubular container for housing hexagonal fuel such as that employed in the Russian design VVER reactors; 
     FIG. 8 is a front view of an alternate embodiment of the shipping container system of this invention shown in a filly open position; 
     FIG. 9 is a front view of the shipping container system shown in FIG. 8 with the top sections partially closed; and, 
     FIG. 10 is a front view of an alternate embodiment of the shipping container system of this invention for accommodating hexagonal fuel assemblies. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The overpack and internal components of the nuclear fuel product containment and transport system of this invention is illustrated in FIG. 1. A tubular container or shell  26  constructed from a material such as aluminum houses a nuclear fuel assembly and is suspended over a V-shaped groove  32  in the overpack  12 , supported on shock mounts  28  that are affixed in a recess  30  in an upper wall section of the groove  32  and spaced along the axial length of the lower overpack section  16 . The shock mounts can be those identified by part no. J-3424-21, which can be purchased from Lord Corporation having offices in Cambridge Springs, Pa. Angle irons  24  can be used at the corners of the tubular container to spread the load on the container walls. The number and resiliency of the shock mounts are chosen to match the weight of the container, which depends upon the nuclear product being transported within the container  26 . The orientation of the lower section  16  of the overpack  12  is fixed by the legs  18  so that the weight of the container  26  holds the container centered in the groove  32 . One capped end  22  forms part of the lower overpack support section  16  while a second capped end  20  is formed as an integral part of the top cover  14 . The end  20  of the upper overpack segment  14  seals against the lip  21  in the lower support section  16 . Similarly, though not shown, the end  22  formed as an integral part of the lower support section  16  seals against a corresponding lip on the upper overpack section  14  in the same manner. Keys  50  on each side of the upper section  14  of the overpack  12  fit in complementary keyways in the lower overpack support section  16  as can be better appreciated from the frontal view shown in FIG.  2 . 
     FIG. 2 shows a frontal view of the shipping container system  10  of this invention with the endplate  20  removed. Both the top segment  14  and bottom segment  16  of the overpack  12  are formed from a hollow stainless steel sheet  34 , for example, an 11 gauge stainless steel shell filled with polyurethane can be employed. Preferably, in this embodiment the polyurethane has a minimum 3 inch (7.62 cm) thickness. In the preferred embodiment, the hollow channel in the overpack  37  is shaped to substantially conform to the outer profile of the tubular container  26  and the walls of the hollow channel  37  can be lined with a neutron-absorbing material such as a half-inch (1.27 cm) of borosilicate. Alternately, the outer surface of the tubular canister  26  can be lined with a neutron-absorbable material such as an eighth inch (0.318 cm) thick layer of borosilicate or a combination of neutron-absorbing material on the walls of the tubular container  26  and the walls of the hollow channel  37  can be employed. FIG. 2 provides a better view of the recess  30  that the shock mounts  28  are mounted in than can be derived from FIG.  1 . Similarly, the keys  50  and keyways  48  that aid in positioning the top Section  14  on the lower support member  16  of the overpack  12  are shown more clearly in FIG.  2 . 
     FIG. 3 illustrates an alternative embodiment to that shown in FIG. 2, which omits the layer of neutron-absorbing material  42  on the hollow channel  37  illustrated in FIG.  2  and shows instead the four sides of the tubular container  26  having a eighth inch (0.318 cm) layer of neutron-absorbing material on all four sides. Additionally, the embodiment shown in FIG. 3 includes an approximately half inch (1.27 cm) layer of thermal insulation interposed between the stainless steel shell  34  and the polyurethane  36 . In this embodiment, the polyurethane will have a minimum thickness of approximately 2,½ inches (6.35 cm). 
     The top segment  14  of the overpack is latched to the bottom support segment  16  in the preferred embodiment using the latch assembly illustrated in FIG.  4 . Both the lip  53  on the upper overpack section  14  and the lip  55  on the lower overpack section  16  include a plurality of axially spaced slots. A latch bar is affixed to either the upper lip  53  or the lower lip  55  in a manner to permit the clamp arm  54  to slide within a corresponding slot in the lip. For example, with the latchbar  56  coupled to the lower lip  55 , the clamp arm  54  would protrude through the corresponding slot in a downward direction and have an enlarged protruding end to anchor the latch bar  56  to the lower lip  55 . The upper clamp arm  54  can have an L-shape as shown in FIG. 4 so that when the lip  53  is seated over its corresponding clamp arm  54 , the latchbar  56  can be moved in a direction into the figure to lock the upper section  14  to the lower section  16  of the overpack  12 . The clamp arm  54  can then be secured in that locked position. An external lever can be used to slide the latchbar to an open and closed position with an approximate four-inch stroke desirable. To facilitate the locking and unlocking action, a low friction coating can be applied to the sliding surface. 
     FIG. 5 illustrates a perspective view of an open tubular container  26  with a fuel assembly  74  positioned therein. The fuel assembly  74  is made up of a parallel spaced array of fuel elements  76  that are maintained in space relationship and in position by gridstraps  78 , a bottom nozzle  77  and a top nozzle which is not shown. The gridstraps are constructed in an eggcrate design to maintain the spacing between the fuel elements  76  that form flow channels for the reactor coolant to flow through during reactor operation. The fuel assembly  74  is seated on a neoprene or cork rubber bottom pad  72 , which is affixed to the bottom  68  of the tubular container  26 . The neoprene or cork rubber pad  72  supports and cushions the fuel assembly  74 . A similar arrangement is provided above the fuel assembly  74  supported by the top end  70  (shown in FIG. 6) of the tubular container  26 . The tubular container  26  has two stationary sides  59  and  62  which are affixed to the bottom  68  and top  70  of the container. The container  26  has two moveable sides  62  and  64  which are hinged to the adjacent edges of the stationary sides  58  and  60  through hinges  66  that rotate around a kingpin  67  (shown in FIG.  6 ). The two moveable sides are in turn connected, when latched, by similar hinges  66 , with the insertion of the kingpin in the hinge forming the latch. In this way, the moveable sides  62  and  64  can be opened from any of the hinged seams to provide access to the interior of the tubular container  26  from a number of different directions to facilitate loading and unloading in different environments that may present obstructions. The tubular container  26  is preferably constructed out of aluminum of a thickness, for example, of 0.375 inches (0.9525 cm) 
     The interior walls of the sides  58 ,  60 ,  62  and  64  are covered with an iron ferrite composite sheet,  88  and neoprene or cork rubber pads with magnetic backing  82  are attached and affixed by the magnetic force at the grid elevations to seat the neoprene or cork rubber side of the pads against the outside straps of the grid at the grid elevations. The magnetic coupling on the pads makes them adjustable to accommodate different nuclear fuel component designs. The neoprene or cork rubber pads are not as hard as the material that the grids are constructed of and secures the grids in position when the sides  62  and  64  are in the closed position without damaging the grids and cushions the fuel assembly during transport. The inside of the tubular container  26  can be used to transport other fuel components such as fuel rods separately by employing inserts within the tubular container  26  that will hold those components securely. FIG. 6 provides a better view of the iron ferrite composite sheet  80  and hinged locations. Alternately, clips on the backs of the neoprene or cork rubber pads can be supported in slots at multiple elevations on the interior walls of the sides  58 ,  60 ,  62  and  64 . Axial adjustment of the pads can be made by moving the pads from slot to slot. 
     FIG. 7 shows a simplified top view of a hexagonal tubular container  26  for transporting hexagonal fuel such as that employed in Russian designed VVER reactors. In this case, there are four stationary sides  58 ,  59 ,  60  and  61  with two moveable sides arranged and hingably connected as previously described. In all other respects, the embodiment of the tubular container  26 , shown in FIG. 7, is the same as that shown in FIGS. 5 and 6. 
     With the fuel assembly or other nuclear fuel component loaded within the tubular container  26 , the moveable sides  62  and  64  can be closed and secured with the kingpins  67  and the tubular member  26  loaded within the V-channel  32  of the lower overpack support section  16 . The upper overpack section  14  then can be lowered into engagement with the lower section  16  and the latch  52  can be secured. In this way, the nuclear fuel product can be shipped in the horizontal position in a secure and safe manner. The overpack provides the versatility to carry many different types of fuel components. 
     FIGS. 9 and 10 illustrate another embodiment of the overpack  12 . The lower section  16  of the overpack  12  is the same in most respects as that illustrated in FIG. 1, except for the keying of the upper section to the lower section and the front closure  20  shown in FIG.  1 . The legs  18  are also different as shown. The upper section  14 , shown in the embodiment illustrated in FIGS. 8 and 9, is split into sections  84  and  86  with each section hinged to the lower section  16 . When the sections  86  and  84  are closed, the mating surfaces  88  and  90  intersect along their keyway and can be latched. The front closure  92  is generally rectangular and formed as an integral part of one of the top half sections  84 . When closed, the front closure  92  seats within a sealing groove  94  in the front of the lower section  16 . Lifting lugs  96  are provided for transferring the cask to and from a transport vessel. The swinging top sections  84  and  86  of the embodiment illustrated in FIGS. 8 and 9 make it easier to open and close the overpack  12  and load the tubular container  26 . 
     FIG. 10 shows the overpack  12  as illustrated in FIGS. 8 and 9 with a hexagonal tubular container or member  26 ′ for transporting a hexagonal fuel assembly. In all other respects, the overpack  12  and tubular container are the same as described above. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.