Abstract:
An unirradiated nuclear fuel assembly transport canister that includes a clamshell type fuel assembly inner liner that has interior dimensions that closely conform to the outer envelope of the fuel assembly to be transported and exterior dimensions that conform to a generic overpack tubular container. The liner is inserted into the overpack tubular container which is in turn supported by a shock absorbing suspension system within a birdcage frame.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a shipping container for a nuclear fuel assembly and, in particular, to such a container for unirradiated nuclear fuel assemblies which have a plurality of fuel rods supported in a geometric array.  
           [0003]    2. Background Information  
           [0004]    In the shipping and storage of unirradiated nuclear reactor fuel elements and assemblies, which contain large quantities and/or enrichments of fissial 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-percent 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.  
           [0005]    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 plurality shockmountings. 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.  
           [0006]    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.  
           [0007]    This and an 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, US Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of Nuclear Material Safety and Safeguards, Washington, DC 20555.  
           [0008]    U.S. Pat. No. 5,490,186, assigned to the assignee of this invention, describes a completely different nuclear fuel shipping container designed for hexagonal fuel and more particularly for fuel designed for Soviet style VVER reactors. Still, other shipping container configurations are required for boiling water reactor fuel.  
           [0009]    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.  
           [0010]    There is a need for such a fuel assembly shipping container that can accommodate a single assembly in a lightweight, durable and licensable design.  
           [0011]    There is a further need for such a shipping container that can be readily loaded in both a horizontal or vertical orientation.  
         SUMMARY OF THE INVENTION  
         [0012]    These and other objects are achieved by the individual fuel assembly containment system design of this invention to safely transport unirradiated nuclear fuel assemblies under normal and hypothetical accident conditions. The shipping container includes an elongated inner tubular liner having an axial dimension at least as long as the 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 wall of the tubular container is constructed from relatively thin shells of stainless steel coaxially positioned with close cell polyurethane disposed in between. Preferably, the inner shell includes boron-impregnated stainless steel.  
           [0013]    The inner tubular liner enclosing the fuel assembly is sideably mounted within the tubular container overpack and the overpack is sealed at each end with end caps. The tubular container 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.  
           [0014]    An elongated external frame, preferably of the birdcage design, is sized to receive the tubular container 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 preferably at least two of the circumferential ribs extending around the tubular container, to isolate the tubular container from a substantial amount of any impact energy experienced by the frame should the external frame be impacted. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    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:  
         [0016]    [0016]FIG. 1 is a perspective view, partially cut away, showing the elongated inner tubular liner of this invention surrounding a fuel assembly, which is partially inserted into a boron sleeve which, in turn, is partially inserted into the overpack of this invention;  
         [0017]    [0017]FIG. 2 is a perspective view of the overpack of this invention supported in the external frame by shock absorbing attachments; and  
         [0018]    [0018]FIG. 3 is a perspective view of a carriage supporting the inner liner enclosing a fuel assembly that is aligned to load the inner liner into the overpack illustrated in FIG. 2.  
         [0019]    FIGS.  4 A- 4 E are schematic representations of different embodiments for supporting the overpack within the external frame. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    The overpack and internal components of the fuel assembly containment and transport system of this invention is illustrated in FIG. 1. A fuel assembly  24  is figuratively shown closely surrounded by the elongated inner liner  18 . The inner liner  18  is shown separated into two halves  22  and  20  so the inner liner can more easily be positioned around the fuel assembly prior to loading into the overpack  10 . Though the liner  18  is shown in a clamshell design, preferably split in two equal half sections  20  and  22 , it should be appreciated that the liner  18  can be separated into two or more sections of equal or unequal circumferential lengths without departing from the scope of this invention. Desirably, the mating interfaces of the interior liner sections  20  and  22  include complementary keyways to facilitate the positioning of the liner sections  20  and  22  around the fuel assembly  24 . The liner  18  is preferably constructed from stainless steel with an external circumferential shape and size that is designed to be closely received within the tubular overpack  10 . The interior of the liner  18  is either stamped or machined to closely conform to the outer envelope of the fuel assembly  24 . A boron sleeve  16  is inserted between the inner liner  18  and the overpack  10  prior to loading of the inner liner. Alternately, the exterior surface of the inner liner  18  can be coated with boron, e.g., through flame deposition or other coating techniques. As another alternate or supplement to the boron sleeve  16 , boron-silicate can be poured into a void space in the inner liner  18  between the inner wall of the liner  18  that mates with the outer surface of the fuel assembly  24  and the outer surface of the liner  18  that is closely received within the overpack  10 . Still another alternate is to attach plates formed out of boron-silicate or borated stainless steel to the inside of the inner liner  18 .  
         [0021]    The overpack  10  is an elongated tubular member whose walls are preferably formed from two coaxially positioned stainless steel shells of approximately ⅛ inch thickness. Other wall thicknesses can be employed that will satisfy the strength requirements. However, greater wall thickness will add to the weight of the overpack  10 . Preferably, each shell is welded or otherwise manufactured to have a continuous, seamless circumference. In this embodiment, as an example without intending to be limited, a three inch thick layer of close cell polyurethane is interposed between the shells in a closely packed arrangement and preferably the inside shell is formed from a stainless steel sheet impregnated with boron, though it should be appreciated that both shells can be formed from boron impregnated stainless steel or another moderator other than boron can be employed for this purpose. Though the overpack is shown with a circular cross-section, other geometries can be used, e.g., an oval or rectangular geometry, though a circular geometry has some structural advantages.  
         [0022]    The overpack is fitted with a number of axially spaced circumferential ribs  12  which add circumferential rigidity to the shells that form the walls of the overpack  10 . Separate shockmount attachments points  14  can be provided along an external axial rail  28  or the shockmount attachment points can be provided directly on the circumferential ribs  12 . Ball bearings  26  are figuratively shown in dotted form and represent a friction reducing mechanism that is employed to assist loading of the inner liner  18  into the overpack  10 . The friction reducing mechanisms can be ball bearings on either the interior surface of the overpack  10  or the exterior surface of the liner  18  or a telescoping railing arrangement that can be fixedly built into the interior wall of the overpack  10  and detachably connected to the liner  18 . It should also be noted that the sections  20  and  22  of the liner  18  can be clamped together once they are positioned around the fuel assembly  24 .  
         [0023]    [0023]FIG. 2 illustrates the overpack  10  enclosed by two endcaps  36  which can be affixed by bolts or a quick hinge and wingnut attachment once the inner liner containing the fuel assembly is loaded. The overpack  10  is shown supported within a birdcage-like frame  30  that is formed from a plurality of axially-spaced circumferential straps  32  that are fixedly connected, such as by welding, to circumferentially-spaced and axially oriented rails  34 . The end locations  38  are formed from angle straps. In this arrangement, the external frame is shown with a square cross-section though it should be appreciated that other geometries, such as a circular cross-section can be employed. The overpack  10  is supported within the frame  30  by at least one shock absorber mechanisms  40 . The shock absorber support can be, for example, part number J-5735-64, supplied by Lord Corporation, Mechanical Products Division, 2000 West Grandview Boulevard, Erie, Pa. 16514. FIGS.  4 A- 4 E illustrate different support arrangements within the birdcage frame  30  that can be used to support the overpack  10 . In FIG. 4A, the overpack  10  is suspended from the midpoint of the sidewalls of the birdcage frame  30  on diametrically opposite sides of the overpack  10 . In FIG. 4B, a single slightly different shockmount support secures the overpack  10  to one sidewall of the birdcage frame  30 . In FIG. 4C, four shockmount supports respectively suspend the overpack  10  from the midpoints of the sidewalls of the birdcage frame  30 . In FIG. 4D, three shockmount supports  40  suspend the overpack  10  from the midpoint of three of the sidewalls of the birdcage frame  30 , and in FIG. 4E, the overpack  10  is suspended from four shockmounts  40  respectively attached to the intersection points of the sidewalls of the birdcage frame  30 . Thus, it can be appreciated that other support arrangements can be configured without departing from the scope of this invention.  
         [0024]    [0024]FIG. 3 illustrates a car transport  42  that can be used to load the liner  18  into the overpack  10  once the liner  18  has been positioned over the fuel assembly. The sidewalls  46  of the car  42  are aligned with the outer frame  30  which aligns the cradle  44  holding the liner  18  with the axis of the overpack  10 . The liner  18  can then be translated axially over rollers in the base of the loading car  42  into the overpack  10  where the frame  30  and car  42  are supported in the horizontal position. Alternatively, the frame  30  can be oriented in a vertical position and a winch  50  can control the cable  48  which is tied to the end of the car  42  to slowly lower the liner  18  into the interior of the overpack  10 .  
         [0025]    Different fuel assembly configurations can be accommodated by supplying liners with different interior geometries to be complementary with the envelope of the fuel assemblies. In its basic form, the invention is a clamshell type fuel assembly package that is inserted in an overpack tube container. An external suspension system and birdcage frame then support the tube overpack. This design allows for a very simple, low cost, lightweight replacement to current fuel transport casks.  
         [0026]    The overpack provides the versatility to carry many different pressurized water reactor fuel designs as well as many fuel types. The overpack can carry clamshells configured for boiling water reactor fuel assemblies and other nuclear products.  
         [0027]    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.