1. Field of the Invention
This invention relates to a shipping container for transporting nuclear fuel components and, in particular, to such a container for transporting unirradiated nuclear fuel assemblies and control rod spider assemblies.
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 U235, it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, nuclear reactor fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments; i.e., weights and weight percent U235 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's rules and regulations, including those defined in 10 CFR §71. These requirements define the maximum critical accident (MCA) that the shipping container and its internal support structures must endure while maintaining the sub-criticality 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 assembly 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 connected 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 having 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 jack posts with pressure pads that are tightened down on 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 that 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. 5454, U.S. Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of the Nuclear Material Safety and Safeguard, 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 a Soviet style VVER reactor. 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 further need for such a nuclear fuel assembly shipping container that can accommodate a single assembly in a light-weight, durable and licensable design.
These and other needs have been partially resolved by U.S. Pat. No. 6,683,931, issued Jan. 27, 2004 and assigned to the assignee of the instant invention. The shipping container described in this latter patent 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 clam shell for placement of the two halves of the liner around the fuel assembly. The exterior 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 and the liner is coaxially positioned within the tubular container 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 end caps. 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 fixably connect the straps to form the frame design. A plurality of shock absorbers are connected between certain ones 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.
Although the shipping container described in the aforementioned '931 patent is a substantial improvement in that it can accommodate different fuel assembly designs through the use of complementary shell liners while employing the same overpack and birdcage frame, that improvement has been taken one step further by U.S. Pat. No. 6,748,042 assigned to the assignee of the instant invention. The '042 patent describes a transport system that provides a liner and overpack system that will achieve the same objectives as the '931 patent while further improving the protective characteristics of the transport system and the ease of loading and unloading the nuclear fuel components transported therein. The shipping container includes an elongated tubular container, shell or liner designed to support a nuclear fuel product such as the fuel assembly therein. The interior of the tubular liner preferably conforms to the external envelope of the fuel assembly. The exterior of the tubular liner 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 side walls to swing open and provide access to the interior of the tubular container. The tubular container or liner is designed to seat within an overpack for transport. The overpack is a tubular package having an axial dimension and a cross-section larger than the tubular liner. The overpack is split into a plurality of circumferential sections (for example, two sections, a lower support section and an upper cover, or three sections, 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 liner. Shock mounts extend from both radial walls of the V-shaped groove to an elevation that will support the tubular liner in space relationship to the groove. The axial location, number, size and type of shock mount employed are changeable to accommodate different loadings. The tubular liner is seated on the shock mounts, preferably with a corner of the liner aligned above the bottom of the V-shaped groove. The top cover section (sections) of the overpack has a complementary inverted V-shaped channel that is sized to accommodate the remainder of the tubular liner with some nominal clearance approximately equal to the spacing between the lower corner of the tubular liner and the bottom of the V-shaped groove. The ends of the overpack are capped and the overpack sections are latched.
Though the transport system of the '042 patent provides a substantial improvement in the protective characteristics and ease of loading and unloading of the nuclear components being transported, further improvement in the ease of loading and unloading the liner is desired. The invention of U.S. Pat. No. 7,474,726 assigned to the assignee of the current invention, was conceived for such purpose. The '726 patent is a variation on the '042 patent that permits loading of the liner from the top as well as the sides. The liner comprises an elongated tubular container that has at least two substantially flat walls with at least one circumferential end having a hinged interface with a stationary wall of the container to provide access through the side of the container. The hinged wall extends axially in a direction of one end of the container and terminates a preselected distance short of the corresponding end of the stationary wall. The stationary wall has a lateral groove on an interior surface thereof at an elevation starting substantially at the elevation of the one end of the hinged wall. An access cover is slidable in the groove in the stationary wall to close off the one end of the container so that the interior of the container may be accessed either through the one end by sliding out the access cover, or from the side by rotating the hinged wall. The access cover can be locked in position and the elongated tubular container has the other end opposite the one end capped and sealed and the tubular container is sized to fit within the overpack of the '042 patent.
While the '726 patent is a further improvement in the design of shipping casks which provides greater versatility in the number of fuel assembly designs that can be accommodated and the ease of loading, further improvement is still desired to accommodate an increased number of fuel assembly and component designs.
Accordingly, it is an object of this invention to further enhance the versatility of the shipping cask design described in the '726 patent.
It is an additional object of this invention to enhance the shipping cask design of the '726 patent so that it will accommodate all Westinghouse designed light water reactor nuclear fuel assemblies.
It is an additional object of this invention to further improve the shipping cask design of the '726 patent so that it will accommodate all CE designed light water reactor nuclear steam supply system fuel assemblies.
Furthermore, it is an object of this invention to further enhance the design of the '726 patent so that it will satisfy European and U.S. fuel transport regulations.
Furthermore, it is an object of this invention to further enhance the unirradiated nuclear fuel transport cask design of the '726 patent so that it will transport control rod spider assemblies in addition to nuclear fuel assemblies.
Additionally, it is an object of this invention to enhance the design of the '726 patent to provide a larger access envelope for the loading of a nuclear fuel assembly into the liner.