Patent Number: 055132324
Section: summary

TECHNICAL FIELD OF THE INVENTION The present invention relates to containers used for transportation and short term storage of spent nuclear fuel. BACKGROUND OF THE INVENTION As the nuclear utility industry matures, there is an ever-increasing need for additional storage space to safely contain spent nuclear fuel. One method that has been developed in recent years for storage of spent nuclear fuels is dry storage in horizontal storage modules, which are shielded bunkers in which containerized spent fuel is stored and monitored for definite periods of time. One conventional technique for horizontal modular dry storage of spent nuclear fuel rods is disclosed in U.S. Pat. No. 4,780,269 to Fischer et al. A basic procedure for dry storage of spent nuclear fuel is to position a dry shielded canister into a shielded transfer cask. The canister and cask are filled with deionized water, which is then lowered into a pool containing the spent nuclear fuel. Spent fuel assemblies are then placed into the canister, and a shielded end plug is positioned to close the canister. The canister and cask are then removed from the pool, and the cask and canister are drained and dried. The exterior of the cask is decontaminated, followed by closure of the cask with a closure plate. The closed transportation cask is then lowered onto a transport trailer and secured by tie-downs. The transport trailer carries the cask to the sight of the horizontal dry storage modules. The cask is opened and docked with an entry port of a dry storage module. The canister is then transferred from the cask into the module, such as by passing a ram through the dry storage module from an end opposite the entry port, through the entry port and into the opened cask. The canister can then be grasped and pulled into the dry storage module, after which both the entry port and access port are sealed. A critical aspect of this process is the safe containment and transfer of the spent nuclear fuel within the canister from the original pool storage to the final dry horizontal storage site. The transport cask must be constructed with adequate structural strength and shielding to both physically protect the dry shielded canister within, and to provide biological shielding to minimize personnel radiation dosages during canister transfer and transport operations. During the canister transfer and transport process, the cask must be able to withstand any foreseeable impact, such as could occur by accidental dropping of the cask from the transport trailer or exposure to tornadoes or other natural disasters. In the United States, federal regulations setting forth requirements that transport casks must meet are found in 10 C.F.R. 72, including subpart G, as well as 10 C.F.R. 71 and 10 C.F.R. 50. In particular, the cask must be able to withstand impacts due to a drop of 30' onto an essentially, unyielding fiat horizontal surface, without structural failure. Even if structurally damaged, no leakage of the contents from the cask is permitted. It is thus important to design casks with high structural integrity. At the same time, it is desirable to maximize the quantity of spent fuel that can be transported within the cask at any given time, and to minimize the cost of constructing the cask. While strength considerations typically warrant constructing the cask from thicker sections of metal and other materials, this requirement may reduce the quantity of spent fuel that can be transported within the cask. External dimensions of the cask are limited by constraints such as the total weight of the loaded cask, and clearances required to transport the casks through tunnels, under bridges and overpasses, and the like. Currently, conventional casks are often constructed from a polished austentitic stainless steel, such as 304 stainless steel, for corrosion prevention. However, such stainless steel is limited in strength and may fail under high stresses. To combat this potential, conventional casks are constructed from thick metal sections, and must be reinforced with gusset plates and other reinforcing members. Additionally, locations on the casks that are subjected to force during transport must be reinforced with additional metal plates welded to the cask structure. For example, conventional casks are outfitted with cylindrical trunnions welded or bolted directly to a cylindrical structural shell of the cask at diametrically opposed locations. These trunnions are grasped by hooks, and serve as pivot points while lifting the cask during the transportation process. Because of the stresses transferred to the cask structure from the trunnions during use, the shell is typically reinforced in the area surrounding the trunnions by welding additional plates of metal. The trunnions themselves are conventionally permanently secured to the structural shell of casks by welding or bolting directly to the shell. In the case of welding, the welded joint is subjected to substantial stress during hoisting of the cask. In the case of bolting the trunnions in place, the bolts are subjected to extreme shear and tensile loads during hoisting of the cask. Again, the trunnions must be heavily reinforced to withstand such loads, increasing the weight and overall dimensions of the cask, and thus decreasing the spent fuel containment capacity and increasing the cost of manufacture. When sealed joints, such as elastomeric (e.g., O-ring) seals or metal seals are utilized, the base metal used to form the structural shell is conventionally machined to form the sealing surfaces. Thus, for example, when 304 stainless steel is used to construct the shell, annular surfaces on the shell are machined and polished to form sealing surfaces. While functioning adequately in most situations, extreme impact to the seal area, such as by accidental dropping of the cask at an oblique angle whereby force is concentrated on the seal area, may result in permanent deformation of the metal seal surface, and subsequent leakage potential. SUMMARY OF THE INVENTION The present invention provides a container designed for use as a cask for short-term containment and transporting of spent nuclear fuel. In the first aspect of the present invention, the container is formed from a structural shell defining a cavity for receiving spent nuclear fuel, and first and second end apertures opening into the cavity. The shell has a first end portion formed of a first material and a second end portion formed of a second material. The first end portion is joined to the second end portion to form the structural shell. A bearing surface is defined on the first end portion of the shell and is engageable to enable hoisting of the container. The first end portion of the shell is constructed from a first material that has a higher load bearing strength than the second material, to handle the hoisting stress. The container also includes a first closure securable to the first end portion of the shell to seal the first end aperture, and a second closure securable to the second end portion of the shell to seal the second end aperture. The container further includes a radiation absorbing shield layer, which may include both gamma radiation and a neutron radiation absorbing materials. The container is thus constructed so that those areas of the container that are subjected to the greatest stress, e.g. the first end portion, is constructed from the strongest material, such as a high-strength metal alloy. However, those portions of the cask that are not exposed to as high a stress are produced from lower cost materials having a strength that is adequate for the lower loads to be imposed on those portions. In a further aspect of the present invention, a cask is provided that includes a tubular inner shell defining a cavity for receiving spent nuclear fuel, and first and second ends. A tubular outer shell having first and second ends is assembled coaxially over the inner shell to define an annular space therebetween. A radiation absorbing material fills the annular space. An annular member defining a central aperture and a first annular sealing surface is secured about its perimeter to the first ends of the inner shell and the outer shell to create airtight joints with both the inner shell and the outer shell. A first closure plate is releasably securable to the annular member and defines a second annular sealing surface corresponding to the first annular sealing surface defined by the annular member. A seal is positioned between the second annular sealing surface of the first closure plate and the first annular sealing surface of the annular member to create an airtight seal between the first closure plate and the annular member. The cask also includes a second closure plate secured proximate its perimeter to the second ends of the inner shell to create airtight joints with the inner shell. In a further aspect of the present invention, a cask is provided that includes a structural shell defining a cavity for receiving spent nuclear fuel and first and second end apertures. A first closure is securable to the shell to seal the first end aperture. A second closure is securable to the shell to seal the second end aperture. A radiation absorbing shield layer is affixed to the shell. First and second pairs of trunnion mounting structures, preferably configured as tubular sleeves are secured in opposing disposition within apertures formed in the structural shell. First and second trunnions, each defining a base and a bearing surface, are included. The base of each trunnion is releasably securable to a corresponding one of the trunnion mounting structures, whereby the bearing surfaces of the first trunnions can be grasped to hoist the container. The second trunnions are used to provide a point of support and rotation for loading and unloading the cask from its conveyance. In a preferred embodiment, the trunnion mounting structures are configured as annular sleeves that are welded to the structural shell of the cask, within which sleeves the base of the trunnions are received. Because of this construction, fasteners such as bolts used to secure the trunnions to the mounting structures are substantially isolated from tensile and shear loads. In a further aspect of the present invention, the trunnion mounting structures are preferably formed from a high-strength material such as is used to form the portion of the outer shell to which the first trunnions are mounted, thereby providing a strong trunnion mounting without requiring additional plate reinforcement. In a still further aspect of the present invention, improved seal joints are included in the cask. Sealing surfaces of the cask are formed utilizing hardened metal weld overlays, thereby providing sealing surfaces that are not readily subject to permanent deformation upon impact of the cask. In the preferred embodiment, sealing surfaces of closure plates on the cask include grooves formed to define a half-dovetailed cross section for receiving seals. This enables use of either metal or elastomeric seals in the joints, and enables assembly of the joints while the cask is in either the horizontal or vertical disposition. In a still further aspect of the present invention, a cask is disclosed that includes a tubular structural shell defining a cavity for receiving spent nuclear fuel and first and second opened ends. The first closure plate is releasably securable to the first opened end of the shell, whereby when secured to the shell, the first opened end of the shell is sealed, and when released from the shell, loading and unloading of spent nuclear fuel through the first open end into the cavity is permitted. The second closure plate is secured to and seals the second open end of the shell. The second closure plate defines a central access aperture. An access cover plate is releasably securable to the second closure plate to seal the central access aperture. When released from the second closure plate, entry of a ram through the access aperture into the cavity of the shell to facilitate unloading of spent nuclear fuel through the first open end of the shell is permitted. Shield plugs filled with a radiation-absorbing material are provided to cover the trunnion mounting structures and central access aperture formed in the cask during short-term storage and transportation. In another aspect, the present invention relates to a skid for transporting a nuclear fuel transportation cask and containment vessel. The skid supports the cask around the neutron radiation shielding material. The skid includes a supporting member and a retaining member that each include a plurality of parallel spaced-apart plates lying in planes perpendicular to a longitudinal axis of the cask which are connected by a plurality of longitudinal fins parallel to the longitudinal axis of the cask. The longitudinal fins are positioned to mate with structural elements associated with the neutron radiation shielding material to transfer loads from the cask to the skid. The present invention thus provides a cask that is less costly to construct, yet that provides improved safety under impact conditions. Exposure of workers to radiation during transport procedures is also reduced.