Patent Number: 054385970
Section: summary

FIELD OF THE INVENTION The present invention generally relates to containers fur storage and transportation of spent nuclear fuel, and in particular, to containers For transportation of spent nuclear fuel across areas accessible to the public. BACKGROUND OF THE INVENTION In a nuclear reactor, the fissionable material gradually becomes spent and must be removed. Since the spent fuel contains fission by products which are highly radioactive, and which generate large amounts of heat, the spent fuel is usually temporarily stored in the reactor's spent fuel pool. The spent fuel pool is a pool of water of sufficient volume to prevent the escape of harmful radiation, and to absorb and dissipate the heat generated by the decaying fissionable material. Alternatively, the spent fuel may be temporarily stored in a hot cell. That is, a heavily shielded structure having the capability to prevent the escape of harmful radiation, while absorbing and dissipating the heat generated by the spent fuel. Generally, there is limited storage space in a nuclear reactor's spent fuel pool, or in its hot cell. Thus, the spent fuel must be moved to a storage site to make room for additional spent fuel. In some cases, there is a desire to shut the nuclear reactor down, and remove all fissionable material, in which case, all of the fissionable material must be removed to a storage site. There are two primary problems in the transportation of spent fuel. The most difficult problem is the transportation of spent fuel that includes failed fuel rod assemblies. Typically, nuclear fuel is formed of numerous small pellets that are inserted into a hollow rod. In some cases the rods become damaged and allow some of the nuclear fuel pellets to escape. These damaged rods are known as failed fuel rods. Further, in some cases during nuclear reaction of the fuel, the pellets disintegrate into sand-sized particles, capable of easily escaping from a failed fuel rod. The fuel rods themselves are arranged into assemblies including several fuel rods. Thus, a fuel rod assembly including a failed fuel rod is termed a failed fuel rod assembly. An important pan of transporting and storing spent fuel is avoiding criticality. This is achieved by carefully arranging the spent fuel rod assemblies so that there is a minimum distance between each assembly, such that there is little chance of neutron multiplication occurring to the point of criticality. In the case of failed fuel rod assemblies, however, fissionable material can escape from failed rods, and potentially accumulate near enough to other fissionable material that criticality is achieved. One attempted solution to the foregoing problem has been simply to store failed fuel rod assemblies indefinitely in a nuclear reactor's spent fuel pool or hot-cell. The problem with storing failed fuel rod assemblies indefinitely, however, is that there is limited storage space in a nuclear reactor's spent fuel pool or in its hot-cell, and in some cases there is a desire to completely shut a nuclear reactor down, and remove all fissionable material, including that contained in failed fuel rod assemblies. Another attempted solution has been to transport failed fuel rod assemblies in fuel transportation containers designed for undamaged fuel rod assemblies. The foregoing attempted solution, however, has required that substantially fewer failed fuel rod assemblies be transported per container, compared to the number of undamaged fuel rod assemblies that can be transported in the same container. By transporting fewer failed fuel rod assemblies, even if some fissionable material escapes from the failed fuel rods, and accumulates near other fissionable material in the container, there is not enough fissionable material in the entire container to pose a significant risk of criticality. The problem with the foregoing solution, though, is it wasteful of resources, because significantly fewer failed fuel rod assemblies can be transported per container, relative to the number of undamaged fuel rod assemblies that can be transported in the same container. Another, attempted solution has been to transport failed fuel rod assemblies in fuel transportation containers designed for transporting fissionable material in the form of rubble. That is, the fissionable material is not in the form of rods, but is in the form of small particles. Thus, the failed fuel rods are broken up into rubble, and placed in the container. The problem with that solution, however, is that the method is inefficient for three principle reasons. First, the failed fuel rod assemblies be broken up. Second, such containers are capable only of transporting comparatively few failed fuel rod assemblies. Finally, the transportation container is only designed for transportation, not storage. Thus, once the fissionable material has been transported to another location, the container must be unloaded in a fuel pool or in a hot cell, and other arrangements made to store the fissionable material. The present invention solves the foregoing problems, and provides a device for transporting and storing failed fuel rod assemblies at a storage site, other than in a spent fuel pool or hot cell. The other major problem with transporting spent nuclear fuel is that United States law imposes stringent safety requirements even on containers used to transport undamaged fuel rod assemblies. The relevant law imposes significantly more restrictive requirements with respect to the transportation of spent nuclear fuel across areas accessible to the public, as opposed to areas inaccessible to the public. State of the an spent fuel transportation containers for areas accessible to the public are casks with individual compartments. The fuel rod assemblies are loaded into individual compartments in the casks in a spent fuel pool or a hot cell. The purpose of the individual compartments within each cask is to ensure sufficient spacing between adjacent fuel rod assemblies to avoid any danger of criticality. The fuel rod assemblies are loaded into the cask in a spent fuel pool or hot cell. Upon reaching the storage location, the fuel rod assemblies must be removed from the cask in a spent fuel pool or hot cell, and then stored. In contrast, state of the art spent fuel transportation containers for areas inaccessible to the public are typically a sealed canister placed within a cask. The fuel rod assemblies are loaded into individual compartments in a canister in a spent fuel pool or a hot cell. The canister is then sealed and placed in a cask. When the cask/canister assembly reaches the storage site, the canister is removed from the cask, stored, and the cask may be reused, which is a much more efficient process. Nonetheless, the cask/canister method cannot be used for transportation in areas accessible to the public because they fail to meet the requirements imposed by U.S. law. Accordingly, there is a need for an invention that provides for the transportation and storage of failed fuel rod assemblies, and for a cask/canister device for transportation and storage of spent fuel across areas accessible to the public. The present invention provides a solution, wherein a cask/canister device can be used, and additionally may be used with existing casks, resulting in much greater efficiency in the transportation over public thoroughfares and storage of spent nuclear fuel. SUMMARY OF THE INVENTION In one aspect, the present invention relates to a container for receiving a structurally damaged nuclear fuel assembly, the container being for the subsequent storage and transportation of the nuclear fuel assembly. The nuclear fuel assembly includes fissionable material, and is received by the container from within a fuel pool. The container includes an elongated receptacle that forms an enclosure. The receptacle includes an open end for receiving the structurally damaged nuclear fuel assembly. A cover is provided to mate with, and close the open end of the receptacle. Further, a drainage passage is defined in the container, so that liquid can be drained from the interior of the receptacle to the exterior of the receptacle. Additionally, the drainage passage includes a restrictor that prevents the passage of fissionable material through the drainage passage. The container may also include an exterior projection for receiving fuel handling tools used to handle the container. In another aspect, the present invention relates to a canister for receiving structurally damaged nuclear fuel assemblies, and for the subsequent storage and transportation of the nuclear fuel assemblies. The nuclear fuel assemblies include fissionable material, and are received by the canister from within a fuel pool. The canister includes a basket assembly having a plurality of apertured plates, and structural members interconnecting the apertured plates. The structural members maintain the plates in a spaced apart relationship, axially aligning the apertures in each plate into a plurality of rows. The basket assembly is received in an exterior shell that forms an enclosure open at one end. The basket assembly is surrounded by the shell, and is oriented such that the longitudinal axis of each row is substantially parallel to the longitudinal axis of the shell. A container is inserted into each row of axially aligned apertures. Each container is for containing a damaged nuclear fuel assembly, and includes an elongated receptacle that forms an enclosure, having an open end. The structurally damaged nuclear fuel assemblies are inserted through the open end of the enclosure into the receptacle. A cover is provided to mate with the open end of the receptacle, and substantially close the open end of the receptacle. Moreover, a drainage passage is defined in each container, for draining liquid out of the container. The drainage passage includes a restrictor that prevents the passage of fissionable material. A lid is also provided to mate with the open end of the shell, thereby closing the open end of the shell. Further, the exterior of each container may also include a projection for receiving fuel handling tools to remove and insert the containers into the canister. In a further aspect, the present invention includes a canister for storing and transporting nuclear fuel assemblies which includes a basket assembly. The basket assembly again includes a plurality of apertured plates, and structural members interconnecting the apertured plates. The structural members maintain the plates in a spaced apart relationship with the apertures in each plate axially aligned into a plurality of rows. An exterior shell, forming an enclosure open at one end, receives and surrounds the basket assembly. The basket assembly is oriented within the shell such that the longitudinal axis of each row is substantially parallel to the longitudinal axis of the shell. A plurality of guide sleeves are provided with the basket assembly, the number of guide sleeves corresponding to the number of rows of axially aligned plate apertures. Each guide sleeve has a longitudinal axis that is generally coincident with a corresponding row, and includes a first structural layer, a neutron absorbing layer, supported by the first structural layer; and a second structural layer, structurally supporting the side of the neutron poisoning layer opposite the first structural layer. A lid is included to mate with the open end of the shell, thereby closing the open end of the shell. Preferably, the first structural layer comprises a hollow steel jacket inserted into each row of axially aligned apertures. Other features of the present invention will become apparent from the following detailed description.