Patent Number: 044501340
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the transfer of nuclear fuel between storage pools and shipping casks. More particularly, the invention relates to an improved method and apparatus which substantially reduces the probability of nuclear contamination during the transfer of nuclear fuel elements between a fluid-filled storage pool and a shipping cask. 2. Description of the Prior Art In the operation of nuclear reactors, a controlled nuclear chain reaction is maintained in a reactor core by fuel elements containing radioactive uranium. Typically, these elements comprise long, thin, tubular structures made of steel, clad with a zirconium alloy and within which are packed a number of pellets containing a radioactive uranium composition. The fuel elements which can be efficiently utilized for extended periods of time eventually fail due to reduced activity or physical integrity. The resulting spent fuel elements must then be removed and replaced to assure safe, efficient reactor operation. After removal from the reactor core, the spent fuel elements are generally transferred to a fluid-filled fuel storage pool for retention pending shipment to a disposal or recycling facility. Similarly, fresh fuel elements can be stored in fluid-filled pools after shipment to the nuclear reactor location but prior to their placement in the core. Spent fuel elements are typically shipped from one point to another in sealed, fluid-filled, shielded containers called casks. Transfer of the fuel elements from the pool to the casks, and from the casks to the pool, must usually be done without removing the fuel from the fluid, using constantly-filtered water, to assure maximum safety. However, the art has experienced substantial difficulty in safely and efficiently effecting such transfer. Water employed in the pool or gas or air surrounding the spent fuel will be contaminated regardless of precautions taken. Moreover, even if a system were designed to have essentially zero contamination in the environment, prudence would still dictate treating it as if it were contaminated to guard against possible anomalies in the system. Among the early prior art transfer systems were those which immersed the cask in the pool to allow transfer without removing the fuel from the water. There were many risks attendant with such transfer systems, not the least of which was the total wetting of the cask exterior with contaminated water. The contaminated wash water produced had to be disposed of. Faced with this problem of cask contamination, there have evolved a number of systems for effecting transfer without wholly immersing the cask. These systems have come to be known as dry cask systems. Unfortunately, known dry cask systems have exhibited various drawbacks. For example, U.S. Pat. No. 3,765,549 presents a system employing a pair of independently-actuatable, concentric bellows mounted beneath a fuel storage pool and circumscribing a hatch therein. According to that disclosure, a fuel cask is positioned directly below the hatch and the bellows are extended downwardly to seat against the cask and form a transfer channel between the pool and the cask. The channel is then flooded, the hatch opened, and transfer effected. The particular arrangement of bellows and supporting devices shown, however, render the system susceptible to serious losses of contaminated material in the event the cask to bellows sealing surfaces do not match perfectly or if the cask sealing surface becomes dirty or damaged in transportation. This system does not provide secondary means for preventing leakage and is susceptible to leakage in the event of moderate seismic disturbances. In an attempt at overcoming certain of the difficulties of the bellows arrangements, U.S. Pat. No. 3,910,006 discloses that a direct contact between a cask and the underside of a transfer pool can be employed. This arrangement is said to eliminate the problems associated with large differential pressures on the bellows and the large amounts of water that the bellows arrangements must employ in the transfer channel. Here again different casks with different fabrication tolerances have to be matched with sealing surfaces beneath the fuel transfer pool and leakage cannot be prevented if casks do not properly match the surface or if their sealing surfaces become dirty or damaged in transportation. This system does not provide a secondary means for preventing leakage and is also susceptible to leakage in the event of a moderate seismic disturbance. In U.S. Pat. No. 3,883,012 there is described yet another dry cask system. In particular it is disclosed that the fuel cask can be positioned within a tank to accommodate casks of varying dimensions and to avoid some of the risks that might still surround the use of systems such as that described in U.S. Pat. No. 3,765,549. While this disclosure suggests lateral seismic restraints on the cask tank, no means are identified for suitably positioning the cask transporter and for restraining the cask tank while at the loading terminal. The art has thus evolved fuel transfer systems culminating in a number of dry cask systems. There remains however, a need for a dry cask system which permits safe and efficient transfer of nuclear fuel elements between casks and fluid-filled storage pools. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved method and apparatus for safely and efficiently transferring nuclear fuel elements between storage pools and dry casks. It is a further and more specific object of the invention to provide an improved method and apparatus for effecting transfer of nuclear fuel elements between storage pools and dry casks supported within cask tanks which are transported between transfer stations and decontamination stations by means which also provide effective sealing between a fuel transfer pool and a cask tank, accurate repeat positioning of the system and seismic restraint during transfer operations. Another object of this invention is to provide a novel port hatch means for efficient transfer and maximum safety from seismic disturbances. It is another object of this invention to provide an improved dry cask method and apparatus for transferring nuclear fuel elements between a storage pool and a dry cask positioned within a cask tank wherein the cask tank is advanced into mating contact with an undersurface of the storage pool. Yet another object of this invention is to provide an improved dry cask method and apparatus for transferring nuclear fuel elements between a storage pool and a cask positioned within a tank wherein the tank is advanced into engagement with a surface of the storage pool and the weights of the cask, the cask tank and a column of water in the transfer pool is removed from a transport means and is supported by a lift means on a transfer corridor floor. The method of the invention in one aspect comprises the steps of supporting and restraining a nuclear fuel element transfer cask in a tank which is supported upon a transport means at a first terminal location; advancing the transport means and supported tank to a second terminal location adjacent a port of a nuclear fuel or transfer pool; supporting and restraining the tank at the second terminal location; establishing a fluid-sealed channel between the port and an interior of the cask; flooding the sealed channel; providing access between the fuel storage pool and the cask through the port; and, transferring fuel elements between the cask and the pool. In accordance with another feature of the method of the invention, the tank is advanced into engagement with a surface of a transfer pool port, and is restrained in contact with the surface. A preferred embodiment of the invention provides for unweighting the tank from the transport means. Yet another object of this invention is the provision of an improved method and apparatus for transferring nuclear fuel elements between a storage pool and dry cask supported within a cask tank, wherein paired bellows arrangements have nuclear shielding material positioned therebetween. It is yet another object of this invention to provide an improved method and apparatus for transferring nuclear fuel elements between storage pools and dry casks supported within cask tanks wherein novel bellows arrangements are efficiently engaged in sealing contact. An apparatus in accordance with the invention for transferring nuclear fuel elements between a fluid-filled storage pool and a fuel element cask, including a tank transport means for supporting and advancing a cask tank between a first terminal location and a second terminal location adjacent a port in a fuel transfer pool. A cask tank means is provided for supporting and restraining the cask within the tank during transport and transfer and for protecting the cask from contamination during the transfer. A port closure means operable between a closed position for sealing the port in the pool, and an open position for enabling the passage of fuel elements between the fuel transfer pool and the cask interior is also provided along with a means for coupling the tank to the fuel transfer pool and for restraining the tank. In a preferred embodiment of the apparatus of the invention, a means is provided for unweighting the cask tank from the transport means at the second location and for advancing a surface of the tank into engagement with a transfer pool port surface and for restraining the tank in engagement with the surface. An alternative embodiment of the apparatus provides means for supporting the cask tank on the transport means and for advancing the cask tank into engagement with the port surface. Other features of the apparatus provide for restraining and accurately positioning the transport means and means integral with the transport means for restraining the tank. In still other alternative embodiments of the invention, the cask tank is supported on the transport means at the second location and extensible coupling means are provided for coupling between the tank and the port. Other features of the apparatus provide for restraining and accurately positioning the transport means and means integral with the transport means for restraining the tank.