Patent Number: 047145838
Section: summary

FIELD OF THE INVENTION This invention relates to a method and apparatus for compacting spent nuclear reactor fuel rods, and more particularly for preparing such spent fuel rods for long-term water pool storage. STATEMENT OF THE PRIOR ART Nuclear reactor installations employ nuclear fuel materials in the form of fuel rods which are supported in fuel rod assemblies. The fuel rods are metal pipes which are filled with nuclear fuel material and are about 0.4-0.6 inch in diameter and from 8 to 15 feet in length. Groups of 64, 128, 220 or more such fuel rods are assembled in a fuel rod assembly which includes grids for alignment and support of the fuel rods, a lower end fitting, an upper end fitting, and guide tubes. The fuel rod assembly is introduced into a nuclear reactor as the fuel source. After the nuclear fuel in the fuel rod assembly is spent to a pre-established level, the entire fuel rod assembly is withdrawn from the nuclear reactor and stored vertically in appropriate metal racks in a wet pool until the radioactive properties have dissipated sufficiently for transfer to other storage locations. Within the fuel rod assembly, the individual fuel rods are spaced apart in a pre-established array, usually a rectangular array. The fuel rod assemblies are spaced apart in the array and are maintained under water in the reactor for the purpose of moderating or slowing the neutrons. In the fuel rod assembly, the ratio of cross-sectional area of fuel rod to cross-sectional area of water is approximately 1:1. At the present time, spent nuclear fuel rod assemblies are withdrawn from the nuclear reactors and are stored vertically in appropriate storage racks under water in storage pools without any deliberate change in the fuel rod assembly. The fuel rod storage pools are filled with the spent fuel rod assemblies whose activity has dissipated as a result of extended storage in the pool. A number of suggestions have been made for removing long-term storage fuel rod assemblies from the pool and for withdrawing individual spent fuel rods from the fuel rod assembly and thereafter for assembling the individual spent fuel rods in new containers or canisters wherein the fuel rods are more closely aligned, i.e., more densely compacted, and for returning such newly-filled canisters to appropriate storage racks within a water storage pool for long-term storage or until appropriate fuel recovery processing is economically feasible. Some of the anticipated difficulties with the proposed fuel rod compacting processes which have been suggested arise from the knowledge that the actual fuel rods are twisted and bent out of alignment as a result of their long-term exposure in nuclear reactors. In some cases, the distortion may be as much as 5 inches in an 8-foot long rod. Such permanent distortion of the fuel rods will interfere with the proposed alignment techniques. The casing of the fuel rods should be handled by using procedures and equipment designed to accommodate embrittlement due to irradiation in the nuclear reactor. A further problem is that the long, thin fuel rods are whippy and therefore likely to impact with each other when pulled from the fuel assembly. Such impacting could cause fuel rod breakage. Moreover, the fuel rods may be difficult to manipulate. A still further problem relates to the inherent safety of compacting spent fuel rods. There is a possibility that the fuel rods might become spaced apart by a critical distance while removed from the fuel rod assembly and before compaction and confinement in a storage canister. Moreover, the fuel rods might be dropped in the water pool or broken due to embrittlement during multimanipulation before confinement in a storage container. Such possibilities should be precluded. SUMMARY OF THE INVENTION According to the present invention, a method and related apparatus are provided for transferring spent fuel rods from a fuel rod assembly in an underwater pool or in a hot cell directly into a fuel rod canister where the density of the fuel rods greatly exceeds the fuel rod density in the fuel rod assembly. As a result of the present invention, the spent fuel storage capacity in a particular water storage pool can be approximately doubled. Moreover, the fuel rod consolidation process of the present invention is carried out without altering the relative position of the fuel rods whereby after consolidation, the identity of a fuel rod is known at each position in the fuel rod canister which facilitates accounting procedures. According to the invention, an end of a fuel rod assembly is removed by cutting or otherwise and grippers of a movable gripper assembly are passed between rows of the exposed end portions of the array of fuel rods to simultaneously grip the fuel rods. The gripper assembly is reciprocated along a rectilinear path between a fuel rod gripping position at the exposed end of the fuel assembly and a remote fuel rod release position which is adjacent an entry end of a fuel rod directing chamber such as a transition funnel which has fuel rod receiving openings corresponding to the array of fuel rods in the fuel rod assembly. The operation of the reciprocating gripper serves to withdraw an increment of length of all the fuel rods in unison from the fuel assembly in one axial direction for entry and passage in the fuel rod directing chamber. The transition funnel at its fuel rod discharge end has a relatively narrow cross section which corresponds to the cross section of the desired compacted bundle of fuel rods presented to the storage container. For each individual fuel rod, there can be a separate guide within the transition funnel for directing a fuel rod from the fuel rod assembly through the transition funnel into a permanent storage container. The fuel rod consolidation process is thus carried out by positioning the transition funnel between the fuel rod assembly and a permanent storage container in a tandem arrangement so that the spent fuel rods pass in only one direction directly from the fuel rod assembly through the transition funnel into the storage container. This tandem arrangement of components can be provided in a hot cell or it can be provided beneath the water surface in a water pool. In the either event, the spent fuel rods move along a generally horizontal path or a generally vertical path. In the latter event, the fuel storage container can be located either above or below the transition funnel. Thus, the storage container can be positioned so that the spent fuel rods either move upwardly into the storage container or downwardly into the storage container. The passageways through the transition funnel direct the spent fuel rods into pre-established storage positions in a compacted array of fuel rods within the container. A second gripper is arranged to reciprocate along a rectilinear path in a gap established between the discharge end of the transition funnel where the fuel rod bundle is gripped and the entry end of a storage canister where the fuel rod bundle is released. The second gripper can embody a construction for gripping the entire bundle of fuel rods since they are in a compacted array. The fuel rods are advanced into the storage container by reciprocating the second gripper in the same manner as the reciprocating motion of the first gripper. When the fuel rods are advanced in a generally horizontal plane from a fuel assembly through a transition funnel into a storage container as well as the aforesaid arrangement wherein the fuel rods are advanced upwardly from the fuel assembly through a funnel into a storage container, it is preferable and, in some instances, it may be desirable to provide additional retention means to support the fuel rods during the return movement of each of the first and second grippers. Such retention means can be located at the entry side of the transition funnel and/or storage container. The retention means may, when desired, include the use of conventional grids provided in the fuel assembly to support the fuel rods at various spaced-apart locations along the length of the fuel assembly. In this regard, such grids conventionally provide resilient spring clips to apply a spring tension force against the outer cylindrical surface of the fuel rod. The spring forces can be utilized to prevent unwanted axial movement of the fuel rods while the grippers are returned from a release position to a gripping position as described hereinbefore. When desired, a gripper at the entry and or exit end of the transition funnel can be attached to the funnel and the funnel with one or both grippers attached thereto can be reciprocated along a rectilinear path between gripping and release positions to move the fuel rods from the fuel rod assembly. Preferably, the individual fuel rods are withdrawn concurrently from a fuel rod assembly so that the leading ends of all of the fuel rods enter into the container at about the same level to facilitate stacking within the container. Preferably, within the container, the array of spent fuel rods is a triangular array which provides maximum fuel rod density in the container. Preferably, the fuel rod density in the container is approximately twice that of the fuel rod density in the fuel rod assembly. The transition funnel is so arranged that the guide tubes therein merge toward one another. As a consequence, the fuel rods, in passing from the fuel rod assembly into the fuel rod container, do not move apart so that critical distances between fuel rods cannot occur. By providing fuel rod containers of the same cross-sectional dimensions as the fuel rod assemblies, the containers can be stored in the same underwater fuel rod storage racks which have been employed for the fuel rod assemblies. In addition, it is possible to transform the consolidated rods to other geometries, i.e., rhombic, to maximize storage in a cylindrical container which can be used for transporting and/or permanent storage at a local or remote storage site. When the present invention is practiced, the capacity of the fuel rod storage pools for spent nuclear fuel rods can be approximately doubled. The structural components of the empty fuel rod assembly are collected and stored for appropriate disposal. Accordingly, it is an object of this invention to provide a method and apparatus for moving spent fuel rods from a fuel rod assembly directly into a fuel rod container for compact storage of the spent fuel rods. It is a further object of this invention to carry out the described method and apparatus including reciprocating grippers that move a distance corresponding to only a small increment of the fuel rod lengths which minimizes the space required to consolidate the fuel rods while causing the fuel rods to move unidirectionally from a fuel rod assembly and into a storage container.