Patent Number: 047042478
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 or more such fuel rods are assembled in a fuel rod assembly which includes grids for alignment and support of the fuel rods, lower end fitting, 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 After the nuclear fuel in the fuel rod assembly is spent to a pre-established level, the entire fuel rod assembly is withdrawn from the nuclear reactor and is stored vertically in appropriate metal racks in a wet pool until the radioactive properties have dissipated sufficiently for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 rods 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 At the present time, spent nuclear fuel rod assemblies are withdrawn from the nuclear reactors and are stored vertically in appropriate storage racks under water in storage pools without any deliberate change in the fuel rod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 filling 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. None of these compacting processes have been carried out except with simulated fuel rod assemblies containing simulated fuel rods. 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 1.5 inches in an 8-foot long rod. Such permanent distortion of the fuel rods will interfere with the proposed alignment techniques. A further problem is that the long, thin fuel rods are whippy and 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. Such possibility should be precluded. At the present time there is a need to compact spent fuel rods which are contained in wet storage pools in the fuel rod assemblies. STATEMENT OF THE INVENTION According to the present invention, a method and related apparatus are proposed for transferring spent fuel rods from a fuel rod assembly in an underwater pool 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. According to the invention, the top end of a fuel rod assembly is removed, by cutting or otherwise, and the exposed fuel rod tops are individually connected to individual pulling elements such as wires or other tensioning devices. The wires are presented in an array which corresponds to the array of the individual fuel rods. The wires are drawn through a fuel pin directing chamber such as a transition funnel which has a relatively wide cross-section at its base corresponding to the array of the tops of the fuel rods in the fuel rod assembly. The transition funnel at its top end has a relatively narrow cross-section which corresponds to an array of fuel rods in a compact storage presentation. For each individual wire there is a separate guide within the transition funnel for directing the wire and its associated fuel rod upwardly from the fuel rod assembly through the transition funnel into a permanent storage container which is positioned above the top of the transition funnel. The wires pass downwardly through the container, through the transition funnel, and are connected at their upper ends to a tensioning device such as a reeling drum to permit pulling the wire and its associated fuel rod upwardly out of the fuel rod assembly into a selected one of the passageways through the transition funnel and thence into a pre-established position in a compacted array of fuel rods within the container. Connection means are provided at the bottom end of each wire for securing one fuel rod. A preferred connection means is a helical tension sleeve gripper which has its upper end secured permanently to the lower end of a wire and has an open sleeve depending downwardly which can be fitted over the top end of a fuel rod to provide a tension connection which is easily established and easily terminated. Preferably the individual fuel rods are withdrawn upwardly concurrently from the common fuel rod assembly so that the upper 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 an equilateral 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 tubes 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-section 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. When the present invention is practiced accordingly, 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 for transferring spent fuel rods from a fuel rod assembly directly into a compact 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 without extracting the fuel rods above the surface of the water in the fuel rod storage pool.