Patent Number: 047215976
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 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 After the nuclear fuel in the fuel rod assembly is spent to a pre-established level, the entire fuel rod assembly is withdrawn from the nuclear reactor and is stored vertically in appropriate metal racks in a wet pool until the radioactive properties have dissipated sufficiently for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 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 At the present time, spent nuclear fuel rod assemblies are withdrawn from the nuclear reactors and are stored vertically in appropriate storage racks under water in storage pools without any deliberate change in the fuel rod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. 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. The casing of the fuel rods is usually embrittled due to irradiation in the nuclear reactor. 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. 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, the top end of a fuel rod assembly is removed, by cutting or otherwise, and the exposed fuel rod tops are individually connected by welding to individual pulling members such as tubes or other tensioning devices. The pulling elements are presented in an array which corresponds to the array of the individual fuel rods. The pulling elements are drawn through a fuel rod 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 pulling element there is a separate guide within the transition funnel for directing the pulling element and the fuel rod which is welded thereto so as to pull the fuel rod 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 pulling elements each includes welding means at their lower ends which can pass downwardly through the container and through the transition funnel. The upper ends of the pulling elements are connected to a tensioning device such as a reeling drum to permit movement of each pulling element and the 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. Welding means are provided at the bottom end of each pulling element for securing one fuel rod. A preferred pulling element is a plastic tube and a preferred welding means is an arc welder, e.g., an inert gas metal-arc type welder having an electrode extending to form an arc gap with the bottom wall of a fixture that is releasably connected to the plastic tube. After the fixture is welded to the fuel rod, the fuel rod is pulled into the canister. Thereafter, the pulling element is separated from the fuel rod by breaking the weld joint or by withdrawing the plastic tube from the fixture. The upper ends of the plastic tube forming the pulling elements are connected to a wench by a cable through a manifold used to supply inert gas to the plastic tubes for the welding process. 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 a rectangular 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. Also, since the fuel rods are advanced under tension they can be straightened in the guide tubes without breakage due to irradiation embrittlement. 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. 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 in an area of the storage pool which is minimized by the fact that the fuel rods are moved unidirectionally from a fuel rod assembly and to a standard canister.