Patent Number: 052788792
Section: summary

TECHNICAL FIELD The present invention relates to consolidating spent nuclear fuel rod assemblies, and, more particularly, to a grid crusher apparatus and method for crushing and storing spacer grids from within spent nuclear fuel rod assemblies. BACKGROUND OF THE INVENTION Nuclear fuel assemblies for powering nuclear reactors generally consist of large numbers of fuel rods contained in discrete fuel rod assemblies. These assemblies or cells generally consist of a bottom end fitting or nozzle, a plurality of fuel rods extending upwardly therefrom and spaced from each other in a square or triangular pitch configuration, spacer grids situated periodically along the length of the assembly for support and orientation of the fuel rods, often a plurality of control guide tubes interspersed throughout the rod assembly, and a top end fitting or cap. Moreover, the assembly is installed and removed from the reactor as a unit. When the nuclear fuel rods have expended a large amount of their available energy, the fuel rods are considered to be "spent," and the fuel rod assembly is pulled from the reactor and temporarily stored in an adjacent pool until the assemblies are transported to a reprocessing center or to permanent or temporary storage. Even though the rods are considered "spent," they are still highly radioactive and constitute a very real hazard both to personnel and to property. In general, there are a number of alternatives available for disposition of the radioactive spent fuel rods, none of which is totally satisfactory. The fuel rod assemblies can be enclosed in a suitable basket and cask arrangement and transported to a storage facility, or possibly, to a reprocessing plant. A second alternative is to store the spent fuel in a dry storage system. Dry storage entails either the use of a large number of metal casks or the building of massive concrete containers either above or below ground, which is a very expensive process, and, where the storage system is above ground, it is often not acceptable to people living or working in its vicinity. A third alternative is the storage of the fuel units in the existing water pool originally designed for temporary storage. This type of storage is the simplest and cheapest, since the fuel rod assemblies can remain in the pool and be left there until the appropriate governmental agency or other agency collects them, often at the end of the life of the nuclear plant. However, such storage pools have a limited capacity, and, where they are adjacent to the nuclear reactor, necessitate the construction of a new pool when one becomes full. Numerous attempts have been made to increase the capacity of a pool through a process known as fuel rod compaction or consolidation. This process, in brief, comprises removing the fuel rods from each fuel rod assembly and placing them in a storage canister where they are placed in rows with minimal spacing. It is possible, with this process, to place the fuel rods from two or more fuel assemblies into a single storage canister, thereby achieving approximately a 2:1 reduction in required pool volume, or, conversely, a 2:1 increase in pool storage capacity. However, successful consolidation has been an elusive goal for a number of reasons. Inasmuch as the pools are approximately forty feet deep, and inasmuch as the rods must remain immersed in the water at all times, all of the consolidation operations must be performed under the shield and cooling water. In addition, even though the rods are kept under water, the process could be quite hazardous to personnel performing the operation. Prior art arrangements for achieving rod consolidation have included a system whereby the rods are pulled out row-by-row, as in, for example, a 14.times.14 matrix of rods, lifted and deposited in a tapered interim storage container, which tapers from a large area top opening to a bottom that has the area of a storage canister. After the intermediate container has the rods from approximately two fuel assemblies deposited therein, the intermediate container is placed over a storage canister, the bottom plate of the tapered container is lowered to cause the rods to slide into the storage canister. If the rods jam or stick, as they often do, they must be pushed from above the pool by operators using long rods. This last operation is made more difficult in that the rods develop on their outside surfaces what is referred to in the trade as "crud". When the fuel rods are pulled, this radioactive crud is scraped off and clouds the water making it difficult for the operators to see what they are doing and contaminating the pool. The method just described has proven to be quite slow and complicated, and can be hazardous to personnel. Another problem associated with nuclear fuel rod consolidation is the disposal of spacer grids situated in the nuclear fuel rod assemblies for supporting the fuel rods and for maintaining the spacing between the fuel rods. The spacer grids are generally rigid metallic material, and there are usually about seven spacer grids in each rod assembly, or as few as three in gas cooled reactor fuel elements. Conventionally, during the process of fuel rod consolidation, the spacer grids have been crushed by a compactor in the pool, and the crushed remains are then placed in a storage canister. Oftentimes, the compactor has a first ram for crushing the spacer grid in a first direction and a second ram for crushing the spacer grid in a second direction which is orthogonal to the first. As a result, the spacer grids are compacted into a rectangular block which are discarded somewhere in the storage canisters. However, crushing the spacer grids has been problematic in the art. During the crushing process, the rigid spacer grids break up and/or shatter, resulting in jamming of the compactor rams and creating a contamination problem in the surrounding pool area. Furthermore, the compactor ram surfaces which come in direct contact with the spacer grids during crushing operation become radioactively contaminated and must be disposed of in the storage canisters. Hence, the disposal and consolidation problem is further compounded. SUMMARY OF THE INVENTION The present invention is a method and apparatus for efficiently and reliably crushing and storing spacer grids from a nuclear fuel rod assembly during nuclear fuel rod consolidation. The apparatus and methodology of the present invention may, for example, be primarily used in an automated nuclear fuel rod consolidation system which comprises a commercially available five or six axes robot mounted on the operations floor along the side of the storage pool. Directly below the robot within the pool, at a depth of, for example, twenty-five feet, is an apertured work table, and resting on the floor of the pool directly below the work table is a header and support base, which includes a manifold for a pair of vacuum filter assemblies which are mounted to, and extend upwardly from, the support base. Extending vertically from the support base and into openings in the work table are a plurality of holders configured to support fuel assemblies or fuel rod canisters, which are accessible from above the work table. A plurality of individual or multiple purpose long reach tools are mounted on racks above and to either side of the work table. Each of the tools has a quick change coupling mounted to its upper end which matches and is adapted to couple with a corresponding quick change coupling on the end of the robot arm. Locating pins are mounted on the top surface of the work table, and a position sensor carried by one of the long reach tools sends signals to the computer to give precise locations on the work table, thereby enabling the computer to determine the exact location of all components in the system. In operation, three or four spent fuel rod assemblies are transferred, under water, to the fuel rod assembly holders as dictated by the number of cells provided in the work table for fuel rod assemblies. Empty canisters are transferred to canister holders and their lids are installed in a well located in the work table. The upper end fittings of the fuel rod assemblies are then cut away by a long reach tool having a cutter on its lower end and placed in a scrap canister. Alternatively, the upper end fittings can be unbolted on those fuel assembly types which permit this type of removal. The computer next directs the robot to couple with a fuel rod transfer tool having a collet for grasping a fuel rod and pulling it out of the rod assembly up into the tool. When this occurs, crud is scraped off of the rod, but, because of the downward water current created by the filter units with their associated pumps, the crud passed down the holder into the manifold and up through the filter, thereby preventing clouding of the water and contamination of the pool. To ensure that the rod transfer tool centers exactly over a rod to be pulled, an apertured funnel guide plate is placed over the fuel rod assembly, which precisely locates every fourth rod in the assembly. The funnel guide plate is indexed by means of locating pins that fit into holes in the work table or by slots on the underside of the plate that engages the top edges of the canister so that ultimately all of the rods are pulled. After the canister is completely filled, the skeleton of the fuel rod assembly, comprising guide tubes and spacer grids, is subjected to compaction. The guide tubes are cut above and below the grids, and each tube section is fed into the tube compactor where it is repeatedly cut and flattened into small pieces and then dropped into the scrap canister. Finally, the spacer grids are introduced into the grid crushing apparatus of the present invention, where the spacer grids are crushed in accordance with a novel methodology. The grid crusher apparatus in accordance with the present invention comprises a ram assembly and a basket driving means. The ram assembly has a sleeve ram and a central ram. The sleeve ram surrounds the central ram and is longitudinally removable within the sleeve ram. The central ram protrudes from the sleeve ram at a ram contact end and is retractable upon application of a preselected force to the central ram. When the central ram is retracted into the sleeve ram, the central ram is flush with the sleeve ram. The basket driving means moves a basket containing a spacer grid towards the ram contact end of the ram assembly. As the basket driving means moves the basket, the spacer grid within the basket is crushed within the basket by the combination of successive forces from the central ram and the sleeve ram, respectively. Essentially, the central ram provides a first ramming force which causes the outer walls of the spacer grid to fold inwardly so that the ram assembly does not get wedged in the basket by the spacer grid which is being crushed, and then a second ramming force to complete the crushing operation. In accordance with the novel methodology of the present invention, the grid crusher apparatus is operated as follows. A spacer grid is placed in the basket. The basket is then moved toward the ram assembly. Next, the central portion of the spacer grid is depressed during the movement of the basket toward the ram assembly. Finally, the remaining outer portion of the spacer grid is depressed after full depression of the central portion. Many other spacer grids can be crushed in the same manner within the same basket until the basket is full, at which point, the basket is placed in a storage canister. Other advantages and features of the present invention will be more readily apparent from the following detailed description, when read in conjunction with the accompanying drawings.