Patent Number: 052746828
Section: summary

FIELD OF THE INVENTION The present invention relates to the consolidation of spent nuclear fuel rods, and, more particularly, to a storage canister for spent rods and the method of loading and sealing it. 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, 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, when one becomes full the construction of a new pool is necessary. 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, most often in a square or triangular array. 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 1:2 reduction in required pool volume, or, conversely, a 2:1 increase in pool storage capacity. Examples of such rod consolidation systems are shown in U.S. Pat. No. 4,659,536 of Baudro; U.S. Pat. No. 4,731,219 of Beneck, et al.; and U.S. Pat. No. 5,000,906 of Ellingson, et al. 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 shielding and cooling water. In addition, even though the rods are kept under water, the process could be quite hazardous to personnel performing the operation. The rods themselves, because of their dimensions, for example, one-half inch in diameter and eighteen feet long, are subject to flexing and bending. As a consequence, placing the rods in a desired location can be quite difficult. Prior art arrangements, such as the Beneck et al. system, 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, and the bottom plate of the tapered container is lowered or removed 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. After a storage canister has been filled, it is generally capped by a lid member. In accordance with U.S. government requirements for storing spent nuclear fuel rods, the lid must be lockable or self-locking with tamper proof, or tamper indicating mechanisms. Meticulous records must be kept by the operator as to just what has been put in the canister and when, and the lock and tamper indicator is intended to insure that what has been put in the canister remains there, undisturbed. An example of a storage canister and a lid locking mechanism is disclosed in U.S. Pat. No. 4,474,727 of Kmonk, et al. SUMMARY OF INVENTION The present invention is a storage canister for spent nuclear fuel rods and the method of loading and locking the canister with a tamper indicating mechanism. 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 placed 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, for example. The funnel guide plate is indexed by means of locating pins that fit into holes in the work table or by slots or channels on the underside of the plate that engages the top edges of the canister so that ultimately all of the rods are pulled. The funnel guide plate is the subject of U.S. patent application Ser. No. 07/831,404, filed Feb. 5, 1992, of which this application is a continuation-in-part. 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 a grid crushing apparatus, where the spacer grids are crushed in accordance with a novel methodology which forms the basis of U.S. patent application Ser. No. 07/570,812, filed Aug. 22, 1990, also a continuation in part of U.S. patent application Ser. No. 07/831,404 filed Feb. 5, 1992. In accordance with the present invention, in a preferred embodiment thereof, the storage canister has a loading configuration and a storage configuration and comprises an elongated hollow tubular member, rectangular or square shaped in cross-section, having a plurality of spacers or dividers therein and affixed thereto, defining an array of rod locations within the canister. The bottom wall of the canister has an opening therein for permitting passage of water therethrough, and an apertured base plate, axially slidably located in the bottom of the canister, is adapted to permit ingress and egress of water through the bottom wall in a first, or loading configuration position, and to block, confine, or restrict passage of water in a second, or storage configuration position. Mounted on the top surface of the base plate and spaced therefrom is a plurality of horizontal apertured plates, spaced from each other. The apertures in the plates are aligned and oriented with respect to each other so that some fuel rods extend through the array with their tips resting on the base plate; some fuel rods pass through the array to where their tips rest upon the first plate of the array which is adjacent to the base plate; some fuel rods pass through the array to where their tips rest upon the second plate in the array, and some rods have their tips resting upon the top plate of the array. As a consequence, adjacent rods are vertically displaced from each other so that the rod grasping tool can grasp a rod without interference from adjacent rods, the highest rods in the array being the last to be inserted. The storage canister has mounted therein a plurality of corrugated plate members or spacers which extend along a portion of the interior length of the canister. Adjacent pairs of the plates define a plurality of longitudinally extending rod locating passages so that when a rod is inserted into a passageway, it maintains a fixed transverse location as it passes down the length of the canister until the tip passes into the array of apertured plates. Thus, the pairs of corrugated plate members define a plurality of rod locations within the canister. The apertures in the plates of the array are oriented so that no two adjacent rods rest upon the same plate, thus adjacent rods are vertically staggered with respect to each other. Each fuel rod is characterized by a weld bead where the rod tip is welded to the rod, both at the top and bottom thereof. The staggered arrangement of the rods permits the rods to be placed in close proximity to each other without the weld beads of adjacent rods interfering with each other, whether during insertion or as finally positioned within the canister. As discussed heretofore, the exterior of the rod is characterized by a deposit of "crud" which, during insertion of the rod into the storage canister, is scraped off as it passes down between the corrugated plates. In order to facilitate removal of the "crud", a central opening or plurality of openings in the bottom wall of the canister communicates with the vacuum pump assembly through the base of the holder so that a current flows downward through the bottom of the holder. A significantly small volume of the surrounding pool water is also drawn in by the vacuum pump. This adjacent water flows upward through the central screened aperture of the base of the canister, and back out of the canister base through additional apertures encircling the central screened aperture in the base, thereby preventing a buildup of crud on the screen which must allow free upward flow later when in the storage configuration. To facilitate a large volume downward flow during the fuel rod loading configuration, the apertured base plate of the array of plates, and the array itself are lifted up from the bottom of the holder by pins or other means in the loading configuration, and, when the canister is full and is lifted out of the holder, the bottom wall of the canister moves up into contact with the base plate so that the apertured base plate rests against the bottom wall, thereby sealing off all the apertures in the wall except the central screen aperture while in the storage configuration. The storage canister of the invention is provided with two slots on each side adjacent the top edge. A closure or cap member has eight locking cams oriented to fit within the slots, and threaded bolt members for rotating the cams to force them into the slots when the closure member is in position within the top end of the canister. Each bolt member has fitted thereon a circular tamper indicator having a detent recess on its underside. Within the top plate of the closure member are spring loaded plungers adapted to fit within the detent recesses of the tamper indicators, and thus, as the bolts are screwed down to force the cams into the slots, the tamper indicator descends until the detent recesses are engaged by the corresponding plungers. The position of the indicators on the bolts in such that the detent is engaged at that point where the cams are firmly seated in the slots, thus locking the closure lid in place. Any further rotation of the bolts, in either direction, will cause permanent distortion of the indicator members inasmuch as the plungers in the detents prevent rotation of the indicator members. Thus, any attempt to unlock the closure after it is properly locked in place will result in a distorted indicator, thereby indicating tampering. The various features and advantages of the present invention will be more readily seen from the following detailed description, read in conjunction with the accompanying drawings.