Patent Number: 041750004
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nuclear equipment and more particularly to methods and apparatus for the non-destructive inspection, manipulation, disassembly and assembly of reactor fuel elements, and the like. 2. Description of the Prior Art To generate power in a nuclear reactor, an adequate quantity of nuclear fuel or fissionable material must be concentrated in the reactor core. This concentration produces heat through a sustained sequence of reactions between neutrons and fissionable nuclei within the core. These fission processes, moreover, leave a residue of nuclear debris within the reactor core that is intensely radioactive. In order to assemble a concentration of this nuclear fuel in a sufficient quantity to propagate the fission process, the fissionable material usually is pressed into pellets, which are loaded into and sealed within hollow metal rods or "pins". The individual pins ordinarily are assembled into a group that constitutes a "fuel element". An array of these fuel elements, when assembled in a side-by-side relation, generally constitute the active portion of the reactor core. Because of the high pressures, temperatures, and other hostile environmental conditions that often are encountered within a reactor core it is occasionally necessary to inspect the individual fuel pins to insure that they have not lost their structural integrity. As a part of the fuel element construction, the individual pins are held in place through two end fixtures that each engage respective pin extremities. Cellular grids also are positioned at intervals along the tubular surfaces between the end fixtures in planes that are generally transverse to the longitudinal pin axes. Typically, these grids are rectangular arrays of interlocking metal plates. The arrays of interlocking plates thus form more or less cube-like cells that receive the individual fuel pins. Protrusions jut from the surfaces of the plates that form the cells. These protrusions engage the adjacent surfaces of the fuel pins and hold the respective pins rigidly in place. Because of the radioactivity that is induced in these fuel elements as a result of nuclear processes, it is necessary to dismantle used and partially used fuel elements under at least ten feet of water. The water, in this instance, provides radiation protection. Clearly, dissasembly and re-assembly operations in these circumstances must be conducted in a tedious, painstaking manner with remotely handled tools and specially trained crews. This situation is further aggravated by the conflicting requirements for a structurally strong fuel element that is capable of withstanding about three years exposure to the extreme physical conditions within a reactor core, and nevertheless be readily dismantled through remote manipulations. Because the need for structural integrity has been paramount, fuel element disassembly heretofore has involved the destruction of many of the components in order to remove, identify and separate those few fuel pins that might require replacement. This is an expensive and wasteful process, especially in view of the high cost of these components. Thus, there is a need for a safe, efficient, and essentially non-destructive technique for dismantling fuel elements. SUMMARY OF THE INVENTION The invention generally satisfies this need for a non-desctructive fuel element disassembly technique. Typically, a deep pool of ordinary water is provided at a nuclear power plant for temporarily storing used fuel elements. The water depth is sufficient to reduce the radiation from the stored elements to a safe level at the pool edge. In accordance with the invention, a traveling bridge spans the width of the pool. The bridge supports a generally parallel strongback that is immersed in the pool water. The strongback is pivoted or hinged on one end to the lower end of one of the pair of beams that protrude down into the water from the bridge. The free end of the strongback is attached to a winch-driven cable. This structural arrangement enables the strongback to be swung selectively from a horizontal position that is parallel to the bridge down into a vertical position within the shielding water that is perpendicular to the span of the bridge. In this vertical position, the strongback accepts a complete fuel element and draws this element into a horizontal position for external inspection under the control of the winch and cable. Closed circuit television, "feeder gauges", periscopes, and the like, preferably are mounted on a trolley on the bridge to execute this external inspection. To examine all sides of the fuel element in question, moreover, it will be necessary in the illustrative case of a fuel element that has a square, or rectangular cross section to rotate the fuel element through 90.degree. one or more times in order to insure that all four lateral sides of the element are exposed for inspection. To accomplish this rotation, the strongback is swung into the vertical position and the fuel element is released to stand erect in the strongback. A fuel element assembly handling tool is lowered from the trolley to engage the end fitting of the fuel element. The tool lifts the fuel element from the strongback and rotates the fuel element through a 90.degree. angle before the tool returns the fuel element to the strongback. The entire hoisting and inspection process then is once more repeated with respect to the newly exposed side. Naturally, these steps may be repeated a number of times to expose and to examine all sides of the fuel element under study. After these external inspections are complete, the fuel rods that are secured within the bundle also must be removed for individual examination, inasmuch as these interior rods are not subject to close observation from the outside of an assembled fuel element. As an initial step in the technique that characterizes this invention, the two end fixtures are released from the fuel element while it is in the vertical position. As fuel elements are presently constituted, end fixture removal might require remotely controlled cutting of a few small welds. The nature of this operation, however, will not irretrievably damage the end fixtures or the interposed fuel rods. In order to remove individual fuel rods from the fuel element without damage, the grip between the protrusions that jut from the surfaces of the grid cells and the portions of the fuel rod surfaces that are in contact with these protrusions must be relaxed. These gripping forces are temporarily relieved by inserting long, thin bars that have rectangular cross sections through the individual grids. These bars are guided into appropriate grid slots by means of a plate that has cone-shaped apertures which align the bars with the grid slots. A remote manipulator inserts the rods through the plate and into the slots in the grid structure. To relax the grip, the bars are rotated through 90.degree. in order to press the long dimension of the rectangular bar cross section against the plates that form the cell surfaces and, in this manner, to push the jutting protrusions away from the associated fuel pin surfaces. In accordance with another feature of the invention, the inserted bars are rotated through the 90.degree. angle with the aid of a special grid cell opener tool. This tool has a pair of parallel sides that embrace opposite longitudinal surfaces of the fuel element in a transverse plane that coincides with the grid structure. A common selectively contractible member is disposed parallel to a third longitudinal side of the fuel element. This member joins the two opposing parallel sides together. A rack and pinion on the bar enable the opposing sides to move toward the adjacent grid and to engage the protruding edges of the rotatable bars in aligned nipples that are pivoted on the inner surfaces of the parallel sides. The protruding bar ends are securely received in matching rectangular slots in the nipples. The nipples, moreover, all are connected to respective cranks that are ganged to move in response to the motion that is produced by a further rack and pinion on the contractible member that joins the two parallel members together. Moving the rack and pinion associated with the nipples rotates the ganged cranks through a 90.degree. angle. This rotation brings the long dimension of the rectangular cross section to bear against the surfaces of the adjacent grid cell and presses the protrusions away from the respective fuel pin surfaces. Thus, the gripping forces that rigidly retain the fuel pins in their relative positions within the fuel element structure are relaxed. The relaxed grip enables the fuel pins to be individually withdrawn without risk of damage from the fuel element structure. In the circumstances, a remotely controlled manipulator can remove the individual pins for inspection and reinsertion or replacement as necessary. To reassemble the fuel element, the pins are replaced or reinserted into the grid structure, as the occasion demands. The ganged cranks are rotated back through the 90.degree. angle in order to shift the long dimension of the rectangular cross section of the bars away from the grid cell surfaces. This movement of the bars releases the stresses that are imposed on the bars and enables the protrusions in the cells to once more jut forward and engage or grip the associated fuel pins. The unstressed bars within the grid structure can be individually extracted by means of a remote manipulator. The end fixtures also can be secured to the extremities of the fuel element. Screws and nuts, moreover, are reconnected with the aid of a conventional manipulator. Welds also can be restored through remote underwater manipulation of appropriate equipment. The reassembled fuel element is ready to be reinserted into the reactor core or kept in the storage pool, as necessary. In accordance with a further characteristic of the invention, the equipment used in these foregoing operations is portable and the entire disassembly and reassembly process is conducted without inflicting substantial damage to the structure and individual components of the fuel element. The portable nature of the tools used to carry out the invention enables a small crew of trained technicians to travel to different reactor sites and carry out efficient and generally non-destructive fuel element inspections. The need for expensive specially trained personnel and individual equipment at each reactor is overcome through the practice of the invention. Consequently, the invention provides a long step forward toward overcoming a major difficulty in the practical application of nuclear power. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention .