Patent Number: 043361030
Section: summary

The invention relates to a method for repairing partly burnt-off fuel elements or assemblies in fuel-element pits of pressurized-water reactors and a device for removing fuel rods in the performance of the method. Irradiated fuel elements which contain defective fuel rods and have not yet attained the planned burn-off thereof can be refitted for further insertion into the reactor by replacing the defective fuel rods. This objective is highly desirable for economic reasons alone. The realization thereof presents difficulties, however, since the fuel elements are highly radioactive objects which can be manipulated by remote control only behind thick shielding walls. Furthermore, the length of the individual fuel rods (about 4 m) presents difficulties particularly during the insertion thereof into a vacated fuel-rod position of a fuel element, since there is danger of bending or buckling due to the small diameter of the fuel rods, which is in the order of magnitude of 1 cm. There is no danger of such an occurrence in the fabrication of new fuel elements, since the fuel rods can be pulled into the skeleton of the fuel element. In addition, consideration should be given, with respect to this problem, to the fact that each of the fuel elements is formed of a very large number of fuel rods, usually more than 200 thereof, so that it is necessary to remove only those fuel rods from the fuel-element assembly which have been recognized with certainty as being defective. The problem therefore arose of finding a method of repairing a partly burnt-off fuel element which requires a minimum of time and requires devices that can be manipulated from outside the reactor pit, the water content of which serves as sheilding. It is accordingly an object of the invention to provide such a method which meets the foregoing requirements, as well as a device for removing and replacing fuel rods which aids in the performance of the method. With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of repairing partly burnt-off fuel elements in a fuel-element pit of a pressurized-water reactor which comprises placing a fuel element previously detected by conventional testing methods as being defective into a holding cage in substantially upright position with the aid of remotely controlled tools, and tipping it over through 180.degree.; removing the then upwardly disposed fuel-element foot from the fuel element so as to expose the ends of the fuel rods received in the fuel element; inspecting the fuel rods, the ends of which are then freely accessible for defects; removing with a fuel-rod exchanging tool those fuel rods at least which are determined to be defective and depositing them in a storage container; and raising replacement rods with the fuel-rod exchanging tool and inserting them into the fuel element without bending stress in exchange for the removed fuel rods. To better understand this method, it should be recalled that a pressurized-water fuel element is formed, generally of a fuel-element skeleton which is constructed of a head and base or foot member and connecting control rod guide tubes and which, in addition, holds spacer grids for mutually spacing the fuel rods from one another in respective nominal positions thereof. The individual fuel rods, which are pulled or withdrawn through the mesh of the spacer grids, are neither fastened to the head plate nor to the base or foot plate and can therefore expand freely in axial direction. The control rod guide tubes are releasably connected to the lower base or foot plate by means of threaded bolts or pins, so that it is relatively easy to remove this base or foot plate, as compared to the removal of the head member, by means of remotely controlled tools. All the fuel rods are also, thereby, freely accessible and can be pulled out of the fuel element one after the other. Because of the relatively great length thereof, however, it is not readily possible to push the fuel rods into the fuel element, as mentioned hereinbefore. In accordance with another mode of the method of the invention, the fuel rods are inspected for penetration of water into the same. In accordance with a further mode of the method of the invention, the removed fuel rods are replaced by new fuel rods. In accordance with an added mode of the method of the invention, the method includes passing the fuel rods, during the removal thereof from the fuel element, through a tube traversed by a flow of water, and feeding the flow of water through a gas separator to a purification system. In accordance with an additional mode of the method of the invention, the method includes inspecting the fuel rods during removal thereof from the fuel element by detecting the physical location of defects therein. In accordance with yet another mode of the method of the invention, the method includes inspecting the replacement rods during the insertion thereof into the fuel element for detecting the physical location of defects therein. In accordance with the device of the invention for removing fuel rods in the performance of the method of the invention, there are provided a pulling linkage, lockable gripper means carried by the pulling linkage and being grippingly engageable with a fuel rod for pulling the fuel rod into a guide tube, a fixed support tube located at an end of the guide tube, guide means for sealingly guiding the pulling linkage through the guide tube and into the support tube; and means connectible to the interior for sucking water out of the interior of the guide tube. In accordance with another feature of the invention the pulling linkage has an end remote from the gripper means, and a rack-and-pinion drive is provided for moving said pulling linkage, as well as locking means for locking and unlocking the gripper means, and actuating and indicating means for the locking means located at said remote end of said pulling linkage. In accordance with a further feature of the device of the invention, the pulling linkage is a pulling tube, and the locking means comprise a locking rod disposed in the pulling tube. In accordance with an additional feature of the device of the invention, the fuel-element pit is filled with water, and float means are provided for weight-relieving the device. In accordance with a concomitant feature of the device of the invention, the fuel-element pit is filled with water and the device is insertable into the water in the fuel-element pit, and float means are provided for supporting and weight-relieving the device upon insertion thereof into the water in the fuel-element pit. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a method of repairing partly burnt-off fuel elements in the fuel-element pit of pressurized water reactors and device therefor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.