Patent Number: 048141380
Section: summary

The invention relates to a method for repairing a nuclear reactor fuel assembly by replacing a fuel rod of the fuel assembly filled with nuclear fuel and retained in a force-locking manner in a gap formed in a lattice-like spacer by means of a spring disposed in the gap, and a replacement rod for performing the method. A method of this type is known from German Published, Non-Prosecuted Application No. DE-OS 26 35 501. In this known method, a defective and irradiated fuel rod filled with nuclear fuel is removed from the nuclear reactor fuel assembly and replaced with a new replacement rod filled with nuclear fuel, which is inserted with a fuel rod changing tool into the gap in the lattice-like spacer in the nuclear reactor fuel assembly in which the defective fuel rod was located. However, this method cannot be used if the spring located in that particular gap of the lattice-like spacer has broken off or no longer has adequate spring force. In that case, neither the fuel rod originally located in that gap nor a replacement rod can be securely retained in the gap of the spacer of the nuclear reactor fuel assembly. However, as a rule the gap of the spacer of the nuclear reactor fuel assembly in which the spring for the force-locking retention of a fuel rod has broken or become unusable cannot merely be left unoccupied, because in the nuclear reactor, the space for the fuel rod missing from the nuclear reactor fuel assembly is filled with water, which changes the moderating action of the water in the nuclear reactor upon the neutrons located there. Furthermore, the missing fuel rod causes a change in the speed of the water flowing as coolant through the nuclear reactor, so that oscillation of the fuel rods still located in the nuclear reactor fuel assembly and consequent further damage to this fuel assembly cannot be precluded. In order to utilize the still considerable burnup potential present in the irradiated fuel rods of a nuclear reactor fuel assembly having a broken spring and located in a gap of the spacer, it is possible to move all of the irradiated fuel rods from the nuclear reactor fuel assembly and shift them into a new fuel assembly skeleton, thereby forming another fuel assembly. The skeleton is substantially formed of new spacers as well as a head and a base or foot piece, which are firmly connected to one another by means of control removal tubes which engage gaps in the spacers. However, this method is very time-consuming and accordingly entails high repair costs. A further factor is waste disposal costs for the irradiated skeleton of the old nuclear reactor fuel assembly. It is accordingly an object of the invention to provide a method for repairing a nuclear reactor fuel assembly and a replacement rod for performing the method, which overcomes the hereinafore-mentioned disadvantages of the heretoforeknown methods and devices of this general type and which makes it simpler and less expensive to repair a nuclear reactor fuel assembly in which the spring for the force-locking retention of a fuel rod in a gap of the spacer is unusable. With the foregoing and other objects in view there is provided, in accordance with the invention, in a nuclear reactor fuel assembly having a fuel rod filled with nuclear fuel retained in a gap formed in a lattice-like spacer by means of a spring disposed in the gap, a method for repairing the fuel assembly, which comprises replacing the fuel rod with a replacement rod, and retaining the replacement rod in the gap of the spacer with a retaining spring. In accordance with another mode of the invention, there is provided a method which comprises placing the retaining spring in a corner of the gap. The repair costs for the fuel assembly remain approximately the same as in the prior art repair method because of the use of the replacement rod. Nevertheless, the nuclear reactor fuel assembly does not have a vacant space where the fuel rod replaced by the replacement rod was located. With the objects of the invention in view, in a nuclear reactor fuel assembly having a fuel rod filled with nuclear fuel retained in a gap formed in a lattice-like spacer by means of a spring disposed in the gap, there is also provided a replacement rod for replacing the fuel rod, comprising a cladding tube having a shell with an opening formed therein, a sliding body movable inside the cladding tube in two longitudinal directions of the cladding tube, a slotted link disposed on the sliding body, a retaining spring guided in the slotted link and extending outwardly through the opening formed in the shell, and a locking device associated with the sliding body for displacing the spring radially outwardly upon movement of the sliding body in one longitudinal direction of the cladding tube so as to retain the replacement rod in the gap formed in the spacer and for displacing the spring radially inwardly upon movement of the sliding body in the other longitudinal direction of the cladding tube so as to release the replacement rod from the gap formed in the spacer. In accordance with a concomitant feature of the invention, the cladding tube has a protruding locking shoulder on the inside thereof, and there is provided an actuating rod with an outer shell surface displaceable inside the cladding tube for moving the sliding body, the locking device being in the form of a tongue spring with a given thickness and length disposed on the outer shell surface of the actuating rod, the tongue spring having two ends extending in the longitudinal direction of the actuating rod, one of the ends of the tongue spring facing in the direction of movement of the sliding body being secured on the actuating rod for expulsion of the retaining spring and the other of the ends of the tongue spring being spaced from the outer shell surface of the actuating rod for gripping the protruding locking shoulder from behind, a clamping sheath with a given length disposed inside the cladding tube, a protruding stop shoulder for the clamping sheath disposed inside the cladding tube and spaced apart from the locking shoulder by a given distance in the direction of movement of the sliding body for expulsion of the retaining spring, the clamping sheath being coaxially displaceable in the longitudinal direction of the cladding tube between the locking shoulder and the stop shoulder, the clamping sheath having an inner shell surface protruding beyond the locking shoulder in radial direction and being radially spaced apart from the actuating rod by a distance greater than the given thickness of the tongue spring, and the given distance between the locking shoulder and the stop shoulder being greater than the sum of the given length of the clamping sheath and the given length of the tongue spring in the longitudinal direction of the actuating rod. 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 for repairing a nuclear reactor fuel assembly and a replacement rod for performing the method, 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.