Patent Number: 048470376
Section: summary

This invention relates to an apparatus for the inspection of nuclear reactor fuel rods, which are combined into fuel rod clusters in a fuel assembly, wherein fingers of a test probe which are fitted with ultrasonic test heads on several levels are insertable into spaces between the fuel rods. Such an apparatus is known from German Published, Non-Prosecuted Application 34 19 765 corresponding to U.S. Pat. No. 4,683,104. In that device, the probe can be moved parallel to the longitudinal direction of the fuel assembly to permit inspection on several levels. The vertical movement of the probe over several meters (a fuel assembly is approximately 4 meters long) requires a precise and consequently elaborate, spindle drive mechanism. For reasons of radiation protection, the fuel assembly as well as the apparatus are disposed in a water-filled pool. The vertical travel of the probe to the various test levels must therefore take place slowly. The test operation which is executed successively at the various levels, especially the required vertical movement of the probe, contributes to a considerable expenditure of time. Since the inspection of the fuel rods takes place with the reactor plant shut down, the inspection time is lost from the time that the nuclear power plant is available. It is accordingly an object of the invention to provide an apparatus for the inspection of nuclear reactor fuel rods, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which reduces the inspection effort, while nevertheless ensuring a precise positioning of the probe in the direction of the spaces to be passed through. With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for the inspection of nuclear reactor fuel rods combined in fuel rod clusters with spaces therebetween in a fuel assembly, comprising test probes each being disposed at a respective level along the fuel assembly, fingers each being part of a respective one of the test probes, ultrasonic test heads each being disposed on a respective one of the fingers, means for inserting the test heads into the spaces between the fuel rods, and means for correcting the insertion position of each of the test probes independently of the insertion position of the others of the test probes, before insertion of the test probes. This apparatus ensures that it will be possible to cope with an inclined position of the fuel assembly or a bowing or bending of the fuel rods (a deviation from normal) without any problems, ie. without the risk of a test probe running into a fuel rod. It is only the separate positioning that allows simultaneous testing on several levels. In accordance with another feature of the invention, there is provided a rack holding the probes assigned to the levels, the inserting means moving the rack together with the probes in a given insertion direction of the probes, and a common drive moving the probes at all of the levels transverse to the given insertion direction. This simply constructed rack, which does not take up any more space than the conventional apparatus with vertically movable probes, allows the joint movement of the probe to be carried out both in the insertion direction and transversely thereto. In accordance with a further feature of the invention, the rack has a lower surface, and there is provided a support plate having brackets, at least two mutually parallel spindle nuts fixed on the lower surface of the rack, and spindles mounted in the brackets and engaging in the spindle nuts for moving the rack and the support plate. This structure provides for integration of the sequence of movement and for the support of the rack. In accordance with an added feature of the invention, the rack has bearing points at each of the levels, shafts disposed at the bearing points at each of the levels, the shafts each having a central region in the form of a spindle, probe carriers in the form of spindle nuts each being disposed on a respective one of the central regions, and pairs of bars each bordering a respective one of the central regions and being fixed relative to respective one of the shafts, the bars jutting out from the shafts in the same direction as the fingers of the probe by a distance at least equal to the diameter of a fuel rod, the bars of the pairs being spaced apart by a distance equal to the nominal width of a fuel assembly, and the shafts being displaceable relative to the bearing points. In this case, simple mechanical side successfully perform the automatic guidance of the probes in the case of fuel assemblies which deviate from normal regarding the vertical disposition thereof. In accordance with an additional feature of the invention, the rack has opposite side walls at which the bearing points are disposed, the shafts pass through the bearing points and the side walls and have free ends protruding from the side walls, and including abutments disposed on the free ends of the shafts, and compression springs each being disposed between one of the abutments and a respective one of the side walls. This is done in order to ensure that once they have assumed their position, the bars do not adjust themselves of their own accord. In accordance with yet another feature of the invention, there are provided worm wheels each being disposed on a respective one of the shafts, tongue and groove connections securing the worm wheels on the shafts, and a worm shaft extending transversely to the shafts and simultaneously engaging all of the worm wheels. Since the position of the probes disposed on the individual levels can be corrected, the uniform movement of all of the probes in their travel from one gap to the next is thus ensured. The tongue and groove connection permits axial movement of the shaft relative to the worm wheel. In accordance with yet a further feature of the invention, there are provided hubs each being disposed on the rack for limiting axial movement of a respective one of worm wheels. The lateral fixing of the worm wheel prevents the worm wheel from following the axial movement of the shaft. In accordance with yet an added feature of the invention, there is provided a base plate for holding a fuel assembly, two struts jutting out from the base plate parallel to each other and to the fuel assembly, spindle drive mechanisms each being disposed transverse to the struts at a respective one of the levels, spindle nuts each being disposed on a respective one of the spindle drive mechanisms, probe carriers each being disposed on a respective one of the spindle nuts, and drive elements each being disposed on a respective one of the probe carriers for inserting the probes into the spaces between the fuel rods. Through the use of this structure, the probes of the various levels can be positioned independently of one another and can also execute the insertion movement independently of one another. In order to effect positioning of the probes, in accordance with yet an additional feature of the invention, there is provided a strip guiding the probe fingers, and an ultrasonic transducer disposed on the strip between the probe fingers, the ultrasonic transducer emitting sound waves in a given insertion direction of the probes and receiving returning echoes providing positional determination of the probes relative to a fuel rod. In this way, the transducer which is constructed as transmitter and receiver receives a maximum echo when the center of the probe coincides with the center of the ultrasonically exposed fuel rod. The insertion movement of the respective probe can then take place immediately. In accordance with a concomitant feature of the invention, there are provided other struts offset by 90.degree. with respect to the first-mentioned struts, and other probes disposed on the other struts at levels different from the first-mentioned levels. This makes possible the simultaneous testing of a fuel assembly from two sides. 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 an apparatus for the inspection of nuclear reactor fuel rods, 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. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.