Patent Number: 054835607
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatic representation of a cross section through a reactor pressure vessel 1, from which a reactor core and all other core internals have been removed. A multiplicity of nozzles 2 pass through a hemispherical bottom 3 of the reactor pressure vessel. For reasons of clearer representation, only a few nozzles 2 are illustrated, although there are actually about 50 nozzles. During operation of the reactor plant the nozzles serve for the introduction of probes 4 of a so-called in-core instrumentation into the non-illustrated reactor core. The probes have free end regions 5 which are fitted with non-illustrated detectors for determining core data. If it is found that a repair or an exchange of nozzles is necessary during the inspection of the nozzles, which takes place at certain time intervals, four shielding containers 6, 7, 8, 9 are inserted into the reactor pressure vessel 1, as is also seen in FIG. 2. The shielding containers generally contain water as the shielding medium and correspond in their cross section approximately to a quadrant sector, so that together they cover the interior cross section of the reactor pressure vessel. Three shielding containers 6, 7, 8 include a quadrantal bottom plate 10. Perpendicularly upward extending side walls 11 lead away from the outline of the bottom plate 10. A termination of the shielding containers 6, 7, 8 is formed by a top plate 12. The top plate 12 has a side facing the periphery of the reactor pressure vessel which is constructed as a supporting flange 13. The supporting flange 13 supports the shielding containers 6, 7, 8 on a reactor pressure vessel flange 14, and in the inserted state the bottom plate 10 of the shielding containers 6, 7, 8 extend approximately as far as a transition of the cylindrical reactor pressure vessel wall with the hemispherical bottom 3. Tubes 15 are set or imbedded into the bottom plate 10 and in each case are fixed by a weld seam 16. The tubes 15 have open ends which extend to the nozzles 2 and each nozzle of a quadrant sector of the bottom 3 has a tube 15 concentrically engaged over it. A closed end of each tube 15 protrudes so far into the respective shielding container 6, 7, 8 that a probe 4 is disposed with its free end region 5 in the shielding region of the shielding container. The position adopted generally corresponds to the probe position during operation of the reactor plant. As is evident from FIG. 2, which is represented on a different scale, shafts 17 pass through a bottom plate 10a of the shielding container 9. A shaft 17 is coaxially engaged over each nozzle 2 of the corresponding reactor pressure vessel quadrant cross section. A sealing element 18 seen in FIG. 1 is disposed between a lower edge of the shaft 17 and the hemispherical bottom 3, so that when working on the nozzle the swarf or chips remain in a controllable region and can easily be sucked away. Each shaft 17 has an upper end which is set or imbedded into a top plate 12a, although its opening cross section remains free. Side walls 11a seen in FIG. 2 extend between the bottom plate 10a and the top plate 12a, so that a closed shielding container 9 is produced which has an inner space that is filled by the shafts and the shielding medium, water, which fully or partially surrounds the shafts. If appropriate, part of a side wall 11a may be composed of the aligned shafts 17 themselves. Similarly to the top plate 12 of the shielding containers 6, 7, 8, the top plate 12a has a side facing the periphery of the reactor pressure vessel which is constructed as a supporting flange 13a for supporting the shielding container 9 on the reactor pressure vessel flange 14. Inserted into each shaft 17 is a water-filled cartridge 19, that is closed on all sides, maintains a small lateral play with respect to the wall of the shaft, and is supported on an upper rim of the shaft. A bottom 20 of the cartridge 19 extends approximately down to the level of the bottom plate 10a. A tube 15 is set or imbedded into the cartridge bottom 20 in the same way as in the bottom plate 10 of the shielding containers 6, 7, 8. It similarly engages over a nozzle 2, so that the probe 4 can be disposed in the shielding region of the shielding container 9 in the same way. Each shaft 17 has a rail 21 underneath the cartridge 19 for longitudinally guiding and, if appropriate, for arresting a carrier 22, which serves for receiving testing devices and/or working tools. If a particular nozzle is to be inspected, repaired or exchanged, the probe 4 assigned to the nozzle is moved outside the reactor pressure vessel and the cartridge 19 of the corresponding shaft is removed with the aid of a non-illustrated lifting tackle, acting upon means 23 for attaching the cartridge 19. With the aid of a supporting frame 24 and a cable winch 25 assigned to the same, the carrier 22 is introduced into the exposed shaft 17 and is fixed on the rails 21 in relation to the nozzle 2. While the testing and the working required for repair of the nozzle can be performed from inside the reactor pressure vessel, in order to exchange the nozzle a tool carrier 26 acting from outside the reactor pressure vessel is additionally required. The wall of the reactor pressure vessel 1 has projections 31 seen in FIG. 2, which are used as a centering aid for the shielding containers. If an inspection of the nozzles 2 is due, all of the probes 4 are parked in guide tubes 27 being disposed outside the reactor pressure vessel and leading away from the nozzles. Once all of the components of the reactor core have been removed from the reactor pressure vessel, the shielding containers 6, 7, 8 and the shielding container 9 that is fitted with cartridges 19, are inserted into the reactor pressure vessel. All of the probes are then made to enter the tubes 15 of the shielding containers 6, 7, 8 and the cartridge 19 of the shielding container 9, so that the end regions 5 of the probes, bearing the detectors, are disposed in the shielding region of the shielding containers 6, 7, 8, 9. By withdrawing a probe 4 from the reactor pressure vessel and exposing the associated shaft 17, the nozzles 2 of the quadrant cross section covered by the shielding container 9 are successively subjected to the required working. Thereafter, the shielding container 9 is exchanged for a shielding container 6, 7 or 8, in order to work on the nozzles of another quadrant sector. As FIG. 2 reveals, the nozzles 2 of a quadrant sector of the bottom 3 are only partially symmetrical with respect to the nozzle positions of another quadrant sector. The bottom 10a of the shielding container 9 therefore has so many shafts that all of the nozzles of a quadrant sector can be worked on with the same shielding container 9. Depending on the situation in the reactor building, the cross section of the reactor pressure vessel may also be covered by shielding containers with a different distribution, such as two, three, five or six sectors, for example. It goes without saying that the use of a one-piece shielding container, engaging over the entire cross section of the reactor pressure vessel, is also possible. In this case, shafts may be provided for all or some of the nozzles. In the latter case, the entire container must be turned in order to be able to test and/or work on all of the nozzles. A structure shown in FIG. 3 provides a single shaft that engages over a plurality of nozzles. For instance, a shaft 17a engages over two nozzles 2, a shaft 17b engages over three nozzles 2 and a shaft 17c engages over four nozzles 2. If one shaft 17a, 17b, 17c is assigned more than one nozzle 2, only one nozzle is accessible for working by withdrawing the probe 4. The probes of the remaining nozzles are made to enter a shielding assigned to the shaft. The cross section of a carrier 22 is adapted correspondingly for this purpose. The shielding cartridges may also be adapted to the cross section of the respective shaft. The repair process for a less badly damaged nozzle 2 is represented in FIG. 4. The used nozzle is cut off just above the inner surface of the bottom 3 and a part 2a remaining in the bottom is widened to form a two-stage bore 28. A nozzle part 2b, that is configured in corresponding stages, is inserted and is connected to the nozzle part 2a remaining in the bottom 3 by means of a supporting weld seam 29. A sealing seam 30 prevents the entry of moisture between the original nozzle part and the new nozzle part. Consequently, any stress crack corrosion which may have begun in the original nozzle part is stopped.