Patent Number: 054835607
Section: summary

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a process for testing, repairing or exchanging nozzles penetrating the bottom of a reactor pressure vessel, wherein each nozzle serves for the introduction of a respective probe of an in-core instrumentation of a water-cooled nuclear reactor. Depending on the power and type of the nuclear reactor, a greater or lesser number (about 50) of nozzles pass through the bottom of the reactor pressure vessel. The probes, which are fitted with detectors on their free end regions, are made to enter the fuel assemblies during reactor operation. They are therefore exposed to a high level of radioactive radiation. If testing, repairing or exchanging of nozzles is necessary, the fuel assemblies of the reactor core and all of the core internals are removed from the reactor pressure vessel. Before taking out the fuel assemblies, the probes of the in-core instrumentation, which can travel in channels of the fuel assemblies, must be parked outside in guide tubes leading away from the nozzles. Since the detectors are highly contaminated, due to their insertion in the reactor core, an undesired radioactive loading is the consequence. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a method and an apparatus for testing, repairing or exchanging nozzles passing through the bottom of a reactor pressure vessel, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and in which radioactive loading by detectors of in-core instrumentation is considerably reduced. With the foregoing and other objects in view there is provided, in accordance with the invention, in a method for testing, repairing or exchanging nozzles penetrating a bottom of a reactor pressure vessel, wherein each nozzle serves for the introduction of a respective probe of an in-core instrumentation of a water-cooled nuclear reactor, the improvement which comprises: a. inserting at least one shielding container into the reactor pressure vessel; PA1 b. introducing all of the probes of the in-core instrumentation into a shielding region of the at least one shielding container; PA1 c. withdrawing at least one of the probes assigned to one of the nozzles to be tested or worked on, from the reactor pressure vessel; PA1 d. exposing a shaft extending through the shielding container and being assigned to at least one of the nozzles; PA1 e. introducing testing devices or tools through the shaft; PA1 f. carrying out at least one of testing and working operations; PA1 g. withdrawing the testing devices or tools from the shaft; PA1 h. shielding the shaft; PA1 i. introducing the probe into the shielding region of the shielding container; and PA1 j. repeating steps c to j for further nozzles. During work being performed on a nozzle assigned to one shaft, the remaining probes are located in the shielding region of the shielding container, so that the radioactive loading inside and outside the pressure vessel is reduced considerably. The method successfully accomplishes an integration of the shielding and working operations. In accordance with another mode of the invention, there is provided a method which comprises cutting off a nozzle to be repaired, just above an inner surface of the bottom; drilling open a nozzle part remaining in the bottom to form a staged nozzle bore; inserting a new staged nozzle part into the nozzle bore; establishing a supporting connection by means of a weld seam; and applying a sealing weld seam at a lower end of the staged bore. With the objects of the invention in view, there is also provided, in a water-cooled nuclear reactor having a reactor pressure vessel with a bottom, a flange and a cross section, and nozzles each penetrating the bottom for the introduction of a respective probe of an in-core instrumentation of the nuclear reactor, an apparatus for testing, repairing or exchanging the nozzles, comprising a plurality of shielding containers together filling the cross section of the reactor pressure vessel, the shielding containers having a supporting flange supported on the reactor pressure vessel flange, and the shielding containers having bottom plates ending above the nozzles; a plurality of shafts passing through the bottom plate of at least one of the shielding container and engaging over the nozzles located in a corresponding partial cross section of the reactor pressure vessel; a water-filled cartridge to be inserted into each of the shafts, the cartridges having bottoms each ending above a respective one of the nozzles; tubes for receiving the probes; at least one of the tubes being embedded in at least one of the bottoms and having a closed end protruding by a predetermined amount into the cartridge and an open end being adjacent one of the nozzles; the bottom plate of the others or the remainder of the shielding containers through which the shafts do not pass, having others of the tubes embedded therein with closed ends protruding by a predetermined amount into the shielding containers and open ends adjacent the nozzles covered by or facing away from the shielding container; carriers for traveling in the shaft and receiving at least one of testing devices and tools; and each of the shafts having means for guiding one of the carriers. The inside diameters of the tubes correspond approximately to that of the nozzles, so that they serve as guide tubes for the probes and so that their free ends, which are fitted with the detectors, pass without any problem into the shielding region of the shielding container. As a rule, the cross section of the reactor pressure vessel is covered by four shielding containers, each including approximately a quadrant sector. If only one quadrant sector is provided with shafts, after the completion of work in one reactor pressure vessel sector, the shielding container can be turned through 90.degree. in order to work on the nozzles of the next reactor pressure vessel sector. In the shafts which are not accessible at that time to the carrier for receiving testing devices or tools, a water-filled cartridge performs the shielding task. Consequently, an apparatus has been created in which the working and shielding are combined in a compact structure. In accordance with another feature of the invention, the means for guiding the carrier are preferably assigned to the region of the shaft beginning underneath the respective cartridge. Consequently, the guiding means do not result in any cross-sectional reduction of the cartridges and consequently do not result in any reduction in the shielding effect. In accordance with a further feature of the invention, the guiding means are rails on which the carrier can be securely clamped. In accordance with a concomitant feature of the invention, there is provided a sealing element disposed between the bottom of the reactor pressure vessel and the lower edge of the shaft, for localizing the swarf or chips generated inside a shaft. 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 and an apparatus for testing, repairing or exchanging nozzles passing through the bottom of a reactor pressure vessel, 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.