Patent Number: 047909765
Section: summary

The invention relates to a device for flushing out a lance-housing tube and for aligning in a reactor pressure vessel of a boiling-water reactor a dry power distribution detector (PDD) lance, which partly protrudes with a pressure-tight lance feedthrough or passthrough from an end flange on the lance-housing tube of the reactor pressure vessel. Neutron flux density in a reactor core is an important characteristic for monitoring and controlling nuclear power generating stations. It is measured with detectors, respective groups of four detectors with a surrounding supporting tube forming a so-called PDD lance. During assembly, these lances are vertically inserted by a gripper of a lifting device into a partially unloaded core while the reactor pressure vessel is opened and flooded, and are held in a mounting at an upper end of the lance by upwardly spring-loaded locking pins. Accordingly, the lower end of the lance with a pressure-tight feedthrough or passthrough and a lance protection tube partly protrudes from an end flange on the lance-housing tube of the reactor pressure vessel. A housing assembled therewith prevents the reactor water from running out during the installation. Instrumentation lances of different construction have become known heretofore. The lance installed most frequently to date is a so-called web wet PDD lance. It remains operative for only a few years and must then be disassembled completely and scrapped. Attempts have therefore been made to replace the wet PDD lances by a relatively new type of lance, the so-called dry PDD lance, which advantageously has an extremely long operating life. Dry PDD lances therefore need not be replaced. In the course of the operation of a reactor, radioactive deposits are formed in the reactor pressure vessel which are disposed in part in the lance-housing tubes, and therefore also on the lance feedthrough or passthrough. If long-lived dry lances are installed which are not to be replaced, the deposits increase continuously so that a consequence is the formation of a locally extremely high radiation level in the periodically accessible region below the reactor pressure vessel. Cleaning the lance-housing tubes, however, has been found not to be economically feasible heretofore because the reactor must be shut down considerably longer than for a change of fuel assemblies and must be partially discharged. Because the lances increase in diameter from top to bottom, they can be pulled out of the core with an upward inclination only if the reactor pressure vessel is opened and flooded. The adjacent fuel assembly cases must moreover be removed first in order to avoid damage. Only then it is possible to lift a lance so that the end flange is flushed out or rinsed by reactor water which runs out under controlled conditions and must then be collected underneath the reactor pressure vessel. The heretofore conventionally used wet lances have a circular cross section so that angular orientation is unnecessary during assembly. Installation under water thereby presents no problems. The new dry PDD lances which are preferred for reasons of cost and environment protection are, however, thicker because of their liquid-tight construction and have a pronounced profile. Their cross section has, for example, the shape of a clover leaf, so that the space available in the core grid is better utilized. A dry PDD lance must therefore be aligned accurately relative to the reactor core in order to avoid damage. Because of the great length and the elasticity of the lances they cannot be inserted under water from above into the reactor pressure vessel. The danger would exist that the profiled lances would become twisted or tilted during the alignment and cause damage to the fuel assembly cases or casings when released. Because the conical sealing seat of the lance is loaded by the lance weight of approximately 250 N (Newton) as well as by a pressure of approximately 50 m water column, a rotation of the lance in the sealing seat would damage the sealing surfaces, especially if residual dirt is occluded. This would require costly repairs. If, on the other hand, the lance were lifted for rotation in order to preserve the sealing surfaces, the installation personnel would be endangered by escaping contaminated reactor water. A flushing operation by conventional means is therefore not feasible economically. It would delay and impede work on the upper side of the pressure vessel when that work may have tight deadlines. It is therefore an object of the invention to provide such a device which, for the first time, ensures a flushing to the lance-housing tubes when the reactor pressure vessel is closed and pressureless and, in addition, aligns the dry PDD lances relative to the reactor core. With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for flushing out a lance-housing tube in a reactor pressure vessel of a boiling-water reactor and for aligning therein a dry PDD lance which partly protrudes with a pressure-tight lance passthrough from an end flange on the lance-housing tube of the reactor pressure vessel, includes a tubular housing surrounding from below a part of the lance protruding from the reactor pressure vessel and sealed by a lance protection tube, the tubular housing being fastenable to the end flange; and a piston arranged in the tubular housing underneath the sealed lance, the piston being vertically displaceable and rotatable. By this device, the lance feedthrough or passthrough and the lance protection tube assembled therewith at the end flange are first enclosed in a pressure-tight manner. Then, the piston is moved and lifted, respectively, by hand in the housing until it tangibly engages the lower end of the lance protection tube. Thereafter, if a dry PDD lance is used, the piston is rotated until it snaps in at the lower end of the lance protection tube. There, the piston, together with the lance, is pushed vertically upwardly. The lance is thereby lifted a short distance from its seat in the end flange so that reactor water runs out and the lance housing tube as well as the sealing seat of the lance is flushed out or rinsed. After the flushing operation is completed, the lance with the device according to the invention is deposited again in the sealing seat. If necessary, the dry lance is aligned beforehand exactly to the reactor core by rotating the piston. With the device according to the invention, the advantage is attained that lances inserted into a reactor pressure vessel are freed of impurities and are subsequently arranged reliably again in a pressure-tight manner in the end flange after shutdown, even when the reactor pressure vessel is closed. Working above the reactor is not impeded. The importance of the device according to the invention increases by the fact that dry PDD lances are used increasingly. Whereas with each change of a short-lived lance, a flushing or rinsing operation is performed simultaneously, the long-lived dry PDD lances must be flushed out at given intervals, which can advantageously be performed with the device according to the invention. In accordance with another feature of the invention, the piston is formed with an entrainer pin cooperatively engaging in a slot formed in the lance protection tube. In accordance with a further feature of the invention, the lance protection tube is formed at an upper end in the interior thereof with a spherical member engageable in a marker slot formed in the lance passthrough. Movements of the piston are thereby transmitted exactly to the lance via the lance protection tube which originally served only for sealing during the assembly. For lifting and lowering the unit formed of the piston, the protective tube and the lance, the device, according to an added feature of the invention is equipped with a cap screw which supports the piston and is cooperatively secured with a thread formed on an outer surface of the tubular housing. This cap screw is provided with handles. An advantage is thereby achieved that the lance will always be lifted sensitively without tools and with little effort even though it is loaded with a water column of about 50 m and its own weight of about 250 N (Newton ). In accordance with an added feature of the invention, the piston is formed with a flow passage for reactor water, and including a valve disposed in said passage. The flow of discharging reactor water flushing out the lance is thereby controlled. If only clean water flows off, the lance is again deposited in the sealing seat. In order that the dry PDD lance should remain or be aligned exactly, and in accordance with a concomitant feature of the invention, a telescoping folding lever, for example, is arranged on the piston of the device. With this relatively long pointer lever, the dry lance is slightly turned, if necessary, before being deposited or seated, thereby assuring an exact alignment or orientation with the geometry of the reactor core and the fuel assembly casings, respectively. The aligned lance is then deposited in the end flange by downwardly screwing the cap screw which supports the piston of the device. Thereafter, the device is drained and detached from the end flange after a test for tightness i.e. for leaks. With the invention, an advantage is achieved, in particular, that dry PDD lances, which remain operative in the reactor for the entire operating period, can be retrofitted even in previously contaminated installations. Not economical shutdown times are thereby shortened and the production of highly radioactive scrap is drastically reduced. 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 device for installing dry PDD lances and for flushing out lance-housing tubes in boiling-water reactors, 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.