Patent Number: 051026147
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the repair of boiling water nuclear reactors. Specifically, this invention relates to a repair procedure for the welding, repairing or replacing control rod drive housings in stub tubes while effecting the required precise angularity, both the angularity and weld occurring in a remote, submerged environment. STATEMENT OF THE PROBLEM Repair and/or replacement of control rod drive housings in boiling water nuclear reactors is sometimes required. For many reasons, such control rod drive housings are some of the most remote and difficult to repair portions of boiling water reactors. In order to understand at least some of these difficulties, three factors will be summarized. First, the assembly of the control rod drive housing and its relation to the other components of the reactor will be set forth. Secondly, the function of the control rod drive housing in supporting the core will be emphasized. Finally, the necessity of the precise alignment for this structural element of the reactor will be set forth. Regarding the control rod drive housing relative to the rest of the core, it is to be understood that the control rod drive housing is a structural element in that it participates in the core support of the reactor. The reactor is typically a cylindrical vessel. It has a bottom closing dome. This dome is transpierced by vertical stub tubes. There is one stub tube for each control rod drive housing. The control rod drive housing is a support member for the reactor core. This housing is attached to the so-called stub tube. The control rod drive housing extends upwardly in the range of 6 inches to 4 feet from the bottom of the reactor dome. The control rod drive housing successively supports the overlying control rod guide tube, the fuel support casting. Each in turn supports 4 overlying fuel bundles. Each fuel bundle forms a portion of a core, the core typically being comprised of many fuel bundles (in the order of 500-600). Naturally, for the repair of the control rod drive housing, all of these supported and overlying components must be removed. However, it is important to remember that when a control rod drive housing is appropriately attached, the attachment must be strong enough for the support of the required and overlying elements. It will be understood that overlying the control rod drive housing there is a core plate. This core plate is located at the bottom of the reactor core. This core plate has a matrix of precisely located circular openings. One such precisely located circular opening overlies each control rod drive housing and each stub tube. Typically, the control rod drive housing supported guide tube fits through this circular hole and fills the circular hole. Thus, the core plate, in cooperation with the guide tube forms a fluid barrier between the reactor plenum adjacent to the bottom of the reactor vessel and the reactor core wherein the steam generating nuclear reaction occurs. The core plate and precisely located holes overlying each control rod drive housing also serve an alignment function. Simply stated, the typically square-sectioned fuel bundles are supported in precise vertical alignment between the fuel support castings on the lower end with 4 bundles supported on each casting. An overlying metal lattice known as the "top guide" supports the upper end of all the fuel bundles. By registering the fuel bundles to the core support casting at the bottom and registering the distal upper end of the fuel bundles to the top guide, a precise dimension may be maintained between each of the square sectioned fuel bundles. This precise space dimension, or spacing between the fuel bundles, defines a cruciformed sectioned aperture between all of the fuel bundles. It is into and out of this cruciformed section aperture that the correspondingly cruciform sectioned control rod moves for controlling the nuclear reactor. In order for the control rod to penetrate the core from below the core into the core, it is necessary that the fuel support casting also define a cruciformed shaped aperture. This complementary cruciformed shaped aperture in the fuel support casting is similarly precisely dimensioned and complementary to the dimension between the overlying fuel bundles. It will be understood that in reaction controlling penetration of the core, precision movement on the part of the control rods is required. Since the control rod is moved by a drive affixed to the control rod drive housing, the alignment of the housing is imparted to the control rod drive and through the control rod drive to the movement of the control rod. Hence, a control rod drive's place within the control rod drive housing imparts to its control rod the acquired alignment. It is understood that the control rod is required to make an excursion of approximately 165 inches from a position of storage within the control rod guide tubes to a position of reactor controlling penetration through the fuel support casting. In such penetration, it passes into the interstices between the fuel bundles, absorbs neutrons, and mitigates core reaction. If alignment of the control rod drive housing is not properly made, the control rod drive, as extended, will, with its 165 inch lever arm, leverage the misalignment. Binding between the control rod and the cruciform-shaped aperture in the fuel support casting and the fuel bundles at their enclosing fuel bundle channels can occur. Idle reactor vessels undergoing an outage must be kept for the most part flooded because of their radioactivity. Typically, the flooding includes flooding of at least the top guide all the way down to the bottom of the reactor vessel. This flooding shields those who work on the reactor from radiation. It is to be understood that the control rod drive and the stub tube to which it is mounted is at the very bottom of the reactor. This is in the order of 60 feet from the top of the normally flooded top guide. Thus, the reader can understand the replacement or repair of the control rod drive housing is remote, requires precise alignment, and must be sufficiently rigid to assure reactor support and proper control of the reaction. Accordingly, this disclosure discloses an apparatus and method for effecting such repair. SUMMARY OF THE INVENTION In an uncovered and non-operating boiling water nuclear reactor having its core, fuel support casting, and control rod guide tubes removed to expose the core plate, an apparatus and method for the submerged repair and replacement of the control rod drive housing at the bottom extremity of the reactor vessel is disclosed. The apparatus registers to a single hole in the core plate through the top guide. The apparatus for aligning includes protruding vanes, typically four in number, for mating to holes in the core plate, the holes overlying each control rod drive housing. A shaft extends from the vanes down to the top of the control rod drive housing. One of the vanes fits over and aligns with the alignment pin protruding from the core plate to enable removal and replacement of the alignment device with lower and reproducible angularity to the precise position of the control rod drive housing. All vanes are provided with set screws for contacting the top of the core plate so that upon removal of the alignment device, replacement of the alignment device can occur with precise reproducibility. A control rod drive housing mating fixture on the lower extremity of the shaft precisely fits and mates to the top of the control rod drive housing. The alignment device includes crossed electronic levels sealed for submersion, which levels remotely transfer through attached wiring the precise angularity between the top of the control rod drive housing on one hand and the corresponding and overlying hole in the core plate on the other hand. For the welding repair process herein, a welding cylinder apparatus fits over the lower end of the shaft of the alignment device. This welding cylinder protrudes downwardly and around the control rod drive housing mating to the stub tube through which the control rod drive housing is inserted. A first flexible seal suspended at a constant elevation from the shaft of the alignment tube is gas proof and permits the cylinder to be gas flooded as it is submerged. A second and optional flexible seal can be provided at the bottom of the chamber for attachment to the stub tube. In the embodiment here illustrated, a rotating raceway is provided interior of the chamber for permitting a TIG welding unit to traverse around a path for weldment defined at the side of the control rod drive housing and at the top of the stub tube. Assuming the repair includes the replacement of a control rod drive housing, the apparatus is first used without the welding cylinder to determine the precise angularity of the old control rod drive housing with respect to the overlying hole in the core plate. The old control rod drive housing is then removed and the stub tube plugged. The alignment device with the welding cylinder attached is submerged and flooded with sufficient gas to maintain the interior dry by equal and opposite pressure to the hydrostatic pressure of the water outside. With the cylinder effecting a flexible seal to the shaft of the leveling device at the top, the cylinder is fitted at the bottom over the corresponding stub tube. Thereafter, the control rod drive housing to be inserted is inserted from below the reactor. Once insertion and alignment to the bottom of the control rod drive housing fitting has occurred, welds can be made between the stub tube end and the control rod drive housing side with alignment constantly being monitored. Both sequenced welding and final positioning of the new control rod drive can occur reproducing the precise angularity and elevation of the old control rod drive housing. OTHER OBJECTS FEATURES AND ADVANTAGES An object of this invention is to disclose a precision alignment device for aligning the top of a control rod drive housing with respect to a precisely positioned overlying hole in the core plate. According to this aspect, there is provided an elongate depending shaft. The shaft includes at its lower end a fixture for fitting to the top of the control rod drive housing. The shaft includes, adjacent its upper end and even with the core plate, four vanes. These vanes straddle the precisely dimensioned hole in the core plate and center the alignment device in the core plate aperture. To ensure reproducibility, at least one of the vanes is slotted to register to an alignment pin adjacent each aperture in the core plate. Affixed to the shaft are crossed electronic levels. These levels are sealed against the flooded environment and remotely output the precise angularity of the shaft to monitoring personnel. The important alignment function is achieved using the precisely measured angularity, combined with the known length of the tool, to establish the relative horizontal position of the housing with respect to the vertical center line of the core plate hole. Preferably the vanes are equipped with set screws and a length measuring device with respect to the core plate so that the alignment of the control rod drive housing and the overlying hole in the core plate is reproducible with respect to the core plate alone. A further object of this invention is to disclose a process of measuring the alignment of the top of a control rod drive housing with respect to a hole in the core plate. Specifically, and with all vertical reactor components removed, save and except the core plate and control rod drive housing, the alignment device is passed through the top guide, down through the respective hole in the core plate and to the top of the control rod drive housing. The control rod drive housing mating fixture at the bottom of the depending shaft mates to and fits with the top of the control rod drive housing. At the same time, the vanes mate and fit to the respective overlying hole with one of the vanes imparting precise reproducible angularity by registration to the alignment pin on the core plate. Thereafter, precise measurements of angularity between the top of the control rod drive housing and the hole in the core plate is provided, preferably by the use of set screws and length measurement devices. The elevation of the top of the control rod drive housing can likewise be determined. An advantage of the disclosed apparatus is that the position of the top of the control rod drive housing is precisely determined with respect to the aperture in the core plate. Consequently, location of the top of the control rod drive housing can be made with reference to the corresponding hole in the core plate only. Because of this reference, repair or replacement of the control rod drive housing can occur with confidence that reproducible alignment will likewise occur. A further object of this invention is to disclose a welding cylinder for repair or replacement of the control rod drive housing. According to this aspect of the invention, the bottom of the depending shaft on the alignment device supports a cylinder. The cylinder is sealed to the shaft of the alignment device at a flexible seal. The cylinder is flooded with gas, supports remote welding and viewing apparatus, and depends from the alignment device. Preferably, the cylinder has a secondary seal for sealing with respect to the stub tube. An advantage of the disclosed apparatus is that repair and/or replacement of the control rod drive housing can remotely occur by apparatus lowered with the alignment device. At the same time, the alignment device imparts the required angularity to the top of the control rod drive housing so that the repair or replacement when made duplicates the original housing angularity before the repair or replacement occurred. A further object of this invention is to set forth a process for the repair and/or replacement of a control rod drive housing. Assuming that the angularity of the control rod drive housing has been previously measured, the welding chamber is affixed to the alignment device. It is flooded with gas sufficient to dispel the water and maintain the chamber dry and lowered past the top guide, through the hole in the core plate, and over the respective stub tube. Once in place, a seal is preferably effected with respect to the stub tube. Once the seal is made, either an in situ control rod drive housing can be repaired, or a new control rod drive housing can be inserted and installed. An advantage of the combination of the alignment device and the welding chamber is that repair and/or replacement of the control rod drive housing can occur with the required rigidity and angularity for support of the reactor core and the alignment of the insertion of the control rod.