Patent Number: 052689417
Section: summary

BACKGROUND OF THE INVENTION This invention generally relates to the servicing of the omega seal site where a control rod drive mechanism attaches to the adapter tube of a nuclear reactor head stalk, and is specifically concerned with welding a canopy seal about the omega seal site to contain the radioactive steam released in the event of an omega seal failure. The core of a modern nuclear reactor of the type used to generate electrical power generally includes an upper internals assembly disposed over a lower core barrel. The lower core barrel houses an array of fuel rod assemblies which generate heat as a result of a controlled fission reaction that occurs in the uranium oxide pellets present in the individual fuel rods. Water is constantly circulated from the lower core barrel through the upper internals and out through outlet nozzles provided in the walls of an upper core barrel in order to transfer the heat generated by the fuel rod assemblies to heat exchangers which ultimately convert this heat into usable, nonradioactive steam. The upper internals assembly includes an upper core barrel arranged in tandem with the lower core barrel of the reactor. The ceiling of the upper core barrel is formed from an upper support plate. The peripheral edge of this support plate is seated around the upper edge of the upper core barrel. Both the support plate and the upper core plate which underlies it include a plurality of apertures for both conducting the stream of hot, pressurized water exiting the fuel rod assemblies to the heat exchangers, as well as for conducting control rod assemblies. Separate guide tubes are provided between apertures in both the support and core barrel plates which are aligned with each other and with one of the fuel assemblies in the lower core barrel. The purpose of these guide tubes is to align and guide the relatively long and flexible rodlets of the control rod assemblies into a particular fuel assembly. The rate of the fission reaction taking place within the fuel rod assemblies is regulated by means of the control rod assemblies. Each of these control rod assemblies is formed from an array of stainless steel tubes containing a neutron absorbing substance, such as silver, indium or cadmium. The stainless steel tubes (known as "rodlets" in the art) are suspended from a spider-like bracket. A reciprocable drive rod is connected to the spider-like bracket for either inserting or withdrawing the rodlets of the control rod assembly deeper into or farther out of each of the fuel rod assemblies in order to modulate the amount of heat generated thereby. These reciprocable drive rods are driven by control rod drive mechanisms which may be of the electromagnetic linear motion drive type devices or hydraulic drive type devices which move the control rods in incremental steps into and out of the reactor core. Each of the control rod drive mechanisms are attached to the reactor vessel head by way of adapter tubes, with the control rod drive mechanism being sealed to the adapter tube by way of an omega-type seal (so-called because of its resemblance in cross-section to the Greek letter omega). Because hot, radioactive primary water is contained within the control rod drive mechanism, a leak may develop during the course of operation between the control rod drive mechanism and the adapter tube due to corrosion. Any such leak will promote further corrosion and lead to greater contamination around the area of the leak due to the radioactive steam released by the leak. Presently, due to the high radiation field and lack of working space, the omega seals must be repaired manually which requires a shut down of the reactor vessel and the removal of the control rod guide mechanism. Applicant has observed that the servicing of broken omega seals requires extensive down time of the reactor vessel, with a large amount of labor costs as well as exposure of servicing personnel to potentially harmful radiation. Clearly, there is a pressing need for a system for both efficiently and effectively servicing broken omega seals that minimizes or eliminates reactor vessel down time, which typically costs the utility over $100,000 per day in lost revenues. Ideally, the system should reduce service personnel exposure to radiation contamination and contain the radioactive steam emitted from the broken omega seal. SUMMARY OF THE INVENTION Generally speaking, the present invention is a system for remotely securing a canopy seal about the omega seal site so as to contain any leakage that might occur between the control rod drive mechanism and the adapter tube. The system includes a stalk measuring device for initially measuring the diameter of the reactor stalk head or adapter tube, a split canopy installation fixture which is adjusted according to the measurement detected by the stalk measuring device to position a two-piece canopy seal about the omega seal site, and a robotic weld arm for performing both upper and lower radial welds about the canopy seal as well as C-shaped vertical welds between the two-piece canopy seal. Because each adapter tube may vary slightly in diameter, and the canopy seal must mate with the adapter tube, the diameter of the adapter tube is initially determined by the stalk measuring tool. This tool is anchored to a carousel mounted above the control rod drive mechanism and lowered below the lower end of the control rod drive mechanism to the omega seal site. The stalk measuring tool includes a pair of caliper arms which are initially calibrated to a predetermined diameter. Once the measuring tool is lowered to the omega seal site, the previously calibrated arms are moved into contact with the adapter tube with the displacement of the arms being detected by sensors that direct this detection to a control center which provides a digital readout of the actual diameter of the particular adapter tube. Once the actual diameter has been detected, the stalk measuring tool is disengaged and removed. The split canopy installation fixture is then adjusted to accommodate the canopy seal and to properly engage the adapter tube in order to hold the canopy seal in place for welding. The split canopy installation fixture cradling the canopy seal is then lowered to the omega seal site where hydraulic cylinders are remotely activated to clamp the installation fixture about the adapter tube and to camlock the fixture in place. Once the fixture is camlocked in place, additional hydraulic cylinders are remotely activated to properly position the canopy seal about the omega seal site. This fixture remains in place while the robotic weld arm descends to the omega seal site and temporarily tacks the canopy seal in place. Once the canopy seal has been sufficiently secured in place, the installation fixture is removed to allow the robotic weld arm total access to the canopy seal to complete the welding process. The robotic weld arm is provided with five degrees of freedom to allow the robotic weld arm the flexibility and ability to work in a confined area. The arm is capable of performing the C-shaped vertical welds between the two canopy seal halves as well as the radial welds about the upper and lower periphery of the canopy seal. The robotic weld arm is also attached to the carousel positioned about an upper portion of the control rod drive mechanism which provides for the robotic weld arms orbital movement about the control rod drive mechanism and the adapter tube. An extension tube controls the vertical movement of the robotic weld arm while magnetic induction motors control elbow and wrist joints of the weld arm. The weld process is viewed by two cameras located adjacent the torch cup of the robotic weld arm. One camera examines the leading edge of the weld while the other inspects the trailing edge of the weld. Additionally, in order to evacuate all of the oxygen encased by the canopy seal, an argon gas purge opening is inserted under the canopy seal after the installation fixture has been removed. The weld arm then performs its predetermined welding sequence, welding the argon purge opening last. By providing the above mentioned canopy seal welding system, leakages experience at the omega seal site can be repaired from a position remote from the leakage site. This system will provide both efficient and effective servicing of ruptured omega seals while minimizing, and in most cases, eliminating reactor vessel down time, and significantly reducing the radiation exposure of service personnel.