Patent Number: 061119288
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nuclear power reactors, and more particularly, to a method and apparatus for sealing leaking welded penetrations to the head of a nuclear reactor vessel. On typical pressurized water nuclear reactors, a plurality of control rod drive mechanisms (CRDM) are mounted on the vessel head. The mechanisms are located in pressure boundary housings and provide controlling motion to the neutron absorbing control rods. The components which make up the housings are assembled to each other and jointed with omega sealed welds. A canopy seal omega weld is between the reactor vessel head CRDM nozzle and the mating part (see FIG. 1). This weld has a tendency to develop cracks as a result of stress corrosion cracking. These cracks typically propagate through the weld until leakage occurs. It is a common design to have reactor vessel head nozzles at every potential control rod position. A majority of the nozzles are used for control rods, a smaller number are used for core-exiting thermocouple instrumentation and several others (spares) are capped. 2. Description of the Prior Art The repairs to leaking canopy seal welds are laborious and time consuming because they must be performed in a confined, highly radioactive area. Typically, the omega seal leaks have been fixed by shutting down the plant, draining down the reactor coolant to a level below the reactor vessel head and applying a weld overlay to the leaking location. This process takes a large amount of time, gives the workers a large dose of radioactivity due to the close proximity and contact with the reactor vessel head, and is no different than the existing weld, which means that it could leak again in the same location. SUMMARY OF THE INVENTION It is an object of the invention to provide a top mount canopy seal clamp assembly (TM-CSCA) as a non-welded mechanical method and device for stopping leaking in a canopy seal weld. The TM-CSCA seals the leaking weld and introduces a compressive load into the canopy seal, which tends to close and arrest the crack propagation. The TM-CSCA seals the leaking weld by compressing a flexible graphite seal over the entire annular canopy seal weld area. The flexible graphite seal material is preferably GRAFOIL.RTM. available from the Union Carbide Corporation. The top mounted canopy seal clamp assembly has three major advantages over weld repair options, in that it will save time, radiation dosage, and re-occurrence of the leak. A distinct advantage of the TM-CSCA over the weld repair options is that installation can be completed without "draindown" of the reactor coolant system. This is a major advantage, because the majority of leaks are found during plant start-up after an outage. For the best weld repair, it is required that there be no water (or moisture) in the nozzle. Therefore, time is saved by not having to "draindown" for repair. The TM-CSCA will also save radiation dosage. The weld repair requires a crew to be within close proximity with the reactor vessel head nozzles (high dose area), for a comparable amount of time. The TM-CSCA is installed from the top of the reactor vessel head lift rig (much lower dose area), with long handle tools. Therefore, the TM-CSCA worker receives a smaller amount of radiation dosage than that worker doing a weld repair. A leak is less likely to reoccur on a nozzle that has been repaired with a TM-CSCA, than a nozzle that has been weld repaired. This is because the TM-CSCA introduces a compressive load into the canopy seal, which tends to close and arrest the crack propagation. The weld repair is no different than the weld that previously exists. Therefore, the TM-CSCA repairs the leak, and prevents future leaks from occurring. The TM-CSCA is installed remotely, from the top of the reactor vessel head (approximately 20 feet from the nozzle), with specially designed long handled tools. The housing is lowered over the nozzle to be repaired, below the penetration or nozzle flange. The first support half is lowered and placed into the housing; then the second support half is installed in the same manner. The flexible graphite seal ring and seal support ring are then lowered into the canopy seal weld to be repaired. The top plate is then lowered to compress the flexible graphite seal ring into place. The cap screws with belleville washers are then installed to provide the necessary retaining loading force.