IN NOVEMBER 2002, Unit 3 at Exelon Corporation’s Dresden nuclear power plant developed an unidentified leakage in the drywell. Based on previous operating experience the plant’s technical staff determined that the leak was from an instrument line on the A-reactor re-circulating loop at the root weld location.
Although the repair is minor, isolating the line to repair the leak was a major safety concern for the plant. Because of the location of the leak the plant determined that the use of a freeze seal for isolating the line was not possible. If an alternative method could not be found, the plant would be forced to remove the reactor head and isolate the leak using internal jet pump plugs.
After researching different methods, Exelon discovered that a similar type of line repair had been carried out at the Hope Creek nuclear plant by Framatome ANP. Because of the success of the Hope Creek repair, Exelon contracted with Framatome to design a plug that could be installed remotely through the instrument line piping and then be able to lock in the stepped bore region of the primary piping. The plug also had to remain in place during the removal and replacement of the damaged instrument line.
As the exact location of the leaking line could not be identified, the plant specified that the isolation device be able to navigate a 90-degree elbow downstream of the weld socket. This required substantial modifications to the isolation process that had been used at Hope Creek. To ensure a successful design, a mock-up of the process was constructed to test the plug before using it in repairing the leak.
The first design, using a bladder, was not successful. However, a second design, utilizing a flexible mechanical plug, was developed and successfully tested in the mock-up. After removing Unit 3 from service on December 6, 2002 a plant walk down by the plant’s personnel confirmed that the leak was from the 3A upper instrument line weld. This particular line had been installed with a five-inch radius rather than using a 90-degree solid elbow.
Success Full Repair
After determining that the plug design could successfully navigate into the area where the pipe was leaking, the instrument line was cut and the mechanical pipe plug was installed by Framatome ANP. A video probe verified that the plug was seated into the step bore socket correctly. Once the plug was in place the rest of the instrument line was removed and the new one installed and welded into place.
Specially designed plug saves critical path time for Dresden Nuclear Power Plant. Photo courtesy Framatome ANP
During this operation the old piping was removed and new piping installed. This was accomplished by feeding the plug down the short section of the instrument line. After the repair was complete the plug was removed from the stepped bore and a new freeze seal installed. Once the freeze seal was in place the plug was then completely removed and the instrument line reconnected.
According to Dresden’s project management and engineering staff the development and implementation of the flexible mechanical pipe plug was successful and saved four to six days on the plant’s critical path outage schedule.