In late November 2002, Exelon Corporation’s Dresden Station Unit 3 was experiencing an unidentified leakage in the Drywell. Based on operating experience, the station’s technical staff determined one of the potential sources of the leakage was an instrument line coming off the A-reactor recirculating loop. It was postulated that the line was leaking at the root weld location. While the repair itself would be minor, how to isolate this line to allow the repair to be made safely was a concern. Because of the suspected location of the leak, it would be impossible to isolate this leak with a simple freeze seal. If an alternative isolation technique could not be found, then the site would be forced to remove the reactor head and isolate the leak using internal jet pump plugs. This approach would extend the unplanned outage.

Exelon discovered that Framatome ANP had successfully performed a repair on a similar type of line at Hope Creek in the fall of 2001. Therefore, the company was contracted to design a plug that could be installed through the instrument line piping and locked in the stepped-bore region of the primary piping remotely. The plug would then remain in place during removal and replacement of the degraded region of the instrument line. The site would handle any freeze seals and welding required.

Because the exact location of the leaking instrument line could not be identified, the station technical staff specified that the isolation device be able to navigate a 90-degree elbow downstream of the weld socket. This required serious modification to the isolation process used at Hope Creek. A mockup was constructed to allow testing and to ensure a successful design. The first design using a bladder was unsuccessful. With only 3 days remaining until the outage, a second design utilizing a flexible mechanical plug was developed and successfully tested in the mockup.

On December 6, 2002, Dresden Unit 3 was removed from service. A plant walk-down performed by site personnel confirmed the leak in the 3A Upper instrument line weld. This particular instrument line was installed with a 5-inch radius rather than a hard 90-degree elbow. A second mock up was prepared to verify that the plug design could successfully navigate this path. Final testing and procedures were completed the same day (Saturday, 12/7), and a crew of five was chartered to Dresden that afternoon. The freeze seal was initiated and the line was initially cut during the afternoon on 12/8. Framatome ANP installed the mechanical pipe plug on the evening of 12/8. A video probe verified that the plug was seated properly into the step bore socket. Once this was verified, the rest of the instrument line was removed and a new line was installed and welded into place. During this portion of the work the old piping had to be removed and new piping installed by feeding the plug umbilical down the short section of the instrument line. In the afternoon on 12/9 the plug was removed from the stepped-bore and a new freeze seal was initiated to allow the plug to be removed from the piping and the final repair work was completed.

Dresden Project Management and Engineering indicated that the successful development and implementation of the flexible mechanical pipe plug allowed Exelon to save between 4 and 6 days of critical path schedule.