Framatome ANP - AREVA

Innovation & TechnologySpecially Designed Plug SavesCritical Path Time3 days remaining until the outage,a second design utilizing a flexiblemechanical plug was developed andsuccessfully tested in the mockup.In late November 2002, ExelonCorporation’s Dresden Station Unit3 was experiencing an unidentifiedleakage in the Drywell. Based onoperating experience, the station’stechnical staff determined one of thepotential sources of the leakage wasan instrument line coming off theA-reactor recirculating loop. It waspostulated that the line was leakingat the root weld location. While therepair itself would be minor, how toisolate this line to allow the repair tobe made safely was a concern. Becauseof the suspected location of the leak,it would be impossible to isolate thisleak with a simple freeze seal. If analternative isolation technique couldnot be found, then the site would beforced to remove the reactor head andisolate the leak using internal jetpump plugs. This approach wouldextend the unplanned outage.Exelon discovered that FramatomeANP had successfully performeda repair on a similar type of line atHope Creek in the fall of 2001.Therefore, the company wascontracted to design a plug thatcould be installed through theinstrument line piping and lockedin the stepped-bore region of theprimary piping remotely. The plugwould then remain in place duringremoval and replacement of thedegraded region of the instrumentline. The site would handle anyfreeze seals and welding required.Because the exact location of theleaking instrument line could notbe identified, the station technicalstaff specified that the isolation devicebe able to navigate a 90-degree elbowdownstream of the weld socket. Thisrequired serious modification to theisolation process used at Hope Creek.A mockup was constructed to allowtesting and to ensure a successfuldesign. The first design using abladder was unsuccessful. With onlyOn December 6, 2002, DresdenUnit 3 was removed from service.A plant walk-down performed by sitepersonnel confirmed the leak in the3A Upper instrument line weld. Thisparticular instrument line was installedwith a 5-inch radius rather than a hard90-degree elbow. A second mock upwas prepared to verify that the plugdesign could successfully navigate thispath. Final testing and procedures werecompleted the same day (Saturday,12/7), and a crew of five was charteredto Dresden that afternoon. The freezeseal was initiated and the line wasinitially cut during the afternoon on12/8. Framatome ANP installed themechanical pipe plug on the eveningof 12/8. A video probe verified that theplug was seated properly into the stepbore socket. Once this was verified,the rest of the instrument line wasremoved and a new line was installedand welded into place. During thisportion of the work the old pipinghad to be removed and new pipinginstalled by feeding the plug umbilicaldown the short section of theinstrument line. In the afternoonon 12/9 the plug was removed fromthe stepped-bore and a new freeze sealwas initiated to allow the plug to beremoved from the piping and thefinal repair work was completed.Dresden Project Management andEngineering indicated that thesuccessful development andimplementation of the flexiblemechanical pipe plug allowedExelon to save between 4 and 6 daysof critical path schedule. ■Advanced Nuclear Power N O 7 April 2003 19