Feeder Grayloc hub local allowable thickness - a comparison of ASME section III FFSG Appendix E Level 2 evaluation

Flow assisted corrosion (FAC) causes high rates of wall loss at outlet feeder pipes. The most affected area for Darlington outlet feeders is close to the Grayloc end fittings. Inspection data since 2007 identifies that thinning near the Grayloc weld is randomly distributed throughout outlet feeders at the Darlington Nuclear Generation Station (DNGS). The extent of thinning is predicted to reduce the remaining wall below the pressure based (PB) thickness limit for a large portion of the feeder population in the near future. Stress analyses must be performed to demonstrate feeder fitness for service (FFS) with reduced wall thickness as per ASME Section III or other accepted Codes and Standards. The stress analyses using both ASME III (Reference 1) and Fitness For Service Guideline for Feeders (FFSG) Appendix E Level 2 (Reference 2) methodologies were performed under the Localized Feeder Stress Analysis Project (LFSA). It has demonstrated that almost all DNGS outlet feeders have sufficient structural integrity to be declared FFS until the planned Darlington reactor refurbish dates. This results in significant reduction in feeder replacement associated economical cost and personnel radiation dosage. This paper presents the generic methodologies and a comparison of the results of ASME III and FFSG Appendix E Level 2. It demonstrates both the advantages and limitation of the FFSG method. (author)