Localized Thinning Assessment - Service Life Extention for Darlington Feeders Feeders

Abstract

Flow assisted corrosion (FAC) causes the highest rates of wall loss at outlet feeder pipes in the regions close to the Grayloc end fittings. Stress analyses have to be performed to demonstrate feeder fitness for service (FFS) with reduced wall thickness as per ASME Section III or other accepted Codes and Standards. Based on inspection data since 2007 it was identified that thinning near the Grayloc weld was 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. With ccurrent projection, more than 230 feeders in all 4 DNGS Units will require replacement/repair by the End of Life (EoL).The results of generic stress analysis performed under the Localized Feeder Stress Analysis Project (LFSA) demonstrated that all DNGS outlet feeders have sufficient structural integrity to be declared FFS when subjected to localized wall loss adjacent to the Grayloc weld below the current approved PB thickness values 2.75mm and 3.33 mm for 2.0’’ and 2.5’’ feeders respectively. In most cases, it is possible to demonstrate that a thickness equivalent to 75% of the current allowable limit is acceptable: 2.07mm and 2.50mm for 2.0’’ and 2.5’’ feeders respectively.This paper presents methodologies employed in LFSA and its generic results. Conservative operational margin is discussed and used in feeder disposition. A disposition procedure is demonstrated on a sample application.