Probabilistic Estimation of Flow-Accelerated Corrosion Rate at the Welded Joints of the Nuclear Piping System

Abstract

Feeder piping is an integral part of the heat transport system (HTS) that supplies the primary coolant from the reactor to the steam generator in CANDU reactors. One of the life limiting factors of the feeder pipes is the highly localized wall thinning caused by the flow-accelerated corrosion (FAC) at the welded joints of the pipes. To ensure the fitness-for-service of the piping system, periodic inspections of the pipe wall thickness and estimation of the FAC thinning rate at the welded joints are needed. A major challenge of FAC rate estimation at the welded joints is that the initial wall thickness is known precisely, since the grinding process before the welding introduces initial thinning in the pipes. Using the nominal wall thickness without considering this initial thinning is likely to overestimate the FAC rate. Another difficult is the sizing error in the wall thickness measurements, which also needs to be properly accounted. This paper develops a sound probabilistic method for the FAC estimation for the welded joints considering both the initial thinning and sizing error. Predictions regarding the lifetime of individual welded joints are also obtained from the proposed method. A practical case study of the problem in a nuclear plant is presented.