Three-dimensional Measurement of Welding Residual Stress of Thick Coolant Pipe Joint in Nuclear Reactor Plant by Inherent Strain Method

Abstract

The objective of this study is to ensure the safety of nuclear reactors. A number of accidents caused by leaks from welded zones at pipe penetration points of the reactor vessel or in coolant pipes have been reported at sites around the world. One of the main causes of such leaks is welding residual stress. It is therefore very important to know the welding residual stress in order to maintain the safety of the plants, estimate plant life cycle and design an effective maintenance plan. In this study, the inherent strain method combined with Finite Element Method (FEM) is applied to measure the welding residual stress accurately.
A mock-up for a welded joint at a coolant pipe of an actual nuclear reactor was manufactured for the study. The inherent strain method is used to measure the three-dimensional residual stress distribution. In this method, the inherent strains are unknowns. When the residual stresses have a complex three-dimensional distribution, the number of unknowns becomes very large. The inherent strain distribution is therefore expressed with an appropriate function, significantly decreasing the number of unknowns.
10 kinds of inherent strain distribution functions are applied to estimate the residual stress distribution of the joint. Applicability of each function is diagnosed. Accuracy and reliability of analyzed results are judged from three points of view, that is, residuals, unbiased estimate of variance of errors and necessary conditions from welding mechanics. Most suitable function is selected, which brings most reliable result.
The characteristics of the three-dimensional residual stress distribution are cleared. The circumferential stress and axial stress are important, related to stress corrosion cracking (SCC). The circumferential stress is estimated to be large tension near the outer surface of welding finish region. The large tensile axial stress is produced near the outer surface, widely in the circumferential direction. Near the inner surface where SCC may occur, both stresses are small tension or compression.