Abstract
The objective of this study is to ensure the safety of nuclear reactors. A number of accidents occurred from the 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. Welded joints in nuclear reactors have complex shapes, and the welding residual stresses have complex three-dimensional distributions.
In this study, the mock-up is idealized and manufactured for the welded joint at the pipe penetration part of actual reactor vessel. The inherent strain method is applied to measure the welding residual stress of the joint accurately. The inherent strain method is an analytical method which solves an inverse problem using a least squares method.
For more accurate estimation, the robust estimation is applied to the inherent strain method. As one of the robust estimation methods, the least squares method with cut is used. The data of the measured strains are selected, based on the residuals. First, all data are used in the analysis. Some data which have large residuals are cut, and the next analysis is performed, using the rest of the data. Such calculation is repeated until a convergence condition is satisfied. A concrete procedure of the calculation is proposed. The welding residual stress of the joint is analyzed by such robust estimation method. As a result, high-accurate estimation is achieved. The significance of the result is discussed in detail.
