Abstract
Fundamental objective of this study is to ensure safety of nuclear reactor. A few accidents of leak from welded zones at pipe penetration part of reactor vessel or at coolant pipe are reported at home or abroad. One of the main causes is welding residual stress. Therefore, it is very important to know the welding residual stress in order to maintain high safety of the plant, estimate plant life cycle and design effective maintenance plan. Welded joints of nuclear reactor vessel have complex shapes, and the welding residual stresses also have three-dimensional complex distributions. In this study, inherent strain-based theory and method are applied to measure the welding residual stresses.
The inherent strain method is one of analytical method as inverse problem, using least squares method, based on finite element method. So the method gives most probable value and deviation of residual stress. Reliability of estimated result can be discussed. In this method, inherent strains are unknowns. When residual stresses are distributed complexly in 3-dimensional stress-state, the number of unknowns becomes very large. So, inherent strain distribution is expressed with appropriate function to decrease largely the number. A mock-up is idealized for a welded joint at pipe penetration part of actual reactor vessel. The inherent strain method is applied to measure residual stress of the joint.
In this paper, applicability of inherent strain distribution function is diagnosed. 10 kinds of functions are applied to estimate the residual stress, and accuracy and reliability of analyzed results are judged from 3 points of view, that is, residuals, unbiased estimate of variance of errors and welding mechanics. Most suitable function is selected, which brings most reliable result.
