2010 Volume 28 Issue 2 Pages 208-215
The objective of this study is to ensure safety of nuclear reactors. A few accidents of leaks from welded zones at pipe penetration parts of reactor vessel or in coolant pipes have been reported 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.
The inherent strain method is an analytical method that solves an inverse problem using a least squares finite element method. The method gives the most probable value and the deviation of the residual stress, allowing the reliability of the estimated results to be discussed. In this method, the inherent strains are unknowns. When 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. In this study, inherent strain theory and method are applied to measure the welding residual stresses for a mock-up of a welded joint at a pipe penetration of reactor vessel.
In this paper, applicability of strain measurement region is diagnosed. 5 kinds of regions are applied to estimate the residual stress, and accuracy and reliability of analyzed results are judged from 4 points of view, that is, residuals, unbiased estimate of variance of errors, welding mechanics and economy. Most reliable and economical measurement region is selected, which brings most reasonable result.