Measurement of welding residual stresses of reactor vessels by inherent strain method – Diagnosis of inherent strain distribution function

Abstract

The fundamental objective of this study is to ensure the safety of nuclear reactors. A few accidents involving leaks from welded zones at the pipe penetration part of reactor vessels or at coolant pipes have been reported at home and 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 the high safety of the plant, estimate the plant life cycle and design an effective maintenance plan. Welded joints of nuclear reactor vessels have complex shapes, and the welding residual stresses also have three-dimensional (3D) complex distributions. In this study, inherent strain-based theory and method are applied to measure the welding residual stresses. The inherent strain method is an analytical method as an inverse problem, using the least squares method, based on the finite element method. So the method gives the most probable value and deviation of residual stress. The reliability of the estimated result is discussed. In this method, inherent strains are unknowns. When residual stresses are distributed complexly in a 3D stress-state, the number of unknowns becomes very large. So, the inherent strain distribution is expressed with an appropriate function to decrease largely the number. A mock-up is idealized for a welded joint at the pipe penetration part of an actual reactor vessel. The inherent strain method is applied to the measure the residual stress of the joint. In this paper, the applicability of the inherent strain distribution function is diagnosed. Ten kinds of functions are applied to estimate the residual stress, and the accuracy and reliability of the analysed results are judged from three points of view, i.e. residuals, unbiased estimate of variance of errors and welding mechanics. The most suitable function is selected, which brings the most reliable result.

Keywords:

• reactor vessel
• welding residual stress
• measurement
• inherent strain method
• inherent strain distribution function
• diagnosis
• finite element method

Acknowledgements

This article describes further development of the studies performed to carry out deeper discussion and further analysis to further improve the accuracy measurement, based on the outcomes of the ‘Technical developments related to increased accuracy of innovative interior residual stress measurements and cracking propagation analyses of 3D welded structures’ (representative NAYAMA Michisuke), a study selected to be part of the 2003 Two-year Plan of the Innovative Practical Nuclear Power Technology Development and Proposal Recruitment Project of the Ministry of Economy, Trade and Industry.