Measurement of welding residual stresses of reactor vessel by inherent strain method: measurement of residual stresses of pipe–plate penetration joint

Abstract

This study aims to ensure the safety of nuclear power plants. The accidents involving leaks from the welded zones at the pipe penetration part of a reactor vessel or at a coolant pipe are reported at home and abroad. One of the main causes is the welding residual stress. So, it is important to know the welding residual stress for maintaining high safety of the plants, the estimation of plant's life cycle and the plan of maintenance. The welded joints of the nuclear power plants have complex shapes, and the welding residual stresses also have complex distributions three-dimensionally. In this study, the inherent strain method combined with finite element method is used to measure the welding residual stresses accurately.

The mock-up is idealized for the welded joint at the pipe penetration part of the actual reactor vessel. The inherent strain method is applied to measure the residual stresses. In this method, the inherent strains are unknowns. When the residual stresses are distributed complexly in a three-dimensional stress-state, the number of unknowns becomes very large. So, the inherent strains are expressed with some functions to decrease the number largely. The theory, the experiment process and the analysed results are explained. The characteristics of the distributions of residual stresses and their production mechanisms are discussed. The inherent strain method gives the most probable values and the deviations of the residual stresses. The deviations are small enough for the most probable values. It assures the high reliability of the estimated results.

Keywords:

• welding residual stress
• measurement
• inherent strain method
• finite element method
• nuclear power plants
• pressure vessel
• welded joint

Acknowledgements

This article is part of the results of ‘Research and development for the improvement of accuracy of crack propagation analysis and a revolutionary internal stress measurement method for three-dimensional welded structures’, a study performed according to a two-year plan from 2003, part of the Recruitment Project to Propose and Develop Innovative and Practical Nuclear Power Technologies of the Ministry of Economy, Trade and Industry.

We would like to express our gratitude to all involved. We also wish to express our gratitude to Mr Yoichi Iwamoto of the Takasago Laboratories of Mitsubishi Industries for his assistance with the tests and Mr Takanori Suwa of Japan General Laboratories (currently of Canon) for his assistance with preparation of the analysis program.