Probabilistic Structural Integrity Assessment Based on Uncertainty of Weld Residual Stress at the Piping Butt-Welds of Nuclear Reactor Components

Stress corrosion cracking (SCC), which affects the structural integrity of reactor component, has been observed at some piping joints made by austenitic stainless steel in BWR plants. It is well known that the SCC behavior is significantly scattered depending upon the various conditions such as materials, piping geometry, crack growth rate, weld residual stress, and so on. Since probabilistic fracture mechanics (PFM) analysis method treats such scatter and uncertainties in the structural integrity evaluation, it is, therefore, useful to apply the PFM analysis to the evaluation of the piping integrity. In JAEA, the PFM analysis code of PASCAL-SP for aged piping has been developed based on Monte Carlo method as described in our previous paper1). Among the conditions related to SCC behavior, weld residual stress near the welded joint is one of the most important factors to assess the structural integrity of piping because the tensile residual stress becomes a driving force of a SCC. Welding conditions such as heat input, welding speed and piping geometry affect weld residual stress distribution at the welded joint of piping. Effect of the welding conditions on the weld residual stress distribution has not yet been evaluated quantitatively. Hence, in this study, an effect of uncertainty of welding conditions, such as scatters of heat input and welding speed during welding, on weld residual stress at the piping butt-welds was evaluated using the simulation method by varying the welding conditions. Probabilistic fracture mechanics analysis using PASCAL-SP was also performed to evaluate the effect of uncertainty of weld residual stress on the break probability of piping. It was clarified that the break probability increased with increasing the uncertainties of residual stress.