Developing Response Functions for Crack Opening Displacement to Calculate Leak-Rates Through Complex Cracks in Pipes With Weld Overlays

Abstract

Leak-Before-Break (LBB) is employed in design of nuclear power reactor piping to eliminate consideration of the dynamic effects of pipe rupture from the plant design basis for the affected piping system. LBB cannot be applied if environmental conditions that could lead to degradation by stress corrosion cracking exists. For Alloy 600/82/182 dissimilar metal welds (DMW) in pressurized water reactor plants, primary water stress corrosion cracking (PWSCC) is found to be active. Application of weld overlay (WOL) of non-susceptible Alloy 690/52/152 material has been shown to mitigate PWSCC growth in DMW. Therefore, LBB can be considered for a DMW with Alloy 690/52/152 overlay. However, WOL sizing design postulates a complex crack which is through wall in the overlay material and part through or full circumferential in the DMW base material. This significantly reduces the critical flaw size and in turn the maximum allowable flaw size for leak rate. The current industry practice conservatively ignores the full circumferential crack in the original pipe material and assumes a through wall crack along the entire pipe thickness. This assumptions leads to significantly reduced leakage due to smaller crack opening. The problem becomes more critical with small diameter pipes. The current work calculates the crack opening displacements (CODs) for a pipe with complex crack. Since it is a function of several geometry and materials parameters, response functions are generated to calculate CODs.