On the shape of stress corrosion cracks in water-cooled nuclear power reactor piping

The evolution of the shape of surface cracks in sensitized Type 304 SS in Boiling Water Reactor (BWR) primary coolant at 288 °C was explored as a function of the electrochemical potential and stress intensity using the Coupled Environment Fracture Model (CEFM). The revised CEFM, which incorporates Shoji's model for calculating the crack tip strain rate and more advanced expressions for estimating the stress intensity factor for semielliptical surface cracks, provides more accurate prediction of the dependence of the crack growth rate on stress intensity factor and offers an alternative explanation for the development of semielliptical cracks than that provided by fracture mechanics alone. The evolution of the shape of surface cracks depends strongly upon environmental variables, such as the corrosion potential and solution conductivity, and the CEFM predicts that the minor axis of the ellipse should be oriented perpendicular to the surface, as is commonly observed, due to the dependence of the crack growth rate on the electrochemical crack length.