An advanced coupled environment fracture model for hydrogen-induced cracking in alloy 600 in PWR primary heat transport environment

A theoretical model for hydrogen-induced cracking (HIC) has been developed to estimate the inter-granular crack growth rate (CGR) in Alloy 600 in aqueous environments. In this model, crack growth is assumed to occur by anodic dissolution at the crack tip with the combined effect of nucleation and growth of voids ahead of the crack tip along a grain boundary, and linkage with the main crack due to stress-induced fracture of the remaining ligaments. Under proper electrochemical conditions, hydrogen evolution at the crack tip results in atomic hydrogen diffusing ahead of the crack tip followed by recombination in voids to generate a pressure within a void that increases with time. The hydrogen pressure adds to the hydrostatic stress on the void due to mechanical loading, resulting in an increase in void rupture frequency and hence in the CGR. Calculations were performed for Alloy 600 in PWR heat transport circuit for different electrochemical and mechanical conditions. The calculated results indicate that the hydrogen evolution at the crack tip under PWR conditions has a significant effect on the CGR.