Assessment of environmentally assisted cracking in PWR pressure vessel steels

The low alloy ferritic steel used for construction of the reactor pressure vessel and other major circuit components of light water reactors such as steam generator shells, has been subject to extensive study with regard to susceptibility to environmentally assisted cracking (EAC). Two forms of EAC are generally distinguished, viz stress corrosion cracking under steady loading, and corrosion fatigue when subjected to a cyclic load. Recent studies have shown that for the well controlled deoxygenated conditions relevant to PWRs, static stress corrosion cracking is most unlikely ( Hurst et al 1985 ) so that the most probable form of environmental cracking in the reactor pressure vessel is the extension of pre-existing flaws by cyclic crack growth due to plant transient loading. Assessment of cyclic crack growth is currently made using the reference crack growth curves given in Section XI Appendix A of the ASME Boiler and Pressure Vessel Code (1980). Separate curves ( Fig. 1 ) are given for ’dry’ buried defects and for ‘wet’ surface breaking defects. In the latter case the presence of the austenitic clad layer on the walls of the reactor pressure vessel (but not for example in PWR steam generators) is ignored in the assessment. The code recognises an observed influence of mean stress on ‘wet’ crack growth by defining separate curves for high and low stress ratios, R.