2005:60 Research and Service Experience with Environmentally-Assisted Cracking in Carbon and Low-Alloy Steels in High-Temperature Water

In this report, the most relevant aspects of research and service experience with environmentally assisted cracking of carbon and low-alloy steels in high-temperature water are reviewed, with special emphasis on the primary pressure-boundary components of boiling water reactors. The main factors controlling the susceptibility to environmentally assisted cracking under light water reactor conditions are discussed with respect to crack initiation and crack growth. The adequacy and conservatism of the current BWRVIP-60 stress corrosion cracking disposition curves, ASME III fatigue design curves, and ASME XI reference fatigue crack growth curves, as well as of the GE environmentally assisted crack growth model are evaluated in the context of recent research results. The operating experience is summarized and compared to the experimental/mechanistic background knowledge. Finally, open questions and possible topics for further research are identified.

In spite of the absence of stress corrosion cracking in the field, several unfavourable critical parameter combinations, which can lead to sustained, fast stress corrosion cracking with crack growth rates well above the BWRVIP-60 stress corrosion cracking disposition curves have been identified. Many of them appear atypical for current BWR plant operation with properly manufactured carbon and low alloy steel components, but some could occur during service, at least temporarily under faulted conditions or in components with fabrication deficiencies. In the opinion of PSI, although there are open questions and potential for improvements in all fields, from a safety perspective, the special emphasis of research should be placed on these conditions, and in particular, on an improved identification/quantification of the boundaries/thresholds for the transition from low to high/accelerated stress corrosion cracking crack growth rates.

In this context, PSI consider that research should be focused on the effects of chloride transients and dynamic strain ageing/yield stress on the stress corrosion crack growth behaviour of carbon and low alloy steel and of weld heat-affected zone materials under BWR normal water chemistry conditions. Additionally, the mitigation effect of hydrogen water chemistry or noble metal chemical addition should be evaluated under these critical conditions.