Degradation of Alloy 800 steam generator tubing and its long-term behaviour predictions for plant life management

Alloy 800 tubing has a good service record in steam generators (SGs) in both German pressurized water reactors and CANDU 6 reactors, however, a recent comprehensive examination of several ex-service SG tubes removed from Darlington Nuclear Generating Station (DNGS) found that these SG tubes (which had experienced shallow pitting in service) were more susceptible to pitting corrosion in laboratory tests than a reference nuclear grade Alloy 800 tubing under SG crevice chemistry conditions. This was an unexpected finding and has raised questions about possible effects of in-service 'aging' on SG tubing. In addition, there has also been recent evidence that a few Alloy 800 tubes have experienced stress corrosion cracking (SCC) in some German pressurized water reactors (PWRs), possibly after many years of degradation-free service, although the inspection history of these tubes is not available to confirm that the reported degradation initiated recently. These findings suggest that Alloy 800 tubing may have some aging degradation susceptibility after many years of service. To provide support for a proactive SG aging management, a survey on the corrosion susceptibility of the archived Alloy 800 tubing from CANDU SGs under plausible crevice chemistry conditions was conducted to assess the potential material degradation issues in CANDU SGs. Experimental work was also performed to investigate the root cause leading to Alloy 800 SG tubing degradation. The results from this study suggested that a combination of negative factors; aggressive chemistry resulting from impurity ingress into the secondary side of the SGs, elevated electrochemical corrosion potential (ECP) during SG transients and surface strain/plastic deformation, might have led to the degradation of the ex-service SG tubing. The studies have shown that each of these conditions in isolation does not cause degradation of Alloy 800 SG tubing; a synergistic combination of factors is required. The OPEX and experimental results imply that Alloy 800 SG tubing may experience measurable aging after many years of service. Although these preliminary results require further confirmation, special attention should be paid to manage the SG tubing degradation in a proactive and predictable manner. Electric Power Research Institute (EPRI) and Dominion Engineering, Inc. (DEI) have developed probabilistic approaches using a degradation free lifetime Weibull distribution based on the OPEX of Alloy 600 SG tubing and a concept of materials improvement factors (MIFs) to 'predict' the long-term service behaviour of other SG tubing. In this paper, a new concept, 'corrosion stress cycle analysis' (CSCA) based on the concept of 'Fatigue Usage Factor' used in mechanical design, is proposed here as a guide for predictive and proactive SG life management. The CSCA approach assumes that an alloy can tolerate a certain number of corrosion stress cycles, i.e., excursions due to off-specification (assuming that the specifications are appropriate) chemistry conditions and SG transients etc. Using the in-service experience, the number of stress cycles that occurred in the history of a SG and in the future can be estimated and the Alloy 800 SG tubing long-term degradation can then be predicted. (author)