Mechanical and Chemical Indicators for SCC in Alloy 800 Steam Generator Tubing

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Stress corrosion cracking (SCC) of Alloy 600 steam generator (SG) tubing is a major issue for both CANDU and PWR nuclear reactors. Replacement Alloy 800 (A800) tubing is also not immune to SCC; cracking has recently been reported in Europe. To improve the prediction of the onset of cracking in Alloy 800 under service conditions, the interplay of mechanical and chemical behaviour of this material needs to be thoroughly understood. The effects of chemical environments are well known; the effects on mechanical stress on a microscopic scale are less well understood and their combined effects therefore have not been well characterized. A current industry-sponsored research project has focused on this aspect for the stress corrosion cracking of Alloy 600 SG tubing materials and the project team has developed methodologies for determining the microstrain, grain boundary precipitation and microstructure of the samples exhibiting intergranular cracking. More importantly, some progress has also been made in understanding how these parameters contribute to the crack growth process. Thus, the next step is to apply some of these methodologies to study the SCC behaviour of Alloy 800 SG tubing material, the current material choice for CANDU reactors and the specified material for Advanced CANDU reactors (ACRs). Three tasks have been proposed for the research program. The first task is to determine the conditions that would induce intergranular cracking in a controlled manner. Outcome from the second task would help us to determine the factors that control the cracking of Alloy 800 SG tubing and would also be used to explain the enhanced SCC resistance of Alloy 800 compared to Alloy 600. The third task focuses on the examination of SG tubing material removed from service. This task is of central importance to degradation assessments contained in the Life-Cycle Management (LCM) plans of CANDU power reactors. The proposed research would take place within the multi-university/industry research consortium that has already been established as part of the current research program.