Introduction
In the US alone, material degradation problems due to Environmentally Assisted Degradation (EAD) - Fatigue, Corrosion Fatigue (CF), and Stress Corrosion Cracking (SCC), have cost the nuclear industry at least $10 billion in the last 30 years because of forced and extended outages, increased inspection requirements, component repairs/replacements, and increased scrutiny by the regulator. As designs demand higher performance and longer lives—up to 80 years proposed--incidences of environmentally assisted degradation will increase. Plant modifications and upgrades, compliance with new regulatory standards, and requirements that designs account for fatigue, are driving the need for fatigue testing.
As Alloy 800 is being considered as a material of choice for new designs, testing in representative CANDU environments and US PWR and BWR environments is required. Currently, there is little information on Incoloy 800 (Alloy 800) in CANDU primary water, or in environments representative of secondary side crevices.
Fatigue and Corrosion Fatigue Issues
Fatigue cracking is a common failure mechanism for many nuclear power plant components, including steam generator tubing materials under conditions of flow-induced vibration with chemical interaction. Data for flaw growth rates, including fatigue cracking and corrosion fatigue cracking, are key inputs to validating fatigue crack growth models, fitness-for-service assessments, and projecting future behavior.
Kinectrics Solution
Near-threshold fatigue crack growth rate tests on chemically and microstructurally representative steam generator materials were performed under air and simulated crevice environments using different fatigue testing variables.
Scope of Work
Manufacturing to obtain chemically and microstructurally representative SG materials
Tests performed according to ASTM E647 with DCPD crack length monitoring
Decreasing DK under Constant R (load ratio) and Constant stress intensity Kmax at frequency between 10-30 Hz.
High (R = 0.7) and low load ratio (R = 0.1) to assess crack closure effects
Room temperature air, elevated temperature air (300°C) and elevated temperature neutral crevice environment (300°C)
Additional Capabilities:
Results
Threshold plots illustrating the relative effects of load ratio, temperature, and environment on threshold stress intensity were determined and, data derived for use in unified fatigue crack growth rate models to improve SG tubing fitness-for-service assessments.