Stress corrosion cracking (SCC) in high temperature water is one of the most critical unknowns for failures in light-water reactors (LWRs). Extensive studies have been conducted to understand the SCC mechanisms of structure materials (Ni-base alloys, stainless steel, carbon steels, etc.). However, the SCC mechanisms are still not currently well understood. It is believed that the local temperature, chemistry, structure, stress and deformation at the crack fronts influence the crack propagation. Validated models to relate these factors with SCC mechanisms are not available. The most recent studies by Pacific Northwest National Laboratory (PNNL) using analytical transmission electron microscopy (ATEM) and atom probe tomography (APT) reveal atomic-scale processes at crack tips that drive penetrative oxidation which is believed to influence SCC crack growth. Therefore, it is possible to understand the mechanisms using advanced materials characterization technologies focusing on the microchemistry and microstructure evolution at/near crack tips. The main goal of the proposed research is to understand the potential mechanisms that drive SCC growth in the primary water environments of pressurized water reactors (PWRs) and to identify the key factors that affect the primary water SCC (PWSCC) growth rate. The research will be performed by extensive characterization of tested-specimens from PWSCC testing systems with carefully-controlled operation conditions (water chemistry, temperature, loading) and extensive comparisons between the results of different materials and the same material in different test conditions or with different pre-treatment processes (code work and heat treatment). The following tested-alloys will be analyzed: Alloys 690, 152, 52, 52M and 52i, which will be provided by GE Global Research Center (GE-GRC). The surface and near surface microstructure and microchemistry, the oxide layer cross-section structure and chemistry, the grain boundary distribution, precipitation and chemistry, the penetrative and selective oxidation, and the vacancy distribution at crack tips and near crack fronts will be extensively analyzed using multi-dimensional materials characterization technologies including TEM, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), electron backscattered diffraction (EBSD), and energy dispersive spectroscopy (EDS).

The main goal of the proposed research is to improve the understanding of the stress corrosion cracking (SCC) mechanisms of Ni-base alloys as structural materials in the applications of PWRs. The research goal directly addresses missions of the Light Water Reactor Sustainability (LWRS) program. Materials aging and degradation, focusing on development of the scientific basis for understanding and predicting long-term environmental degradation behavior of materials in nuclear power plants, is one of the Research and Development (R&D) pathways of the program. The proposed research will result in microstructure and microchemistry at /near crack tips, and then provide fundamental data to understand the mechanisms that drive SCC, which will be applied to assess the performance of steam generators and weldments where Ni-Base alloys have been used.