NSUF 21-4346: Mechanistic Insight Through TEM Characterization of Intergranular Irradiation-Assisted Stress Corrosion Crack Tips in As-Irradiated vs. Post-Irradiation Annealed Specimens
Light water reactor neutron irradiation causes radiation-induced defects within the grain interior (e.g., Frank loops, perfects loops, radiation-induced solute clusters and precipitates) and radiation-induced segregation (RIS) at the grain boundaries (e.g., depletion of Cr, enrichment of Ni, and Si) that leads to intergranular irradiation-assisted stress corrosion cracking (IASCC).
A general mechanistic framework has been established. However, the guidance that this general mechanistic framework provides is not quantitative, and it cannot clearly assess relative IASCC susceptibilities based on radiation-induced microstructural changes. For example, even in the laboratory, when a surprisingly high crack growth rate is measured on a specimen, one cannot assess its behavior by comparing its irradiated microstructures to that of similar specimens exposed to comparable fluence. As another example, although two key studies, based on comparison of as-irradiated and post-irradiation-annealed specimens, concluded that radiation-induced defects in the grain-interior are the dominant contributors to IASCC a recent post-irradiation annealing study suggests that radiation-induced segregation best explains its observations.
The studies to date have tended to focus on specific aspects within the mechanistic framework, and on general microstructural/microchemical changes in the bulk or at grain boundaries. This proposal focuses on the crack tip and its local vicinity where all the aspects should be acting in concert and leverages recent advances in TEM characterization.
The objective is to assess the current mechanistic framework of IASCC at the crack tip. Greater insight is anticipated by focusing on the crack tip and its local vicinity (the “scene of the crime”) in examining radiation-induced changes in microstructure/microchemistry, as well as their consequences on localized deformation (dislocation channels) and the protective oxide that forms at the crack tip. This insight will enhance the mechanistic framework and inform the direction of future research.
The source material is a type 304SS extracted from a BWR core shroud that experienced 2.1-3.4 dpa, from which SS-J2 tensile samples were fabricated. A subset of the SS-J2 samples were post-irradiation annealed at ONRL in a vacuum furnace at 553±1 °C for 60.7 minutes to remove radiation-induced defects in the grain interior but retain RIS at grain boundaries. Subsequently, both as-irradiated and PIA samples underwent constant extension rate testing (CERT) at the University of Michigan, funded by the Electric Power Research Institute (EPRI).
In this proposal, four of the SS-J2 samples, tested by CERT, will sent to INL (at EPRI expense) for comparative investigation: (1) an as-irradiated sample tested in BWR normal water chemistry [oxidizing condition]; (2) an as-irradiated sample tested in BWR hydrogen water chemistry [reducing condition]; (3) a PIA sample tested in BWR normal water chemistry (oxidizing condition); (4) a PIA sample tested in HWC hydrogen water chemistry (reducing condition).
Crack tip specimens will be extracted by FIB milling for TEM characterization: one TEM crack tip specimen will be examined from each of the four SS-J2 samples (4 TEM crack tip specimens total). The images (diffraction-based images and compositional maps) and data produced will provide the following:
• Sizes, densities, and characteristics of irradiation-induced defects in the grain interior, including high-sensitivity compositional mapping by EDS and EELS to assess the presence of solute clusters not detectable by TEM diffraction • Quantification of dislocation channels impinging on the grain boundary • RIS-modified composition of the grain boundaries, including minor elements such as Si by EDS and EELS • Morphology, composition, and EELS determination of the oxidation states of the oxide at the crack tip vicinity
Additional Info
| Field | Value |
|---|---|
| Abstract | Light water reactor neutron irradiation causes radiation-induced defects within the grain interior (e.g., Frank loops, perfects loops, radiation-induced solute clusters and precipitates) and radiation-induced segregation (RIS) at the grain boundaries (e.g., depletion of Cr, enrichment of Ni, and Si) that leads to intergranular irradiation-assisted stress corrosion cracking (IASCC). A general mechanistic framework has been established. However, the guidance that this general mechanistic framework provides is not quantitative, and it cannot clearly assess relative IASCC susceptibilities based on radiation-induced microstructural changes. For example, even in the laboratory, when a surprisingly high crack growth rate is measured on a specimen, one cannot assess its behavior by comparing its irradiated microstructures to that of similar specimens exposed to comparable fluence. As another example, although two key studies, based on comparison of as-irradiated and post-irradiation-annealed specimens, concluded that radiation-induced defects in the grain-interior are the dominant contributors to IASCC a recent post-irradiation annealing study suggests that radiation-induced segregation best explains its observations. The studies to date have tended to focus on specific aspects within the mechanistic framework, and on general microstructural/microchemical changes in the bulk or at grain boundaries. This proposal focuses on the crack tip and its local vicinity where all the aspects should be acting in concert and leverages recent advances in TEM characterization. The objective is to assess the current mechanistic framework of IASCC at the crack tip. Greater insight is anticipated by focusing on the crack tip and its local vicinity (the “scene of the crime”) in examining radiation-induced changes in microstructure/microchemistry, as well as their consequences on localized deformation (dislocation channels) and the protective oxide that forms at the crack tip. This insight will enhance the mechanistic framework and inform the direction of future research. The source material is a type 304SS extracted from a BWR core shroud that experienced 2.1-3.4 dpa, from which SS-J2 tensile samples were fabricated. A subset of the SS-J2 samples were post-irradiation annealed at ONRL in a vacuum furnace at 553±1 °C for 60.7 minutes to remove radiation-induced defects in the grain interior but retain RIS at grain boundaries. Subsequently, both as-irradiated and PIA samples underwent constant extension rate testing (CERT) at the University of Michigan, funded by the Electric Power Research Institute (EPRI). In this proposal, four of the SS-J2 samples, tested by CERT, will sent to INL (at EPRI expense) for comparative investigation: (1) an as-irradiated sample tested in BWR normal water chemistry [oxidizing condition]; (2) an as-irradiated sample tested in BWR hydrogen water chemistry [reducing condition]; (3) a PIA sample tested in BWR normal water chemistry (oxidizing condition); (4) a PIA sample tested in HWC hydrogen water chemistry (reducing condition). Crack tip specimens will be extracted by FIB milling for TEM characterization: one TEM crack tip specimen will be examined from each of the four SS-J2 samples (4 TEM crack tip specimens total). The images (diffraction-based images and compositional maps) and data produced will provide the following: • Sizes, densities, and characteristics of irradiation-induced defects in the grain interior, including high-sensitivity compositional mapping by EDS and EELS to assess the presence of solute clusters not detectable by TEM diffraction • Quantification of dislocation channels impinging on the grain boundary • RIS-modified composition of the grain boundaries, including minor elements such as Si by EDS and EELS • Morphology, composition, and EELS determination of the oxidation states of the oxide at the crack tip vicinity |
| Award Announced Date | 2021-06-07T16:15:39.847 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Alina Zackrone |
| Irradiation Facility | None |
| PI | Jean Smith |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 4346 |