SSM perspective
Background
Reactor pressure vessel internal components of austenitic stainless steel located close to the core are subject to a high neutron flux. Irradiation by fast neutrons induces changes in the material that affect microstructure, mechanical properties and microchemistry. These changes eventually lead to an increased susceptibility to irradiation assisted stress corrosion cracking (IASCC). Neutron irradiation also affects the water environment through radiolysis, which creates oxidizing species in the water that could affect oxidation and corrosion of the material.
Flux thimble tubes that had been in service in Ringhals 2 have been studied in order to improve the understanding of the corrosion process of reactor pressure vessel core internals and to gain insight regarding the effect of neutron irradiation on the metal oxidation and initiation of irradiation assisted stress corrosion cracking.
Results
The oxides formed on stainless steel exposed in the core region of a PWR consisted of a duplex layer structure, with an outer porous layer of fine spinel grains and an inner dense layer of epitaxially grown spinel. Three dose levels, 0, 50 and 100 dpa, were examined and no significant effect of dose was observed on the oxide microstructure. Small spots were observed in the oxide near the metal oxide interface of the irradiated samples. However, it was not possible to confirm any compositional or structural differences of these features from the surrounding oxide. In addition, the presence of Ni rich metallic particles in the oxide was detected by atom probe tomography.
The Transmission Electron Microscope (TEM) lamellae of all three dose levels had one or two oxide penetrations along metal/metal grain boundaries. There were no differences in size or density of this feature among the dose levels examined, although additional examinations would be needed to verify this statement statistically. However, regions elevated in Ni were observed immediately ahead of the oxide penetrations in the irradiated samples.
Clusters or precipitates rich in Ni-Si, presumably γ’ (Ni3Si), sometimes including Mn, Cu and P, were observed in the metal of the irradiated samples. A lower cluster density was observed at grain boundaries. There was no effect of dose on the cluster size and density. The cluster size observed in this study was in agreement with literature data, while the number density was lower. Bulk silicon content, irradiation parameters such as fluence, flux and temperature, as well as differences in data evaluation are factors that can contribute to the differences observed.
The microstructure of the irradiated metal was consistent with observations reported in the open literature. Radiation induced segregation with depletion of Cr and enrichment of Ni and Si was observed in the irradiated conditions.
Cavities were observed in the irradiated samples. The number density of this feature in the sample at 50 dpa agreed with the range reported in the literature, whereas the density at 100 dpa clearly was higher. Additional examinations are suggested to confirm the observation and to determine if any dimensional changes have occurred.
Relevance
By studying flux thimble tubes irradiated for up to 34 years in Ringhals 2, valuable insight has been gained regarding the effects of neutron irradiation to very high doses on the microstructure and oxidation of austenitic stainless steel in pressurized water reactors.
Need for further research
To confirm some of the results of this study over the full range of dose, and to determine if they are general and not isolated observations, the author has identified a few areas for further study.