CHARACTERIZATION OF OXIDE FILM ON THE SURFACE OF SCC IN PLR PIPE BY MICRO RAMAN SPECTROSCOPY AND ITS IMPLICATION TO CRACK GROWTH CHARACTERISTICS AT ONAGAWA NUCLEAR POWER PLANT

摘要

Micro Raman Spectroscopy (MRS) is known as one of the most convenient techniques for surface analysis. In this study, a MRS system was applied to analyze oxides on a stress corrosion cracking (SCC) fracture surface at the site of a nuclear power plant in Onagawa, Japan. The sample was cut from a cracked component of the Primary Loop Recirculation (PLR) Pipe of the Onagawa Nuclear Power Plant in operation for 14.6 EFPY. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) were also applied for observation and elemental analysis of the fracture surface. The oxide film on the fracture surface and the oxide particles on the film were observed with SEM. These oxides were identified as one or more of the following spinel oxides: NiFe_2O_4, Fe_3O_4, and FeCr_2O_4). These were present over the whole crack surface. It is worthwhile to note that α-Fe_2O_3 was detected near the crack mouth and Cr enrichment in the oxides was detected at the crack tip. Although α-Fe_2O_3 formed on the surface of austenitic stainless steel exposed to oxygenated water, since conditions in the shallow crack maintained a low pH and a low electrochemical potential, Cr enrichment and a lack of α-Fe_2O_3 would be expected at the crack tip. On the other hand, when the crack opening became large α-Fe_2O_3 was observed. Since oxides formed appeared to correspond to the size of the crack opening, oxide analyses were performed on the fracture surfaces of two different size specimens. SCC was generated in these specimens with different crack growth rates in a simulated BWR environment in the laboratory. α-Fe_2O_3 was observed near the crack tip in the small specimen with a slow crack growth rate. A correlation between the distribution of oxides and the size of the crack opening was extrapolated. It is likely that the existence of α-Fe_2O_3 near the crack tip in the plant sample is associated with either blunting or a large crack opening caused by crack growth retardation. Finally, it is assumed that the crack in the plant sample grew in the circumferential direction rather than the radial direction since α-Fe_2O_3 was observed closer to the tip of the central part of the crack rather than to the side edge.