In English

Structural Characterization of Oxide Films Formed on Stainless Steel of Type 304L in Simulated PWR Primary Water

Cecilia Fager
Göteborg : Chalmers tekniska högskola, 2015. 48 s.
[Examensarbete på avancerad nivå]

ABSTRACT Many different materials are used in Pressurized Water Reactors (PWR). In PWR high temperature high pressure corrosive environments a part of the corrosion products of these materials would release into the reactor coolant, some of which may get neutron activated to form a variety of radionuclides. The radionuclides, in the form of either solids or dissolved chemical species, circulate in reactor circuits and can deposit onto the oxide films being formed on the metal surfaces, leading to activity build-up in the plant and thus increases the dose rate. Zinc injection is known to be capable of reducing the corrosion rate of reactor materials and the activity build-up and is therefore being increasingly implemented in light water reactors worldwide. However, the fundamental mechanism behind the effects of zinc has not yet been fully understood. In this work the oxide films formed on three test coupons of stainless steel of type 304L that had been exposed to simulated PWR primary water conditions have been examined. Each test coupon had been subjected to different exposure environments such as with or without zinc injection, and Co-60 radiotracer was used to measure activity deposition rates under various water chemistry conditions. In order to understand the different activity deposition rates on the three test coupons, a structural characterization has been carried out using X-Ray Powder Diffraction (XRD), Laser Raman Spectroscopy (LRS), Scanning Electron Microscopy (SEM), equipped with Energy Dispersive X-ray Spectroscopy (EDS) and Focused Ion Beam (FIB), and High Resolution Analytical Transmission Electron Microscopy (HR ATEM). The presence of spinel phase (Me3O4) in all oxide films has been confirmed by the XRD and partly also by the LRS measurements. High resolution SEM topography examinations have revealed two kinds of surface oxide grains, (1) relatively large Fe-rich oxide crystals of regular shapes being sparsely distributed on the corroded surfaces, (2) tiny spherical Si-rich oxides which formed agglomerates locally. Cross-section examination with the FIB/SEM technique has revealed a thin oxide film, with a thickness of approximately 10 nm, inward growing crater oxides, with depth of approximately 200 nm, that were present beneath surface oxide grains. High resolution micrographs of the oxide films and zinc concentration profiles across the thin oxide films have also been obtained with HR ATEM. Combined with the present experimental findings the Point Defect Model (PDM) as developed by Macdonald has been used to explain the inhibition effect on the Co-60 incorporation in the oxide films by zinc injection. Suggestions for future work are also proposed.

Nyckelord: PWR, SS304L, XRD, LRS, SEM, TEM, EDS