Abstract
The water chemistry of the secondary coolant in the majority of Nuclear Power Plants is controlled by AVT (All Volatile Treatment), wherein volatile amines are used to maintain the alkaline pH required for minimizing the corrosion of structural materials which one of them is Carbon Steel. In this treatment, ammonia, morpholine and ethanolamine are commonly used as conditioning reagents. In this context, experiments were carried out by exposing carbon steel A106 B samples in a simulated secondary coolant in order to study the nature of the oxide films. The tests were performed in a static autoclave at 260°C using two media: I) hydrazine + morpholine and II) hydrazine + ethanolamine during different exposure periods up to ≈1020 h. The oxide film characterization was mainly studied using Scanning Electron Microscopy and X-ray diffraction. A chemical descaling procedure was used to obtain the material weight loss (W) of samples, the adherent and released oxide. The XRD analyses, for all exposures studied, showed that magnetite was the corrosion product formed in the films grown in both media. The material weight loss, after descaling, could be fitted by a law of the type W = ktn, up to 1020 h of exposure tested, resulting in n = 0.42, k = 6.24 for films grown in medium I) and n = 0.39, k = 6.08 for films grown in medium II) respectively (W is in mg/dm2 and t in h). The higher corrosion product release measured in the medium with morpholine could be important in power plant operation.
Highlights
Structural materials of the primary and secondary cooling systems of Water Cooled Nuclear Reactor (WCNR) are exposed to severe high temperature and pressure conditions, so that the materials employed in these plants have to take into account a useful design life of at least 30 years
The morphology of the oxide films were studied by SEM (Scanning Electron Microscopy) and were analyzed by conventional X-ray diffraction with a diffractometer equipped with monochromator and Cu Kα radiation
SEM micrographs have shown that the oxide films present a similar surface morphology in both media, but the crystal sizes grow with the exposure time [9]
Summary
Structural materials of the primary and secondary cooling systems of Water Cooled Nuclear Reactor (WCNR) are exposed to severe high temperature and pressure conditions, so that the materials employed in these plants have to take into account a useful design life of at least 30 years. Secondary circuit of WCNR employs demineralized water to limit the concentration impurities and some chemical conditioning products to achieve an appropriate pH and redox potential to minimize the normal general corrosion produced by the interaction of the materials with the coolant. Many WCNRs use morpholine as reagent amine in the secondary medium, but other reactors have used morpholine during many years since their start of they had changed to other amine i.e.: ethanolamine. Embalse Nuclear Power Plant (Embalse NPP) is a CANDU 600 nuclear reactor and the secondary circuit of it is built mainly with carbon steel A106 B; for this reason, long-term works have been done in order to know and mitigate several corrosion aspects of this material. It is very important to diminish the release products to the medium in the power plants for the cleanliness of the system and to diminish the transport of radioactivity products that are normally incorporated in the oxide films grown on the materials which part of them are after released to the cooling medium and transport them to the different parts of the circuit or components of it
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