The Synergistic Impurities Effect on the Candu Steam Generator Corrosion

Lucan, Dumitra; Velciu, Lucian; Jinescu, Gheorghita

Dumitra Lucan, Lucian Velciu, Gheorghita Jinescu

The Synergistic Impurities Effect on the Candu Steam Generator Corrosion

Abstract:

The concentration of the impurities and deposits formation on the steam generator structural materials can cause several types of corrosion. The failure of the steam generator as a result of tubing and tubesheet degradation by corrosion has been a major cause of Pressurized Water Reactor (PWR) plant unavailability. Steam generator problems have caused major economic losses in terms of lost electricity production through forced unit out ages and, in cases of extreme damage, as additional direct cost for large-scale repair or replacement of steam generators. Steam generator materials (especially the tubes material) are susceptible to failure by a variety of mechanisms, the most majority of which are related to corrosion in the presence of deposits containing aggressive impurities. The feedwater that enters the steam generators under normal operating conditions is extremely pure, but nevertheless contains low levels (generally in the µg/l concentration range) of impurities such as iron, chloride, sulphate, silicate, etc. When water is converted to steam, the non-volatile impurities are left behind. As a result the concentrations of different impurities in the water from the bulk steam generator are considerably higher. Nevertheless, the concentrations of corrosive impurities are still low and therefore the bulk water is not significantly aggressive towards steam generator materials. The excellent performance of CANDU steam generators can be attributed, in part, to their design and performance characteristics, which typically involve higher recirculation ratios and lower heat fluxes and temperatures than other PWR steam generator. The purpose of this paper consists in assessment of corrosion behavior of the tubesheet material (carbon steel SA508 class 2) at the normal secondary circuit parameters. The testing environment was the demineralized water containing silicon compounds, sulfates and chlorides at pH=9.5 regulated with morpholine and cyclohexylamine (All Volatile Treatment - AVT). The paper presents the results of metallographic examinations and the results of electrochemical measurements. The experimental results allowed us to establish the contribution of the impurities concentration on the evolution of the corrosion processes.

Keywords:
steam generator tubesheet; corrosion; carbon steel SA 508 cl.2; water chemistry; impurities concentration

Issue: 2016, Volume 67, Issue 4

Pages:

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This article is published under the Creative Commons Attribution 4.0 International License

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