Effect of water chemistry improvement on flow-accelerated corrosion in PWR secondary systems

Flow-accelerated corrosion (FAC) of carbon steel (CS) piping has been one of the main issues in light-water nuclear reactors (LWRs). Wall thinning of CS piping due to FAC increases the potential risk of pipe rupture as well as the costs for inspection and replacement of damaged pipes. In particular, corrosion products generated by FAC of CS piping can lead to steam generator (SG) tube corrosion and degradation of thermal performance when they enter and accumulate on secondary side of pressurized water reactors (PWRs). To maintain SG integrity by suppressing the corrosion of CS, high pH all-volatile treatment (AVT) chemistry (High-AVT) (feedwater pH 9.8{+-}0.2) was adopted in Tsuruga-2 (1 160 MW PWR, in commercial operation since 1987) in July 2005 to replace the conventional Low-AVT chemistry (feedwater pH 9.3). After adopting High-AVT, the accumulation rate of iron in the SG decreased to one-quarter of that observed under conventional Low-AVT. As a result, the previously declining SG thermal efficiency was improved. However, it has become clear that High-AVT chemistry is ineffective against FAC in the regions where the flow turbulence is much greater. By contrast, wall thinning of CS feedwater pipes due to FAC has been successfully controlled by oxygen treatment (OT) for long time in boiling water reactors (BWRs). This is due to the fact that the magnetite film formed on CS surfaces under AVT chemistry has a higher solubility and porosity in comparison with hematite film, which is formed under OT. In this paper, the behavior of FAC under various pH and dissolved oxygen concentrations is discussed based on actual wall thinning rates of BWR and PWR plants and on experimental results in a FAC test-loop. It has been established that FAC is suppressed even under extremely low dissolved oxygen concentrations such as 2 {mu}g . kg{sup -1} under AVT conditions in PWRs. Based on this result, we propose oxygenated water chemistry for PWR secondary systems, as it can mitigate FAC of CS piping without any adverse effects on the SG integrity. Furthermore, the applicability and effectiveness of this concept developed for FAC suppression in PWR secondary systems is discussed based on results of an in-plant test at Tsuruga-2. (orig.)