**1. Introduction**

In general, corrosion is defined as the degradation of a material by means of chemical reactions with the surrounding environment. Several types of corrosion occur in a variety of situations in the nuclear power plants. Some of these types are common such as rusting of steel when located in moist environment, and the other type of corrosion such as flow accelerated corrosion required special treatment due to their impact on the plant safety and reliability. FAC degradation mechanism results in thinning of large areas of piping and fittings that can lead to sudden and sometimes to catastrophic failures, as well as a huge economic loss. FAC is a process caused by the flowing water or wet steam damaging or thinning the protective oxide layer of piping components. The FAC process can be described by two mechanisms: the first mechanism is the soluble iron production (Fe2+) at the oxide/water interface, while the second mechanism is the transfer of the corrosion products to the bulk flow across the diffusion boundary layer. Although the FAC is characterize by a general reduction in the pipe wall thickness for a given piping component, it frequently occurs over a limited area within this component due to the local high area of turbulence. The rate of the metal wall loss due to FAC depends on a complex interaction of several parameters such as material composition, water chemistry, and hydrodynamic.

In general, erosion processes or mechanisms can be categorized as:


© 2012 Ahmed, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Ahmed, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

can however be avoided by a combination of improvements in: plant design, drying the steam, and the use of more corrosion-resistant steels.

Flow Accelerated Corrosion in Nuclear Power Plants 155

failures did not start before the severe elbow rapture downstream of a tee occurred at Surry Unit 2 power plant (USA) in 1989, which caused four fatalities and extensive plant damage and resulted in a plant shutdown. In 1999, an extensive steam leakage from the rupture of the shell side of a feed-water heater at the Point Beach power plant (USA) was reported by Yurmanov and Rakhmanov [2] (Figure 1). In 2004, a fatal pipe rupture downstream of an orifice in the condensate system due to FAC occurred in the Mihama nuclear power plant Unit 3 (Japan) [2]. More recently, the pipe failure downstream of a control valve at Iatan fossil power plant in 2007 resulted in two fatalities and a huge capital of plant loss as reported by Moore [3]. Although, a combination of lab research and attempts to correlate lab results with plant experience has been the major efforts made towards the study of FAC mechanism since the 1970's, the lab research only focused on understanding the mechanisms, and correlating experimental results in order to reduce the lab effort and to develop usable forms for the plant engineers. Several well cited correlations used to predict the actual corrosion rates due to FAC in piping systems and incorporated in computer software such as CHECWORKS developed by Electric Power Research Institute (EPRI). Following the abovementioned accidents, most utilities around the world have been following EPRI guideline of improving the flow water chemistry to slow down the rate of damage. Also, in the event of disposition of highly susceptible or damaged areas, utilities

1. Replace individual worn components with other components of same material.

3. Replace entire worn lines with other components of same material.

approximately 70% that was identified to be due to the proximity.

2. Replace individual worn components with other components of FAC resistant material.

4. Replace entire susceptible lines or the more susceptible portions with other components

The recent review by Ahmed [4] highlighted the significant research conducted on investigating the effect of fluid chemical properties on flow accelerated corrosion (FAC) in nuclear power plants. He concluded that the hydrodynamic effects of single and two-phase flows on FAC have not been thoroughly investigated for many piping components. In order to determine the effect of the proximity between two components on the FAC wear rate, Ahmed [4] has investigated 211 inspection data for 90o carbon steel elbows from several nuclear power plants. The effect of the velocity as well as the distance between the elbows and the upstream components was discussed. Based on the analyzed trends obtained from the inspection data, the author indicated a significant increase in the wear rate of

Furthermore, the repeated inspections in nuclear power plants have shown that piping components located downstream of flow singularities, such as sudden expansion or contractions, orifices, valves, tees and elbows are most susceptible to FAC damage. This is due to the severe changes in flow direction as well as the development of secondary flow instabilities downstream of these singularities [4]. Moreover, in two-phase flows, the significant phase redistributions downstream of these singularities may aggravate the problem. Therefore, it is important to identify the main flow and geometrical parameters

have typically taken the following initial steps:

of FAC resistant material.


On the other hand, degradation mechanisms involve combined effect of chemical and mechanical processes can be summarized as:

