Various mechanisms degrade components and power piping in nuclear power plants. The mechanism with the greatest consequence has been flow-accelerated corrosion (FAC). FAC has caused ruptures and leaks and has led to numerous piping replacements. United States utilities use a combination of EPRI guidance, software, and aggressive inspection programs to deal with FAC. However, current technology does not detail guidance for erosive forms of attack including, cavitation erosion, flashing erosion, droplet impingement, and solid particle erosion. These forms of degradation have caused shutdowns, and leaks have become a maintenance issue. This brief will present a description of erosive damage mechanisms found in nuclear power plants.
Flow-accelerated corrosion (FAC) is a degradation process that attacks carbon and low-alloy steels. The process has been studied extensively for the last thirty years. Nevertheless piping leaks and ruptures continue to occur. Recent fatal accidents at the Mihama nuclear station in Japan and at the Iatan fossil unit in Missouri demonstrate that plant operators must continue to maintain mitigation programs in order to maintain a high degree of confidence against unanticipated pipe rupture caused by FAC. While FAC is a complex phenomenon, most of its controlling elements are fairly well understood with U.S. nuclear plants using well-developed analytical methods to increase the efficacy of their inspection programs. However, there remains one particular controlling element — described 10 years ago — that is still not widely included in plant inspection programs. This is the entrance effect (or leading-edge effect), which refers to the accelerated attack downstream of a FAC resistant material to FAC susceptible material joint (e.g., a butt weld). This paper provides an update to the previous PVP Conference paper on this subject [1]. Recent plant experience, developments in analysis and the implications to existing plant programs will be discussed.
Flow-acceleration corrosion (FAC) is a degradation mechanism that impacts carbon steel under conditions often found in both nuclear and fossil power plants. FAC damage is normally found between about 90 – 230°C. However, damage at higher temperatures and occasionally lower temperatures has been reported. Although not common, low temperature degradation can result in shutdowns and costly maintenance activities. Described are reports of damage to piping and equipment in the condensate system and downstream of a blowdown demineralizer at several Pressurized Water Reactors (PWR). In all cases, the piping contained flowing deoxygenated, neutral water at about 120°F (∼ 50°C). Also, experience with non-FAC degradation was reported at one unit. This paper also describes damage to systems in two Boiling Water Reactors (BWR). In these cases, there was a low concentration of dissolved oxygen in neutral water. Damage to BWRs may be less common, but damage rates similar to those found in PWRs have been observed. Based on this work, it is recommended that plant operators perform susceptibility analyses and if necessary inspections in areas where there is neutral water with a low concentration of dissolved oxygen. PWR units that run their polishers full-time are especially vulnerable. Implications to system re-design will also be presented.
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