Evaluation Methods for Corrosion Damage of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics (V) Flow-Accelerated Corrosion under Single- and Two-phase Flow Conditions
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The characteristics of flow-accelerated corrosion (FAC) in the wide sense are overviewed. Overlapping effects of flow dynamics and corrosion are important issues to determine the reliability and lifetime of major structures and components of light water reactor plants. FAC in single- and two-phase flows and liquid droplet impingement (LDI) in two-phase flow are typical phenomena due to both interactions. In order to evaluate local wall thinning due to FAC and LDI, a 6-step evaluation procedure for each has been proposed.(1) For FAC wall thinning evaluation, corrosive conditions along the flow path and mass transfer coefficients at the structure surface under single- and two-phase flow conditions were calculated to evaluate high-risk zones for FAC occurrence, and then wall thinning rates were calculated with the coupled model of static electrochemical analysis and dynamic oxide layer growth analysis at the identified high-FAC-risk zones.(2) For LDI local wall thinning evaluation, trajectories of liquid droplets in high-velocity steam, their collision densities and velocities on the pipe inner surface were calculated to evaluate high-risk zones for LDI occurrence, and then local wall thinning rates were calculated at the identified high-LDI-risk zones. For the region with steam velocity higher than 200 m/s, the wall thinning rate was determined mainly by erosion (LDI (erosion)), while for that with velocity lower than 100 m/s, it was determined by corrosion (LDI (corrosion)).(3) The FAC wall thinning model was applied for two-phase flow as well as single-phase flow, while the FAC wall thinning model was applied for LDI (corrosion). The empirical formula was applied for LDI (erosion) after discriminating LDI (corrosion) and LDI (erosion) based on steam velocity and quality.
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