Shunsuke Uchida

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

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.

Evaluation of Wall Thinning Rate Due to Flow Accelerated Corrosion With the Coupled Models of Electrochemical Analysis and Double Oxide Layer Analysis

Systematic approaches for evaluating flow accelerated corrosion (FAC) are desired before discussing application of countermeasures for FAC. Future FAC occurrence should be evaluated to identify locations where a higher possibility of FAC occurrence exists, and then, wall thinning rate at the identified FAC occurrence zone should be evaluated to obtain the preparation time for applying countermeasures. Wall thinning rates were calculated with the coupled models of static electrochemical analysis and dynamic double oxide layer analysis. Anodic current density and electrochemical corrosion potential (ECP) were calculated with the electrochemistry model based on an Evans diagram and ferrous ion release rate determined by the anodic current density was applied as input for the double oxide layer model. The thickness of oxide layer was calculated with the double oxide layer model. The dependences of mass transfer coefficients, oxygen concentrations ([O2 ]), pH and temperature on wall thinning rates were calculated with the coupled model. It was confirmed that the calculated results of the coupled models resulted good agreement with the measured ones. The effects of candidates for countermeasures, e.g., optimization of N2 H4 injection point into the feed water system, on FAC mitigation was demonstrated as a result of applying the model.Copyright

Validation of Code System DRAWTHREE-FAC for Evaluation of Wall Thinning due to Flow Accelerated Corrosion by PWR Feed Water Piping Analysis

The code system DRAWTHREE-FAC for evaluation of pipe wall thinning due to flow accelerated corrosion was validated by comparison of calculations with measurements at the secondary piping of a PWR plant. Distributions of flow velocity and temperature along the whole piping were calculated with the system code RELAP5 and corrosive conditions were calculated by a N2 H4 -O2 reaction analysis code. Precise flow turbulence at major parts of the piping was analyzed with a 3D computational fluid dynamics (CFD) code to obtain mass transfer coefficients at structure surfaces. In the CFD calculation, the κ-e method was applied. Since the κ-e method can not give detailed flow behavior in a boundary layer, the results were extrapolated with a wall function, a power law, and analogy of non-dimensional numbers to obtain mass transfer coefficients in the boundary layer. Then, wall thinning rates were calculated by coupling models of static electrochemical and dynamic oxide layer growth. The wall thinning calculation was focused on T-junction portions of a PWR feed water line. The wall thickness of the PWR secondary piping was measured by the ultrasonic testing. The calculated residual wall thicknesses after thinning agreed with the measurements within ±20% difference.Copyright