Evaluation Methods for Corrosion Damage of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics (III)

Abstract

Flow accelerated corrosion (FAC) is divided into two processes: a corrosion (chemical) process and a flow dynamics (physical) process. The former is the essential process to cause FAC and the latter is the accelerating process to enhance FAC occurrence. The chemical process in the surface boundary layer is analyzed to evaluate FAC rate. Contributions of flow dynamics on wall thinning rate due to FAC are expressed as a function of mass transfer coefficient but not that of flow velocity. FAC evaluation procedures were divided into 5 steps as follows. (1) Flow pattern and temperature in each elemental volume along the flow path were obtained with 1D computational flow dynamics (CFD) codes, (2) corrosive conditions, e.g., oxygen concentration and electrochemical corrosion potential (ECP) along the flow path were calculated with a hydrazine oxygen reaction code, (3) precise flow patterns and mass transfer coefficients at the structure surface were calculated with 3D CFD codes, (4) danger zones were evaluated by coupling major FAC parameters, and then, (5) wall thinning rates were calculated with the coupled model of static electrochemical analysis and dynamic double oxide layer analysis at the identified danger zone. Anodic and cathodic current densities and ECPs were calculated with the static electrochemistry model and ferrous ion release rate determined by the anodic current density was used as input for the dynamic double oxide layer model. Thickness of the oxide film and its characteristics determined by the dynamic double oxide layer model were used for the electrochemistry model to determine the resistances of cathodic current from the bulk to the surface and anodic current from the surface to the bulk. Two models were coupled to determine local corrosion rate and ECP for various corrosive conditions. The calculated results of the coupled models had good agreement with the measured ones.

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

“…15) Successively, the models validated in Ref. 15) were applied to the comparison of wall thinning rates with those observed at an actual NPP, confirming that the models predicted the wall thinning rates with the accuracy of factor two, which had been the target of the project. 16) On the other hand, a whole feature has not been discussed in detail for FAC in the wide sense.…”

“…The models and programs have been verified using experimental data. 14,15) Heymann's Re (-) droplet diameter: 100 µm Fig. 12 Estimation of erosion rate based on Heymann's formula As shown in Fig.…”

“…Wall thinning rate due to FAC has been calculated with the coupled models of the static electrochemistry analysis and dynamic oxide layer growth analysis 14,15) (Table 2). Anodic and cathodic current densities and ECP were calculated with the static electrochemistry model, and ferrous ion release rate determined using the anodic current density was applied as input for the dynamic oxide layer growth model.…”

“…The major parameters 13,15,16,29) to determine FAC are divided into material parameters, flow dynamics parameters, and environmental parameters.…”

“…[14][15][16] In this paper, the characteristics of FAC in the wide sense are overviewed. The focus is on the discrimination of FAC and LDI regions based on flow dynamics evaluation, and separation of LDI (erosion) and LDI (corrosion) based on liquid droplet trajectory analysis, followed by discussions on procedures for LDI risk zone analysis and wall thinning evaluation.…”