Determination Procedures of High Risk Zones for Local Wall Thinning Due to Flow-Accelerated Corrosion

A modified six-step evaluation procedure has been proposed to evaluate local wall thinning due to flow-accelerated corrosion (FAC). In step 1, the one-dimensional (1-D) distribution of flow turbulence and the temperature along pipes in cooling systems were analyzed with a 1-D system simulation code to obtain approximate mass transfer coefficients at structure surfaces, prior to using a three-dimensional (3-D) computational fluid dynamics (CFD) code for precise flow turbulence analysis of the major parts. In step 2, corrosive conditions were calculated with a N₂H₄-O₂ reaction analysis code. In step 3, high FAC risk zones were determined for further evaluation for wall thinning rates, based on five parameters: temperature, pH, oxygen concentration, mass transfer coefficient, and chromium content. Then, in step 4, the 3-D CFD code was used to calculate precise mass transfer coefficients at the high FAC risk zones. In step 5, the wall thinning rates were calculated using a coupled model of electrochemical analysis and oxide layer growth analysis by applying the corrosive conditions and the mass transfer coefficients. Finally, in step 6, the residual lifetime of the pipes and the applicability of countermeasures against FAC were evaluated.

This paper introduces procedures for determining major FAC parameters and evaluation procedures for high FAC risk zones by synthesizing the parameters in step 3. The procedures for determination of high FAC risk zones in a pressurized water reactor secondary cooling system are also demonstrated.