Verification and Validation of One-Dimensional Flow-Accelerated Corrosion Evaluation Code

A six-step procedure based on three-dimensional (3-D) computational fluid dynamics codes and a coupled model of electrochemistry and oxide layer growth models was proposed to estimate local wall thinning due to flow-accelerated corrosion (FAC), and they were applied to evaluate wall-thinning rates, residual lifetimes of the pipes, and applicability of countermeasures against FAC. A verification and validation (V&V) evaluation based on a comparison of calculated and measured wall thinning confirmed that the wall-thinning rate could be predicted with an accuracy within a factor of 2 and that residual wall thicknesses after 1 year of operation could be estimated with an error of <20%.

To mitigate one of the disadvantages of the 3-D FAC code, which is the large amount of computational time needed, and to evaluate FAC occurrence probability for entire plant systems, a one-dimensional (1-D) FAC code was developed by applying 1-D mass transfer coefficients and geometrical factors. High-FAC occurrence zones along entire cooling systems and the effects of countermeasures on mitigating the risks could be evaluated within a small amount of computer time. Prior to application of the easy-to-handle FAC code for plant analysis, its accuracy and applicability should be confirmed based on V&V processes. From comparison of maximum wall-thinning rates calculated with the 1-D FAC code, those calculated with the 3-D FAC code, and measured results for experimental loops and secondary piping of an actual pressurized water reactor plant, it was confirmed that the calculated wall-thinning rates agreed with the measured ones within a factor of 2.