Verification and Validation of Evaluation Procedures for Local Wall Thinning Due to Flow-Accelerated Corrosion and Liquid Droplet Impingement

A six-step evaluation procedures have been proposed to evaluate the local wall thinning due to flow-accelerated corrosion (FAC) and that due to liquid droplet impingement (LDI). Corrosive conditions were calculated with a N₂H₄-O₂ reaction analysis code. Precise flow turbulence at major parts of the system was analyzed with the three-dimensional computational flow dynamics code to obtain mass transfer coefficients at structure surfaces. Then, wall thinning rates were calculated with the coupled model of electrochemical analysis and oxide layer growth analysis by applying the corrosive conditions and the mass transfer coefficients.

To apply computer simulation codes for wall thinning due to FAC and LDI to evaluate residual life and the effectiveness of countermeasures, accuracy and applicability of the codes were confirmed based on verification and validation processes. From comparison of the calculated wall thinning rates due to FAC with hundreds of measured results for 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 and the accuracy of the evaluation model for residual pipe wall thickness after 1 year of operation had an error of <20%. Finally, just the FAC simulation code was applied to evaluate the effects of oxygen injection into the feedwater line.

From comparison of the calculated wall thinning rates due to LDI with measured results for vent lines of an actual boiling water reactor plant, it was confirmed that the calculated local wall thinning rates agreed with the measured ones within about a factor of 2, though there were still some outside that region.