Application of Coupled Electrochemistry/Oxide-Layer-Growth Model to Water Chemistry Improvement against Flow Accelerated Corrosion in PWR Secondary System

In order to apply a computer simulation model of flow accelerated corrosion to evaluate the effects of water chemistry improvement on wall thinning of PWR secondary piping, applicability and accuracy of the model were confirmed based on verification and validation processes.  Verification process of the model was reported previously.  In the paper, validation process of the model is discussed.

Corrosive conditions were calculated with a N₂H₄-O₂ reaction analysis code.  Precise flow turbulence at major parts of the system was analyzed with 3D CFD codes 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.  As a result of comparison of the calculated wall thinning rates with hundreds of measured results at the secondary piping of a PWR plant, it was confirmed that the calculated wall thinning rates agreed with the measured ones within a factor of 2.  Then, it was recommended that suitable amount of oxygen was injected into the suitable location along the feed water line to result sufficient mitigation of FAC without any serious adverse effect on steam generator tubing.