Evaluation Methods for Corrosion Damage of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics (II): Evaluation of Corrosive Conditions in PWR Secondary Cooling System

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 can be analyzed to evaluate FAC rate. In this paper, corrosive conditions along the flow path of the PWR secondary cooling system were evaluated. To do this, flow velocity and temperature in each elemental volume along the flow path were obtained with 1D computational flow dynamics (CFD) codes, distribution of oxygen concentration along the flow path was calculated with a oxygen hydrazine reaction code, and then electrochemical corrosion potential (ECP) was evaluated by using the Evans diagram. In the proposed calculation procedures for corrosive conditions, the oxygen hydrazine reactions were divided into bulk and surface reactions and the oxidation reaction of hydrazine on the surface was considered to obtain ECP under hydrazine coexisting conditions. Calculations of precise flow patterns and mass transfer coefficients at the structure surface made with 3D CFD codes and calculations of wall thinning rates made with the coupled model of static electrochemical analysis and dynamic double oxide layer analysis agreed with the calculations of corrosive conditions to evaluate FAC rate.

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