Transport processes in steam generator crevices—I. General corrosion model

Abstract

A new model for describing transport processes in Pressurized Water Reactor (PWR) Steam Generator (SG) tube/tube support plate crevices has been developed. The internal and external systems are coupled using the principle of charge conservation, recognizing that anodic and cathodic reactions may occur at any location. This model includes the influence of convective transport, diffusion, and migration of species in the crevice on the evolution of crevice properties. Calculations show that under normal operating conditions after an initial time period, the concentrations of all species and potential of the metal reach limiting values at the center of the crevice. After this period, the space in the cavity can be subdivided into two parts: (1) a region near the crevice mouth where concentration and potential gradients exist, and (2) a region near the crevice center where the concentrations of the various species and metal potential attain constant values. In the latter, the cathodic current density compensates for the corrosion current. The limiting values of the specie concentrations and the metal potential do not depend on the corrosion potential of the metal, but do depend on the available superheat and the bulk composition of the solution. For a sufficiently negative corrosion potential, the net current in the cavity can be negative, which must be balanced by a positive current on the external surface.