ABSTRACT

The conditions under which it is possible to neglect the change of the saturation of the liquid in SG crevices were obtained. It has been shown that in the entry region of the SG crevices the liquid velocity can be described by a linear function. The calculations show that the species concentrations and the potential reach quasi-asymptotic limits throughout the cavity; the limits are determined by the available superheat. After the attainment of these limits, a constant crevice corrosion rate is predicted. The condition was determined whereby the transport processes in the external environment determine the corrosion current density in the crevice. Due to the low value of conductivity and a relatively large crevice width assumed in the calculations, this condition can actually be fulfilled, if the cathodic processes in the crevice do not essentially reduce the current, which flows out of the crevice.

INTRODUCTION

Steam generator ( SG) corrosion is one of the major problems associated with operating Pressurized Water Nuclear Reactors. Among other factors, corrosion is due to the segregation and concentration of ? impurities in the tube/support plate and tube/tube sheet crevices. Although the SG feedwater contains impurities at extremely low levels, impurities accumulate (along with corrosion products) in cracks, crevices, and sludge piles by a thermo-hydraulic (or concentration) mechanism 1. In the case of SG crevices, the temperature gradient across the SG tube produces boiling in the crevice, which in turn results in the flow of water into the cavity. The coupling of diffusive and convective fluxes within the crevice, and the lower volubility of the impurities in steam compared with water, leads to impurity concentrations in the crevice that may be many orders in magnitude greater than those in the feedwater.

Review articles on these concentrating mechanisms can be found in the literature 2-4. The most complete model ~oiling crevice model (beM)] of Millett and Fenton 1?5computes a concentrating factor for species in a porous crevice by solving the appropriate heat, mass, and momentum transfer equations. The model yields sensible predictions for concentration factors, the hydrodynamic velocity, temperature, and wetted length as a function of available superheat. However, the driving forces in this model are purely thermal and mechanical in nature, and no electrochemical effects are considered. Accordingly, the model cannot yield complete information on the chemical composition of the crevice or electrochemical properties, including the concentrations of corrosion products, pH, conductivity, potential distribution, and the rate of corrosion.

In a previous article a crevice model was proposed that combines the attributes and essential features of the beM with those of the coupled environment fracture model (CEFM) which was developed by Macdonald and Urquidi-Macdonald to describe stress corrosion cracking. This new Coupled Environment Crevice Model (CECM) includes the influence of convection on the transport of species in the crevice, in addition to the diffusion and migration driving forces that were previously considered. The equations describing heat and momentum transfer are taken from the beM. Because the species fluxes depend on the hydrodynamic and temperature fields, the equations describing energy, mass, momentum, and charge transfer in the system must be solved simultaneously.

Due to the mathematical complexity, the practical application of this (and the other models) for quantitatively describing corrosion processes in SG crevices is not a simple task. For example, even in the l-D case, only the solution of the hydrodynamic problem (without temperature

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