CONVECTIVE MASS TRANSFER THROUGH AN UNREACTIVE POROUS DEPOSIT LAYER UNDER HIGH TEMPERATURE CONDITIONS

Abstract

Fe–Cr–Ni alloys have experienced localized degradation, such as stress-corrosion cracking (SCC), when used for steam generator tubes in nuclear power plants. The tube surface can be covered by a porous deposit layer resulting primarily from fouling. This porous layer acts as a barrier to the mass transfer for the chemical species in the main fluid to the tube surface. Thus, it influences the interfacial chemistry at the metal surface and the susceptibility of Fe–Cr–Ni alloys to SCC. While the chemistry of the main fluid can be controlled and monitored, this interfacial chemistry must be determined indirectly. Numerical models can be used to predict the interfacial chemistry and provide insight to SCC initiation and propagation. In the present work, a numerical model has been developed to calculate the mass-transfer rate of a chemical species, such as dissolved oxygen (DO), from main fluid to tube surface through an unreactive porous layer under single-phase liquid flow conditions. Major features of the model were validated against available literature data at room temperature (25 °C). The numerical results for high pressure (5 MPa) and high temperature (250 °C) conditions show that the effect of advection on the mass-transfer rate of DO through an unreactive porous layer dominates over that of diffusion.