Theoretical simulation of hydrodynamic parameters for understanding FAC in PHWR

Recent inspections of outlet feeder pipe in PHT systems of CANDU reactors in Canada showed measurable wall thinning, termed as flow accelerated corrosion (FAC), at locations close to the end fitting. The wall thinning rates observed at the feeder outlet bends were higher than the design corrosion allowance, thereby requiring frequent in service inspection of these pipe sections. However, this phenomenon was not observed in inlet feeder pipe bends. FAC is attributed to enhanced mass transfer of iron (probably as ferrous ions) through the diffusion layer at the oxide - water interface on carbon steel in high temperature high velocity water and under reducing and iron unsaturated conditions. This phenomenon is complex and is a function of metallurgical, environmental and hydrodynamic factors. Hydrodynamic factors are often ignored in the analysis of corrosion kinetics, or hydrodynamic regime used does not effectively simulate the environment of interest. There is evidence that shear stress can remove fouling deposits and erode soft dissolving solids. However its impact on FAC needs further investigation. The aim of this paper is to study and understand hydrodynamic parameters using computational fluid dynamics (CFD) code. The hydrodynamic parameters studied are the turbulent kinetic energy, wall shear stress under single phase flow at different bend angles and varying pipe diameters. Computational fluid dynamics code STAR-CD, was used to predict hydrodynamic flow field in different bend angle sections and varying diameters for different magnitudes of velocity. The velocity profile prevailing along the length of the bend was obtained using this code. The velocity was found to be maximum at the beginning of the intrados and end of the extrados bend. Similar data was generated on turbulent kinetic energy and wall shear stress for different bend angles and different bend sizes. The paper also gives an idea of distribution of wall stress for different velocity profiles. It is planned to use this data for devising laboratory scale simulating systems for studying the FAC phenomenon under different PHWR PHT system temperature and chemistry conditions. The specific contribution from each of the critical factors can then be clearly identified. (author)