Flow-accelerated corrosion (FAC) in a pressurized water reactor (PWR) feedwater piping system is considered the leading degradation process that has been blamed for a number of accidents and incidents. One of the challenging issues in estimating FAC in piping is the hydrazine (N2H4) oxygen (O2) reaction rate and its subsequent impact on the wall mass transfer. N2H4 is injected in the system to maintain a stable pH in the water and also to act as an O2 scavenger. Previous research results indicate a varying degree of FAC rate dependence on the presence of N2H4 in the system. The N2H4-O2 reaction is also a complex function of temperature and pipe material. The present paper presents a two-part analysis that uses computational fluid dynamics (CFD) tools to investigate the N2H4-O2 reaction and its subsequent impact on mass transfer. In the first part of the analysis, chemistry and flow of water with dissolved O2 and N2H4 is simulated to assess different reaction mechanisms available in the literature. Results obtained from this study are compared with available experimental data for benchmarking. The numerical results were able to capture the general pattern of reaction rate as a function of temperature. Numerical simulations were also carried out to accommodate the surface effects on the reaction, but results indicate that such accommodations will yield useful results only if the geometrical extent of the near-wall-zone, where surface effects are prevalent, is well known. In the second part of the analysis, numerical simulations were carried out for a U-bend pipe. A number of restrictive assumptions were made to assess the dependence of O2 mass-transfer rate on N2H4-O2 reaction. Hydrodynamic results show the secondary flow pattern within the bend section. Results are also presented for the Sherwood number ratio at the pipe wall with and without reaction. Results indicate that the N2H4-O2 reaction decreases the O2 flow rate toward the wall. This calculation also shows that secondary flow in the bend affects the wall mass transfer pattern.

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