Investigation on the oxidation behavior and multi-step reaction mechanism of nuclear graphite SNG742

Abstract

ABSTRACT Gas chromatography and numerical methods were used to investigate the oxidation behavior of SNG742 nuclear grade graphite at 500–1100°C with different oxidizing gas flow rates. A four-step overall reaction mechanism was used to describe the graphite oxidation reaction process, which reasonably explains the peak of CO mole fraction at around 700°C with lower flow rates. The kinetic parameters of apparent oxidation reaction and four-step reaction mechanism of SNG742 graphite were obtained through curve fitting of the measured data. The apparent activation energy of SNG742 nuclear graphite is 200.7 ± 14.2 kJ/mol. The effects of temperature and gas velocity on the oxidation rate were analyzed. The graphite oxidation process was effectively simulated using CFD method with porous media model and the four-step reaction mechanism. The simulation results show that increasing the gas flow rate can reduce the thickness of the diffusion boundary layer and influence the diffusion control regime of graphite oxidation. The four-step overall reaction mechanism can better describe the graphite oxidation process than the one-step overall reaction. The porous medium model can effectively simulate the mass diffusion process and surface chemical reaction in the interior porous region of the graphite.