Abstract
Matrix graphite (MG) with incompletely graphitized binder used in high-temperature gas-cooled reactors (HTGRs) is commonly suspected to exhibit lower oxidation resistance in air. The oxidation performance of MG in both normal and off-normal events is important for the integrity evaluation and safety analysis of fuel elements in HTGR. In order to reveal the oxidation performance, the oxidation behavior of newly developed A3-3 MG at the temperature range from 500 to 950 °C in air was studied and the effect of oxidation at two typical temperatures on the compressive strength of oxidized MG specimens were characterized. Results show that temperature has a significant influence on the oxidation behavior of MG. The transition temperature between Regime I and II is ~700 °C and the activation energy (Ea) in Regime I is around 185 kJ/mol, a little lower than that of nuclear graphite, which indicates that MG is more vulnerable to oxidation. The mass-normalized oxidation rate in Regime I can be described as: ORm=2.59×109×exp(-22362.13/T) h-1. Oxidation at 550 °C in the kinetic regime causes more damages to compressive strength of MG than oxidation at 900 °C in the boundary layer control regime. Comparing with the strength of pristine MG specimens, the rate of compressive strength loss is 77.3% after oxidation at 550 °C and only 12.5% for oxidation at 900 °C. Microstructure images of SEM and porosity measurement by Mercury Porosimetry indicates that the significant compressive strength loss of MG oxidized at 550 °C in the kinetic regime may be attributed both the uniform pore formation throughout the bulk and the preferential oxidation of the binder.
