Oxidation behavior of matrix graphite and its effect on compressive strength

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. 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 on the compressive strength of oxidized MG specimens was characterized. Results show that temperature has a significant influence on the oxidation behavior of MG.The transition temperature between Regimes I and II is ∼700°C and the activation energy (E α) in Regime I is around 185 kJ/mol, a little lower than that of nuclear graphite, which indicates MG is more vulnerable to oxidation.Oxidation at 550°C causes more damage to compressive strength of MG than oxidation at 900°C. 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 SEMand porosity measurement byMercury Porosimetry indicate that the significant compressive strength loss ofMGoxidized at 550°Cmay be attributed to both the uniformpore formation throughout the bulk and the preferential oxidation of the binder.