Ablation of high purity and nuclear reactor core graphite in oxygen and air

Abstract

An important factor in the use of graphite in space applications is its rate of oxidation or ablation at high temperature in high flow environments during re-entry. This paper presents a study of the kinetics and mechanism of ablation of two types of graphite at temperatures of 1200° to 1332 °C in oxygen and air, at pressures of 5 and 9.5 torr, and at flow rates of 7.6 × 10 18 to 1.6 × 10 20 atoms of oxygen per second. A rate of ablation of 5.7 × 10 19 atoms of carbon per cm 2-sec was observed at an oxygen pressure of 9.5 torr and at 1332 °C. These fast ablation rates were fitted to a reaction mechanism based on the mobile adsorption of oxygen as the rate limiting step, with a heat of activation of 39,000 calories per mole. Comparison of the rate data with collision theory calculations showed 1.9% of the oxygen collisions resulted in reaction. Mass spectrometer analyses of the reaction products indicated carbon monoxide as the primary reaction product.