An experimental investigation into the conversion of nuclear graphite by oxygen and carbon dioxide: Kinetics and change in pore structure

The study on the thermochemical conversion of release-measured nuclear-grade graphite recovered from the thermal column of a shut-down nuclear research reactor focuses on kinetic processes of gasification and oxidation of this graphite. More specifically, the kinetic parameters were determined and the evolution of the graphite structure during carbon conversion was investigated. For oxidation, the three conversion regimes of heterogeneous reactions in the temperature range of 500 °C to 950 °C were observed. The activation energy is approx. 288 kJ/mol in the kinetic regime limited by the chemical reaction (Regime I), approx. 185 kJ/mol in the pore diffusion regime (Regime II) and approx. 10 kJ/mol in the mass transfer regime (Regime III). With an increase of the partial pressure of oxygen, an increase of the pre-exponential factor in Regime I from 6.6 × 1012 1/s at 0.2 bar to 1.8 × 1013 1/s at 0.5 bar and to 2.0 × 1013 at 0.8 bar of oxygen was determined. A similar effect was observed in Regime II. A reaction order of 0.96 ± 0.16 was obtained for the oxidation reaction. In case of CO2 gasification at temperatures of 900 °C to 1400 °C, two regimes were identified. In Regime I, an activation energy of approx. 294 kJ/mol and a pre-exponential factor of 3.5 × 107 1/s were determined, the corresponding parameters being approx. 156 kJ/mol and 8.3 × 102 1/s for Regime II, respectively. Similar activation energy values for both oxidation and gasification reactions indicate that the structure of the nuclear graphite remains stable with proceeding carbon conversion. From the analysis of the development of the inner surface area and particle size distribution at increasing conversion rates, it can be concluded that the gasification of nuclear graphite mainly takes place on the outer surface. Using the ETV-ICP-OES analysis, it was found that the trace elements, Ca, Fe, S, V and Zn, were evaporated and no longer detectable in partially gasified nuclear graphite, whereas elements such as Ba, Cr or Mn are being accumulated in the solid resiude.