A kinetic investigation of conversion of nuclear graphite by hydrogen and water vapor at absolute pressure variation up to 40 bar

Abstract

This study investigates the thermochemical conversion of release-measured nuclear-grade graphite recovered from the thermal column of a decommissioned nuclear reactor in the context of the disposal of this nuclear waste. The focus is on determining kinetic data for graphite conversion to the gas phase and the influence of absolute pressure on kinetic parameters using TGA. During gasification with H₂ and for all tested pressures (1, 5, 20 and 40 bar) an activation energy of approx. 268 kJ/mol was determined for the kinetically-controlled regime (Regime I), and approx. 134 kJ/mol for the pore diffusion-controlled regime (Regime II). For hydrogasification of synthetic graphite (reference material), Regime I with an E_A of approx. 140 kJ/mol and Regime III (mass transfer-controlled) with an E_A of approx. 9 kJ/mol were observed. Due to the absence of any internal surface area, synthetic graphite showed no Regime II, rather Regime I passed directly into Regime III. An increase of pressure in order to accelerate the conversion was only successful up to a certain point. Further increasing the pressure even led to a lower reaction rate can even be observed. For the gasification of nuclear graphite with water vapor, Regime I was found for 1, 5, 20 and 40 bar. The activation energy was approx. 214 kJ/mol.