Combustion is the most reliable technique to remove the matrix graphite during reprocessing of spent HTGR (high temperature gas-cooled reactor) 233U nucl ear fuel. The effects of temperature and oxygen concentration on the specific combustion rate of the unirradiated ThO2-UO2 spherical graphite element were studied experimentally. A combustion kinetics model is presented for calculating the specific combustion rate. The combustion process would be controlled by both the reaction and mass transfer for temperature in the range of 750 900 and 750 Reynolds numbers, Re = 56.9 454 9. The Ranz-Marshall correlation was used to predict the apparent mass transfer coefficient and to better fit the experimental data.