To investigate the geometrical effects on nuclear graphite oxidation in the regime where the chemical effect is the ratecontrolling process, we introduce the concept of internal surface density into the Arrhenius-type reaction model and suggest how to determine it experimentally. Using the 16 different samples of IG-110 graphite, which have different ratios of external surface to volume, we obtained the value of internal surface density as 17 260 m(-1). We found out that the external surface reaction is very small compared to the total reaction for the IG-110 graphite: the ratio of the external surface reaction to the total reaction was within 5%. Finally, we propose the following reaction equation for this material: r'"(kg/m(3)s) approximate to 255200 . exp (-218000/R.T) . P-O2(0.75). (c) 2005 Elsevier B.V. All rights reserved.