Oxidation experiments and theoretical examinations on graphite materials relevant for the PBMR

Graphite oxidation due to gas impurities in normal operation and ingressing oxidants in accidents plays a key role in the material and safety behaviour of HTRs. An overview is presented of the theoretical background concerning graphite oxidation, mainly in regimes I and If. Some differences between the classical oxidation model, based on effective diffusivity and chemical reaction on the inner graphite surface, are discussed. These differences may be due to the complex pore system in graphite, which cannot be approximated by one single diffusivity. Based on these theoretical results, a procedure for measurements on candidate graphites to be used in PBMR is proposed. Regime I measurements are selected for material characterization because of the strong sensitivity to chemical influences. First results measured in air at 650-750 degrees C at the Graphite Oxidation Laboratory. GOLab, Research Centre Julich, are outlined. Graphites examined so far are the SGL grades NBG-10 and NBG-18. Whereas NBG-10 is significantly more oxidation resistant for all specimens and at all temperatures than the former German nuclear graphite V483T5, taken as a standard, the scatter of oxidation rates of NBG-18 is even larger, but is on average also satisfactory. In contrast to the classical model, preliminary low-temperature oxidation experiments on NBG-10 reveal a significant rate dependence on specimen size. Additional experiments in regime I and in regime II are proposed for PBMR graphites, as those for clarification of the deviations to the classical oxidation model. The latter probably requires a broader discussion in the graphite community. (C) 2008 Rainer Moormann. Published by Elsevier B.V. All rights reserved.