Burn-off production of CO and CO2 in the oxidation of nuclear reactor-grade graphites in a flow system

The oxidation behaviors of IG-110 and PGX graphites were experimentally investigated in the temperature range from 500 to 1,500degC for the oxygen concentration of 10 and 30 wt% (1.37 and 5.09 mol%) and the reacting gas velocities of 6.63 and 19.9 m/s (estimated at 900degC). The graphite specimens were prepared to simulate the annular coolant in the core of the HTGR. It has been found that the upstream surface is more oxidized for both IG-110 and PGX graphites, and that the oxidized surface of PGX is rougher than that of IG-110. The oxidation of IG-110 starts at about 700degC and reaches the maximum above 800 to 900degC accompanied by the maximum production of CO2 around 850degC and a monotonic increase in CO with increasing temperature. The oxidation behavior of PGX is similar to that for IG-110, except that the oxidation starts at lower temperature and produces more CO2 over 750 to 950degC while the concentration of CO becomes lower in this temperature range. The present result for the product ratio of CO to CO2 deviates from the linear line in the Arrhenius plot in the region where the production of CO2 increases. However, it agrees best with the Rossberg correlation among the previous empirical correlations. New empirical correlations for the product ratio of CO to CO2 have been obtained form the present data which exclude those in the region of CO2 increase. (author)