The exact mechanism of radiolytic oxidation in a car­bon dioxide-cooled reactor is complex and has been a matter of debate for some time; the most satisfactory explanation has been given by Best et a/.26 However, in its most simplistic form the mechanism can be described as follows:

In the gas phase,

ionizing radiation

CO2 —————— ^ CO + O* [I]

CO + O* ! CO2 [II]

where O* is an activated state-oxidizing species.

Thus, after ionization the carbon monoxide and oxidizing species rapidly recombine back into carbon dioxide and to an uninformed observer, carbon

dioxide would appear to be stable in an irradiation field. However, in the presence of graphite which typically contains ~10% porosity, 10% of which is initially accessible to the carbon dioxide gas, at the graphite pore surface (mainly internal) carbon atoms are oxidized. This can be simplistically described as

O* + C! CO [III]

The principal oxidizing species is still under debate, but the most favored candidate is the negatively charged ion, CO^.

4.11.7.3 Inhibition

The rate of oxidation can be reduced by the addition of carbon monoxide (CO) and moisture (H2O) and can be greatly reduced by the addition of methane (CH4), as illustrated in Figures 9 and 10. As described above the radiolytic oxidation process pro­duces CO and if CH4 is added, moisture will be one of the by-products of the reaction.