Introduction to Chemical Properties
3. Detailed Reports
Chemical Properties of nuclear-grade graphite become important whenever the installed components are able to interact with their environment through chemical reactions.
The principal issues of concern are:
Thermal Oxidation - which may be with oxygen (air), carbon dioxide, or steam according to circumstances. Early reactors, especially plutonium production piles in the UK and France, were air cooled. Magnox and AGR reactors were carbon dioxide cooled. In both cases, however, the temperature of operations was such as to reduce the thermal oxidation to near zero. HTRs are helium cooled, but the high graphite temperature means that, should there be any ingress of oxidant, some graphite oxidation will occur. In the case of a pebble-bed reactor, the most likely oxidant is steam (water) introduced with the new fuel pebbles which will have absorbed moisture on their surfaces. Accident scenarios open up other possibilities for graphite oxidation.
Thermal Oxidation may be catalysed.
Radiolytic Oxidation - which may be again with the same oxidants. This is temperature-independent except for the effect of temperature upon gas density, since it is the absorption of ionising-radiation energy within the gas within graphite pores which is responsible for the reaction and, if the gas density increases, so does the flux of oxidising species reaching the graphite surfaces. Radiolytic oxidation has been of most significance in the UK Magnox and AGRs, and the French UNGG reactors, all of which were operated with carbon-dioxide coolants. The most highly-rated French reactor and the longest-lived UK reactors have achieved in excess of 40% weight loss locally due to these effects.
Other Chemical Reactions - whenever graphite comes into contact with a chemical substance with which it can potentially react, the consequences need to be assessed. Accident scenarios may generate such issues. An example is the accidental introduction of lead metal into a graphite-moderated reactor, which raises potential catalytic issues for thermal oxidation and thus affects safety case arguments. Molten salt or liquid-metal reactors in which coolant comes into contact with a graphite component obviously present other options. We also take into account under this heading the issue of leaching of radioisotopes from irradiated graphite (dismantling and decommissioning issue) where specific chemical reactions may control the process.
3. Detailed Reports