The oxidation of moderator graphites irradiated in carbon dioxide containing carbon monoxide, methane and water

The Advanced Gas Cooled Reactor (AGR) was introduced for the second generation of British nuclear power stations. It was recognised that problems of compatibility between the carbon dioxide coolant and the moderator graphite would arise because of the increased power rating of the reactor compared with the first generation MAGNOX system. This led to the realisation that it would be necessary to reduce the rate of oxidation of the moderator to acceptable levels by the addition of inhibitors to the coolant and to this end carbon monoxide and methane were chosen. This paper describes experiments which have been made in a materials testing reactor at AERE Harwell in which moderator graphite reaction rates have been measured in carbon dioxide containing carbon monoxide at concentrations between 0.03% and 2% and methane concentrations up to 600 vpm. The effect of impressing a flow of coolant through the graphite structure, the so-called ventilation effect, and the role of coolant temperature and pressure have also been assessed. The results confirm the inhibiting power of methane and carbon monoxide on the graphite/CO2 reaction and demonstrate that the application of ventilation in the presence of these inhibitors enhances their effect. A minimum or 'terminal' oxidation rate may be achieved by the CAGR Gilso carbon graphites when irradiated in the presence of 200 vpm methane, or more, under appropriate conditions. (author)