A graphite oxidation experiment to high weight loss under CAGR conditions

The reaction of moderator graphite with radiolysed carbon dioxide coolant has been the subject of intensive study during the development of the UK Magnox and Civil Advanced Gas Cooled Reactor programme. Irradiation of carbon dioxide gives rise to short lived, energetic, species and graphite undergoes significant oxidation, even at moderator temperatures of around 670 K. Reaction is essentially confined to the internal porosity and, since this increases with oxidation, exponential weight loss of the structural material can occur. To retain adequate moderator strength over the station life, inhibitors are added to the coolant. The accepted mechanism of inhibition by the major carbon monoxide and methane additives is a competition with the graphite pore wall for the oxidising species. Further inhibition may result from the formation of a sacrificial surface layer, particularly derived from the oxidation of methane. It follows that, in the presence of inhibitors, oxidation should be confined to the smaller pores; where the probability is highest that the majority of oxidising species will reach the pore wall. In order to test this heterogeneous weight loss model, an ambitious experiment was mounted using the Anglo-French BFB loop in the Siloe reactor at CEN Grenoble in France. This experiment has now been completed and the results of some of the measurements which have been made are reported in this paper. (author)