In terms of its physical properties, carbon dioxide is the best available gaseous coolant, and consequently it was chosen for the large U. K. Magnox and AGR power stations. In the Magnox reactors, the graphite moderator has a maximum temperature of only about 350°C. At these temperatures C02 is unreactive with graphite, nor does it react with the canning material, the circuit steels, or the fuel (uranium metal). When the temperature is increased, difficulties arise be­cause of the chemical reaction:

C02 + C ^ 2CO

Here, the C (carbon) represents the moderator graphite blocks, and the reaction slowly removes the moderator from the reactor, decreasing the strength of the graphite core. The above reaction is induced not only by higher temperatures but also by increasing nuclear radiation.

The occurrence of the carbon dioxide-graphite reaction is a potentially very serious limitation since the structure of the core, including the alignment of the fuel channel, is dependent on the physical strength of the graphite blocks. In the case of the advanced gas-cooled reactor, there have been two approaches to solving this problem:

1. The inlet (relatively cold) carbon dioxide is fed through the moderator struc­ture to the entrance of the fuel channels, thus keeping the moderator at a lower temperature.

2. Carbon monoxide and methane are added to the carbon dioxide to inhibit the above chemical reaction. The mechanisms by which this inhibition is achieved are complex. One mechanism is that the additives produce a thin layer of carbon on the graphite, and this carbon layer reacts sacrificially with the coolant, preventing attack on the bulk structural graphite. A difficulty here is that the carbon may, under certain circumstances, be deposited on the fuel elements themselves. As we saw in the preceding section, heat trans­fer from the fuel elements is critically dependent on small isolated rib rough­nesses on the surface of the cladding. Smoothing out of these roughnesses by carbon deposition would negate their enhancement of heat transfer and lead to a rise in the fuel element temperature. Very precise chemical control is therefore required in AGRs.