Advanced Gas-Cooled ReactorNuclear reactor cooled with gas (helium, carbon dioxide).. A total of 14 reactorSystem used to initiate, maintain and control a fission chai... units of this type are in operation in England and Scotland. AGR reactors use enriched uraniumUranium in which the percentage of the fissionable isotope U... as fuelSee 'nuclear fuel'., graphite as moderatorMaterial used to "decelerate" fast neutrons to low energies,... and CO2 as cooling gas.