Graphite-Gas Reactor

a thermal-neutron nuclear reactor in which graphite is the moderator and a gas, usually carbon dioxide and less frequently helium, is used as the coolant. The main advantages of the gaseous coolants are good nuclear-physical properties and the capability of being heated to high temperatures, which makes it possible to increase the efficiency of atomic power plants to 40 percent and higher.

A characteristic feature of graphite-gas reactors is the relatively low energy intensity (the quantity of heat removed from a unit volume of the core), which is mainly explained in terms of the inferior moderating properties of graphite as compared, for example, to water. This leads to considerable size of the graphite-gas reactors. For example, the core of the British Dungeness B atomic power plant, with a capacity of 660 megawatts (MW), is 9.4 m in diameter and 8.2 m high. The increased dimensions of the core and the presence of excess gas pressure—4.5 meganewtons per sq m, 45 kilograms-force per sq cm—present special design requirements for the reactor. The fuel elements are placed in the cylindrical channels of the graphite lining. The core is enclosed by a strong housing consisting either of steel or of prestressed reinforced concrete, which contains the pressure exerted by the coolant. The core, along with the steam generators and gas ducts, is sometimes enclosed by a single reinforced-concrete housing. The neutron shielding surrounding the core protects the generators and gas ducts from activation, thus making them accessible for maintenance when the reactor is shut down. The interior surface of the concrete housing is covered by heat insulation to protect it from overheating. Special cooling systems are also used.

Graphite-gas reactors are the main type of reactor in British and French nuclear power engineering. Atomic power plants with this type of reactor have also been constructed in Italy and Japan. An atomic power plant using helium as the coolant has been put into operation in the USA.