Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGR)
Two views of the Sizewell nuclear power plant in Suffolk, Great Britain. In the foreground, there is the part with the Magnox reactor being decommissioned; in the background a newer operating part B with a single pressurized water reactor.
Gas cooled reactors (GCR) are used exclusively in Great Britain and Japan. The older type, known as the “Magnox” (for “magnesium oxide”), is being gradually phased out and no new blocks are being constructed. There are 14 operating reactors of the advanced type (AGR), which also uses graphite as the moderator. These reactors were developed in Great Britain.
The GCR reactor is fueled by natural uranium housed in rods covered with magnesium oxide. The reactor core is a steel spherical vessel 8 meters high and 14 meters in diameter encased in concrete. There are several thousand fuel channels each containing several fuel rods. These reactors may be refueled under power.
Brick shaped graphite is used as the moderator. The fuel channels transverse the graphite bricks. Carbon dioxide CO2 is used as the coolant pressurized to 2.75 MPa at a temperature of 400 °C. The modern type AGR may reach temperatures of up to 650 °C. The gas transfers its energy in a steam generator to the secondary water circuit. The generated steam powers a turbine. This type of power station is a double-circuit power plant.
One of the oldest nuclear power plants was the Oldbury plant. It was a Magnox GCR type reactor and was decommissioned in 2012.