1.4.1 Magnox Reactors

The first Magnox reactors built in the UK were at Calder Hall and Chapelcross. These were just 50 MW plants; eight units being built in total which were commissioned between 1956 and 1960. These first plants were originally envisaged for the purpose of producing plutonium but were also operated to produce electricity. They were followed by a series of higher rated plants commissioned between 1962 and 1971. The most highly rated plant was Wylfa at 590 MW operating at a gas pressure of about 27 bars. Many of the earlier plants are now shut down but the later plants are still in operation.

The Magnox reactor core consists of a ‘pile’ of graphite blocks or bricks which contain channels. Carbon dioxide at a pressure of typically a few tens of bars flows through these channels, which also contain the fuel elements or control rods. The fuel elements consist of natural uranium bars clad with a magnesium alloy known as Magnox. These are machined into a ‘herringbone’ pattern in order to optimise heat transfer. A metallic fuel was adopted; i. e. natural uranium was used. The magnesium alloy was specifically chosen because it did not have a significant absorption of neutrons, enabling natural, rather than enriched uranium to be used.

Typical geometric and operating parameters are defined to limit the internal temperature of the elements to about 650°C, a critical temperature at which deformation of the uranium crystal lattice occurs. Similarly, the can temperature is limited to 420°C, associated with the use of Magnox alloy. A typical Magnox core is about 8 m high and 14 m in diameter. The core exit gas temperature is about 400°C.

On exiting the core, the coolant flows directly to the steam generator and then is pumped back to the reactor. The efficiency of the steam cycle is around 31%.

In the early Magnox designs, the vessel was made of steel and the steam generators (heat exchangers) were external to the pressure vessel. In Oldbury and Wylfa, the heat exchangers were placed inside the pressure vessel, constructed with pre-stressed concrete (Smitton, 2000).

Magnox reactors have in general operated very successfully in the UK over a period of many decades. However, from an economic perspective they have a low power density with high fuel costs.