What is a Magnox Reactor?

The first Magnox reactor was built at Calder Hall, next to Sellafield, and opened by the Queen on 17^th October 1956. It is a British designed reactor – only two were sold abroad – one to Japan and one to Italy.

As with most nuclear reactors they are fuelled by uranium. When a uranium atom is split, two or three neutrons fly off at a tremendous speed, splitting more uranium atoms causing a chain reaction. During this chain reaction, the uranium fuel becomes very hot. In Magnox reactors carbon dioxide gas is used to carry the heat away and heat water into steam, which is then used to generate electricity in turbines, just as in a conventional power station.

In order to keep the reaction under control, Magnox reactors have a graphite core or ‘moderator’, which slows down the neutrons. Uranium fuel elements are inserted into vertical channels in this graphite core. (The uranium fuel is clad in a special magnesium alloy ­ hence the name ‘Magnox’). Control rods can also be inserted to absorb the neutrons and stop the chain reaction. These control rods are raised when the operator wants to start up the reactor again. The core is enclosed inside a pressure vessel made out of steel or concrete.

Unlike most other reactors, Magnox reactors use natural uranium. Uranium dug out of the ground, only contains about 0.7% of the split-able (fissile) type of uranium (uranium-235), so 99.3% of the atoms are non-fissile (uranium-238). Most of the world’s nuclear reactors use fuel containing ‘enriched uranium’, which means the uranium-235 content has been increased to about 2 or 3%.

Magnox stations are Britain’s oldest nuclear power stations. Some have already closed, but the rest will not all be closed until 2010. The remaining Magnox stations are at least 30 years old. The reactors are currently operated by Magnox Electric plc - a subsidiary of British Nuclear Fuels plc (BNFL). BNFL freely admits these stations are loss-making. As from 1st April 2005, ownership of these reactors will transfer to the Nuclear Decommissioning Authority, but BNFL will continue to operate them for the time-being. They are not very efficient, and produce more dangerous nuclear waste per unit of electricity than more modern stations, and radiation doses to members of the public living nearby are higher too.

The remaining Magnox stations generated just over 4% of the electricity consumed in the UK in 2001-2  (16.8TWh). These reactors were originally designed to operate for 20 ­ 25 years. The danger is that by continuing to operate them for forty years or more, corrosion and ageing problems will lead to more accidents. They should be closed now, rather than waiting until 2010.