Magnox

Magnox is a type of nuclear power / production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The name comes from the magnesium-aluminium alloy (called Magnesium non-oxidising), used to clad the fuel rods inside the reactor. Like most other "Generation I nuclear reactors", the Magnox was designed with the dual purpose of producing electrical power and plutonium-239 for the nascent nuclear weapons programme in Britain. The name refers specifically to the United Kingdom design but is sometimes used generically to refer to any similar reactor.

As with other plutonium-producing reactors, conserving neutrons is a key element of the design. In magnox, the neutrons are moderated in large blocks of graphite. The efficiency of graphite as a moderator allows the Magnox to run using natural uranium fuel, in contrast with the more common commercial light-water reactor which requires slightly enriched uranium. Graphite oxidizes readily in air, so the core is cooled with CO₂, which is then pumped into a heat exchanger to generate steam to drive conventional steam turbine equipment for power production. The core is open on one end, so fuel elements can be added or removed while the reactor is still running.

The "dual use" capability of the Magnox design led to the UK building up a large stockpile of fuel grade/"reactor grade" plutonium, with the aid of the B205 reprocessing facility. The low-to-interim burnup feature of the reactor design would become responsible for changes to US regulatory classifications after the US–UK "Reactor-grade" plutonium detonation test of the 1960s. Despite improvements to the design in later decades as electricity generation became the primary operational aim, magnox reactors were never capable of competing with the higher efficiency and higher fuel "burnup" of pressurised water reactors.

In total, only a few dozen reactors of this type were constructed, most of them in the UK from the 1950s to the 1970s, with very few exported to other countries. The first magnox reactor to come online was Calder Hall (at the Sellafield site) in 1956, frequently regarded as the world's first commercial nuclear power station,^[1] while the last in Britain to shut down was Reactor 1 in Wylfa (on Anglesey) in 2015. As of 2016^[update], North Korea remains the only operator to continue using Magnox style reactors, at the Yongbyon Nuclear Scientific Research Center. The Magnox design was superseded by the Advanced Gas-cooled Reactor, which is similarly cooled but includes changes to improve its economic performance.