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30-01-2009, 03:55 PM
Magnox is a now obsolete type of nuclear power reactor which was designed and used in Britain, and exported to other countries, both as a power plant, and, when operated correctly, as a producer of plutonium for nuclear weapons. The name magnox comes from the material used to clad the fuel rods inside the reactor. Magnox reactors are pressurised carbon dioxide cooled, graphite moderated reactors using natural uranium (i.e. unenriched) as fuel and magnox alloy as fuel cladding. The design was continuously refined, and very few units are identical. On-load refuelling was an economically essential part of the design, to maximise power station availability by eliminating refuelling downtime. This was particularly important for Magnox as the unenriched fuel had a low burn-up, requiring more frequent changes of fuel than most enriched uranium reactors.
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04-09-2012, 01:43 PM
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Magnox reactors are pressurised, carbon dioxide cooled, graphite moderated reactors using
natural uranium (i.e., unenriched) as fuel and magnox alloy as fuel cladding. Boron-steel
control rods were used. The design was continuously refined, and very few units are
identical. Early reactors have steel pressure vessels, while later units (Oldbury and Wylfa)
are of prestressed concrete; some are cylindrical in design, but most are spherical. Working
pressure varies from 6.9 to 19.35 bar for the steel pressure vessels, and the two prestressed
concrete designs operated at 24.8 and 27 bar. No British construction company at the time
was large enough to build all the power stations, so various competing consortia were
involved, adding to the differences between the stations; for example nearly every power
station used a different design of Magnox fuel element.
On-load refuelling was considered to be an economically essential part of the design for the
civilian Magnox power stations, to maximise power station availability by eliminating
refuelling downtime. This was particularly important for Magnox as the unenriched fuel had
a low burnup, requiring more frequent changes of fuel than enriched uranium reactors.
However the complicated refuelling equipment proved to be less reliable than the reactor
systems, and perhaps not advantageous overall.
The first Magnox reactors at Calder Hall were designed principally to produce plutonium
for nuclear weapons. The production of plutonium from uranium by irradiation in a pile
generates large quantities of heat which must be disposed of, and so generating steam from
this heat, which could be used in a turbine to generate electricity, or as process heat in the
nearby Windscale works, was seen as a kind of "free" by-product of an essential process.
The British government decided in 1957 that electricity generation by nuclear power would
be promoted, and that there would be a building programme to achieve 5,000 to 6,000 MWe capacity by 1965.
Although Sir John Cockcroft had advised the government that electricity generated by nuclear power would be more
expensive than that from coal, the government decided that nuclear power stations as alternatives to coal-fired power
stations would be useful to reduce the bargaining power of the coal miners' unions, and so decided to go ahead. In
1960 a government white paper scaled back the building programme to 3,000 MWe, acknowledging that coal
generation was 25% cheaper. A government statement to the House of Commons in 1963 stated that nuclear
generation was more than twice as expensive as coal. The "plutonium credit" which assigned a value to the
plutonium produced was used, initially secretly, to improve the economic case, although the operators of the power
stations were never paid this credit.
Gas cooled reactors
The accepted term for all of these first-generation, carbon dioxide-cooled, graphite-moderated reactors, including the
Magnox and UNGG, is GCR for Gas Cooled Reactor.
The Magnox was replaced in the British power station program by the Advanced gas-cooled reactor or AGR, which
was derived from it. A key feature of the AGR was the replacement of magnox cladding to allow higher
temperatures and greater thermal efficiency. Stainless steel cladding was adopted after many other alloys had been
tried and rejected.