Magnox

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Magnox is an obsolesent type of nuclear power reactor; when operated on a short fuel cycle (which is non-commercial) they can also produce plutonium for nuclear weapons. In all 26 were built in the UK where the design originated. In addition one was exported to Japan and one to Italy.

At the end of 2003 eight remain in operation, all of them in the UK, and all are planned to closed by 2010. Two (Chapel Cross & Calder Hall) were owned by the UKAEA and were used to produce weapons-grade plutonium; the rest were owned by CEGB and were operated on commercial fuel cycles.

Magnox is also the name of the alloy mainly of magnesium with small amounts of other metals used for the fuel cladding in Magnox reactors, and which gave them their name. This material had the advantage of a low neutron capture cross-section, but it had two big disadvantages:

It limited the maximum temperature and hence thermal efficiency of the plant.
It reacted with water, making storage of spent fuel under water a short-term solution only.

Magnox fuel incorporated cooling fins to provide maximum heat transfer despite the low operating temperatures, making it expensive to produce. While the use of uranium metal rather than oxide made reprocessing more straightforward and therefore cheaper, the need to reprocess a short time after removal from the reactor meant that the fission product hazard was severe and the expensive remote handling facilities were required.

The term magnox is also sometimes loosely used to refer to:

Nine UNGG power reactors built in France, all now permanently shut down. These were carbon dioxide cooled graphite reactors with natural uranium metal fuel, very similar in design and purpose to the British Magnox reactors with one notable exception: The fuel cladding in the French reactors was magnesium-zirconium alloy, not magnox!
Two or three North Korean reactors, all based on the declassified blueprints of the Calder Hall Magnox reactors:
- A very much smaller 5MWe reactor, operated from 1986 to 1994 and the subject of fears that plutonium from the spent fuel might be used in the North Korea nuclear weapons program.
- A 50MWe reactor, construction commenced in 1985 but halted since 1994 and as of 2003 never operated.
- A rumoured uncompleted 200MWe reactor.

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, and the extra neutron losses caused by the stainless steel were blamed for the AGR being relatively unsuccessful economically when compared to the older Magnox design.

General description

The Magnox reactors were carbon dioxide cooled, graphite moderated reactors using natural (unenriched) uranium metal as fuel and magnox alloy as fuel cladding. Early reactors had steel pressure vessels, later units (Oldbury & Wylfa) were of reinforced concrete.

On-load refuelling was an essential part of the design, to maximise power station availability by eliminating refueling downtime.

List of Magnox reactors in the UK

Berkeley in Gloucestershire, 2 units 138 MWe each, first grid connection 1962, shut down 1989
Bradwell near Southminster, Essex, 2 units 123 MWe each, first grid connection 1962, shut down 2002
Calder Hall near Whitehaven, Cumbria - 4 units 50 MWe each, first grid connection 1956, shut down 2003
Chapelcross near Annan, Dumfriesshire, 4 units 50 MWe each, first grid connection 1959, shut down 2003
Hunterston A near Kip, north of Wemyss Bay 2 units 160 MWe each, first grid connection 1964, shut down 1990
Hinkley Point near Bridgwater, Somerset, 2 units 235 MWe each, first grid connection 1965, shut down 1999
Trawsfynydd in Gwynedd, 2 units 195 MWe each, first grid connection 1965, shut down 1991
Dungeness A in Kent, 2 units 220 MWe each, first grid connection 1965
Sizewell A near Leiston, Suffolk, 2 units 210 MWe each, first grid connection 1966
Oldbury near Thornbury, South Gloucestershire, 2 units 217 MWe each, first grid connection 1967, planned to be shut down in 2008.
Wylfa on Anglesey, 2 units 495 MWe each, first grid connection 1971

ja:マグノックス炉