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Magnox

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Magnox nuclear reactor

Magnox is an obsolete type of nuclear power reactor; when operated on a short fuel cycle (which is uneconomic) they can also produce plutonium for nuclear weapons. In all 11 power stations totalling 26 units were built in the UK where the design originated. In addition one was exported to Japan and one to Italy. North Korea also developed their own Magnox reactors based on the UK design, which was made public at an Atoms for Peace conference.

At the end of 2003 eight UK Magnox power stations remained in operation, all were planned to be closed by 2010. Two (Chapelcross and Calder Hall) were owned by the UKAEA and were once used to produce weapons-grade plutonium in their early life before being used for commercial electricity generation; 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 aluminium and other metals used for cladding the unenriched uranium metal fuel, as a non-oxidising covering to contain fission products. Magnox is short for Magnesium non-oxidising. 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 expensive remote handling facilities were required.

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

  • Three North Korean reactors, all based on the declassified blueprints of the Calder Hall Magnox reactors:
    • A smaller 5 MWe experimental reactor at Yongbyon, operated from 1986 to 1994, restarted 2003. Plutonium from the spent fuel may have been used in a North Korea nuclear weapons program.
    • A 50 MWe reactor also at Yongbyon, construction commenced in 1985 but halted in 1994 and never operated.
    • A 200 MWe reactor at Taechon, construction also halted in 1994.
  • 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!

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.

Contents

General description

The Magnox reactors were pressurised carbon dioxide cooled, graphite moderated reactors using natural (unenriched) uranium metal as fuel and magnox alloy as fuel cladding.

The design was continuously refined, and very few units were identical. Early reactors had steel pressure vessels, later units (Oldbury & Wylfa) were of reinforced concrete; some were cylindrical but most spheres. Working pressure varied from 6.9 to 19.35 bar for the steel pressure vessels, and the two reinforced concrete designs operated at 24.8 and 27 bar.

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

The Magnox reactors have many benign features because of their low power densities and gas coolant, so do not have secondary containment. Loss of coolant accidents considered in the design would not cause largescale fuel failure, so the Magnox cladding would retain the bulk of the radioactive material, assuming the reactor was rapidly shutdown (a SCRAM). As the coolant is already a gas, pressure buildup from coolant boiling is not a risk.

List of Magnox reactors in the UK

List of Magnox reactors exported from the UK

  • Latina, Italy, 1 unit 160 MWe, first grid connection 1963, shut down 1987 following Italian referendum on nuclear power
  • Tokai (or Tokaimura), Japan, 1 unit 166 MWe, first grid connection 1966, shut down 1998

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