Magnox

From Wikipedia, the free encyclopedia

You have new messages (last change).
Schematic diagram of a Magnox nuclear reactor showing gas flow. Note that the heat exchanger is outside the concrete radiation shielding. This represents an early Magnox design with a cylindrical, steel, pressure vessel.
Schematic diagram of a Magnox nuclear reactor showing gas flow. Note that the heat exchanger is outside the concrete radiation shielding. This represents an early Magnox design with a cylindrical, steel, pressure vessel.

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 accordingly, as a producer of plutonium for nuclear weapons. The name magnox comes from the material used to clad the fuel rods inside the reactor.

Contents

[edit] General description

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 reinforced 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 reinforced 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.

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.

[edit] Safety

The Magnox reactors have a considerable degree of inherent safety because of their sturdy design, low power density, and gas coolant. As such, they do not require or possess secondary containment features. Loss of coolant accidents—considered in the design—would not cause large-scale fuel failure as 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, explosive pressure buildup from boiling is not a risk, as happened in the catastrophic steam explosion at the Chernobyl accident.

In the older steel pressure vessel design, boilers and gas ducting are outside the concrete biological shield. Consequently this design emits a significant amount of direct gamma and neutron radiation, termed direct "shine", from the reactors. For example the most exposed members of the public living near Dungeness Magnox reactor in 2002 [1] received 0.56 mSv, over half the International Commission on Radiological Protection recommended maximum radiation dose limit for the public, from direct "shine" alone. The doses from the Oldbury and Wylfa reactors, which have concrete pressure vessels which encapsulate the complete gas circuit, are much lower.

[edit] Reactors in use

Sizewell A Magnox nuclear power station
Sizewell A Magnox nuclear power station

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 another 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.

The first Magnox power station, Calder Hall, was the world's first commercial nuclear power station. First connection to the grid was on 27 August 1956, and the plant was officially opened by Queen Elizabeth II on 17 October 1956 [2]. When the station closed on 31 March 2003, the first reactor had been in use for nearly 47 years [3].

However the first two stations (Calder Hall and Chapelcross) were originally owned by the UKAEA and primarily used in their early life to produce weapons-grade plutonium, with two fuel loads per year [4]. From 1964 they were mainly used on commercial fuel cycles, however it was not until April 1995 that the UK Government announced that all production of plutonium for weapons purposes had ceased [5].

The later and larger units were owned by CEGB and operated on commercial fuel cycles.

In operation it was found that there was significant oxidation of mild steel components by the high temperature carbon dioxide coolant, requiring a reduction in operating temperature and power output. For example the Latina reactor was derated in 1969 by 24%, from 210 MWe to 160 MWe, by the reduction of operating temperature from 390 to 360 centigrade.

As of 2007, just two Magnox power stations remain in operation, Oldbury will close in 2008 and Wylfa in 2010.

[edit] Magnox

Main article: magnox (alloy)

Magnox is also the name of an alloy—mainly of magnesium with small amounts of aluminium and other metals—used in cladding unenriched uranium metal fuel with a non-oxidising covering to contain fission products. Magnox is short for Magnesium non-oxidising. This material has the advantage of a low neutron capture cross-section, but has two major disadvantages:

  • It limits the maximum temperature, and hence the thermal efficiency, of the plant.
  • It reacts with water, preventing long-term storage of spent fuel under water.

Magnox fuel incorporated cooling fins to provide maximum heat transfer despite 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 fuel a short time after removal from the reactor meant that the fission product hazard was severe. Expensive remote handling facilities were required to address this danger.

The term magnox may also loosely refer to:

  • Three North Korean reactors, all based on the declassified blueprints of the Calder Hall Magnox reactors:
  • 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 except that the fuel cladding was magnesium-zirconium alloy.

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.

[edit] Decommissioning

The Nuclear Decommissioning Authority (NDA) is responsible for the decommissioning of the UK Magnox power plants, at an estimated cost of £12.6 billion. There is currently debate about whether a 25 or 100 year decommissioning strategy should be adopted. After 80 years short-lifetime radioactive material in the defueled core would have decayed to the point that human access to the reactor structure would be possible, easing dismantling work. A shorter decommissioning strategy would require a fully robotic core dismantling technique.[6]

In addition the BNFL Sellafield site which, amongst other activities, reprocessed spent Magnox fuel in its B205 plant, has an estimated decommissioning cost of £31.5 billion. Magnox fuel is produced at Springfields near Preston; estimated decommissioning cost is £371 million. The total cost of decommissioning Magnox activities is likely to exceed £20 billion, averaging about £2 billion per productive reactor site.

Calder Hall was opened in 1956 as the world’s first commercial nuclear power station, and is a significant part of the UK’s industrial heritage. The NDA is considering whether to preserve Calder Hall Reactor 1 as a museum site.

[edit] List of Magnox reactors in the UK

[edit] Magnox reactors exported from the UK

[edit] See also

v • d • e
Nuclear technology
Nuclear engineering Nuclear physics • Nuclear fission • Nuclear fusion • Radiation • Ionizing radiation • Atomic nucleus • Nuclear reactor • Nuclear safety • Nuclear chemistry
Nuclear material Nuclear fuel • Fertile material • Thorium • Uranium • Enriched uranium • Depleted uranium • Plutonium
Nuclear power Nuclear power plant • Radioactive waste • Fusion power • Future energy development • Inertial fusion power plant • Pressurized water reactor • Boiling water reactor • Generation IV reactor • Fast breeder reactor • Fast neutron reactor • Magnox reactor • Advanced gas-cooled reactor • Gas-cooled fast reactor • Molten salt reactor • Liquid-metal-cooled reactor • Lead-cooled fast reactor • Sodium-cooled fast reactor • Supercritical water reactor • Very high temperature reactor • Pebble bed reactor • Integral Fast Reactor • Nuclear propulsion • Nuclear thermal rocket • Radioisotope thermoelectric generator
Nuclear medicine PET • Radiation therapy • Tomotherapy • Proton therapy • Brachytherapy
Nuclear weapons History of nuclear weapons • Nuclear warfare • Nuclear arms race • Nuclear weapon design • Effects of nuclear explosions • Nuclear testing • Nuclear delivery • Nuclear proliferation • List of states with nuclear weapons • List of nuclear tests

[edit] External links

Retrieved from "http://en.wikipedia.org../../../m/a/g/Magnox.html"