Editing Magnox (section)

===Magnox===
[[File:Calder Hall nuclear power station (11823864155).jpg|thumb|Calder Hall, United Kingdom –  The world's first commercial nuclear power station.<ref>{{cite news|title=Osborne hails UK nuclear deal with China as 'new dawn'|url=http://www.ft.com/cms/s/0/9fffdb6a-367c-11e3-aaf1-00144feab7de.html#axzz3H9SBizea |archive-url=https://ghostarchive.org/archive/a1Yct |archive-date=10 December 2022 |url-access=subscription|access-date=25 October 2014|publisher=FT|date=17 October 2013|quote=the country that built the first civil nuclear power station}}</ref> First connected to the national power grid on 27 August 1956 and officially opened by Queen Elizabeth II on 17 October 1956.]]
As the UK nuclear establishment began to turn its attention to [[nuclear power]], the need for more plutonium for weapons development remained acute. This led to an effort to adapt the basic Windscale design to a power-producing version that would also produce plutonium. In order to be economically useful the plant would have to run at much higher power levels, and in order to efficiently convert that power to electricity, it would have to run at higher temperatures.

At these power levels, the fire risk is amplified and air cooling is no longer appropriate. In the case of the magnox design, this led to the use of [[carbon dioxide]] (CO<sub>2</sub>) as the coolant. There is no facility in the reactor to adjust the gas flow through the individual channels whilst at power, but gas flow was adjusted by using flow gags attached to the support strut which located into the [[diagrid]]. These gags were used to increase flow in the centre of the core and to reduce it at the periphery. Principal control over the reaction rate was provided by a number (48 at Chapelcross and Calder Hall) of [[boron]]-steel control rods which could be raised and lowered as required in vertical channels.

At higher temperatures, aluminium is no longer structurally sound, which led to the development of the [[Magnox (alloy)|magnox alloy]] fuel cladding. Unfortunately, magnox is increasingly reactive with increasing temperature, and the use of this material limited the operational gas temperatures to {{convert|360|C}}, much lower than desirable for efficient steam generation. This limit also meant that the reactors had to be very large in order to generate any given power level, which was further amplified by the use of gas for cooling, as the low [[thermal capacity]] of the fluid required very high flow rates.

The magnox fuel elements consisted of refined uranium enclosed in a loose-fitting magnox shell and then pressurized with [[helium]]. The outside of the shell was typically finned in order to improve heat exchange with the CO<sub>2</sub>. Magnox alloy is reactive with water, which means it cannot be left in a cooling pond after extraction from the reactor for extended periods. In contrast to the Windscale layout, the magnox design used vertical fuel channels. This required the fuel shells to lock together end-to-end, or to sit one on top the other to allow them to be pulled out of the channels from the top.

Like the Windscale designs, the later magnox reactors allowed access to the fuel channels and could be [[Online refuelling|refuelled while operating]]. This was a key criterion for the design because its use of natural uranium leads to low [[burnup]] ratios and the requirement for frequent refuelling. For power use, the fuel canisters were left in the reactor as long as possible, while for plutonium production they were removed earlier. The complicated refuelling equipment proved to be less reliable than the reactor systems, and perhaps not advantageous overall.<ref name=hawley-2006>{{Cite conference |url=http://www.world-nuclear.org/sym/2006/restore/haw-rest.htm |title=Nuclear Power in the UK – Past, Present & Future |author=Robert Hawley |conference=[[World Nuclear Association]] Annual Symosium |year=2006 |archive-url=https://web.archive.org/web/20081214183208/http://www.world-nuclear.org/sym/2006/restore/haw-rest.htm |archive-date=14 December 2008}}</ref>

The entire reactor assembly was placed in a large pressure vessel. Due to the size of the pile, only the reactor core itself was placed within the steel pressure assembly, which was then surrounded by a concrete confinement building (or ''biological shield''). As there was no water in the core, and thus no possibility of a steam explosion, the building was able to tightly wrap the pressure vessel, which helped reduce construction costs. In order to keep the size of the confinement building down, the early magnox designs placed the [[heat exchanger]] for the CO<sub>2</sub> gas outside the dome, connected through piping. Although there were strengths with this approach in that maintenance and access was generally more straightforward, the major weakness was the radiation 'shine' emitted particularly from the unshielded top duct.

The magnox design was an evolution and never truly finalised, and later units differ considerably from earlier ones. As neutron fluxes increased in order to improve power densities problems with [[neutron embrittlement]] were encountered, particularly at low temperatures. Later units at [[Oldbury nuclear power station|Oldbury]] and [[Wylfa]] replaced the steel pressure vessels with [[prestressed concrete]] versions which also contained the heat exchangers and steam plant. Working pressure varies from 6.9 to 19.35{{nbsp}}[[Bar (unit)|bar]] for the steel vessels, and 24.8 and 27{{nbsp}}bar for the two concrete designs.<ref name=hse-msr-2000>{{cite report|page=27 (Table 3)|url=http://www.hse.gov.uk/nuclear/magnox.pdf|title=Report by HM Nuclear Installations Inspectorate on the results of Magnox Long Term Safety Reviews (LTSRs) and Periodic Safety Reviews (PSRs)|date=September 2000|publisher=[[Health and Safety Executive]]|author=Nuclear Installations Inspectorate|access-date=21 March 2010 |archive-url=https://web.archive.org/web/20060526203659/https://www.hse.gov.uk/nuclear/magnox.pdf |archive-date=26 May 2006}}</ref>

No British construction company at the time was large enough to build all the power stations, so various competing consortiums were involved, adding to the differences between the stations; for example, nearly every power station used a different design of magnox fuel element.<ref name=magnox-story>{{Cite report |url=http://www.nuclearsites.co.uk/resources/upload/Magnox%20Brochure2.pdf|title=The Magnox Story|date=July 2008|publisher=Springfields Fuels Limited |archive-url=https://web.archive.org/web/20110613090944/http://www.nuclearsites.co.uk/resources/upload/Magnox%20Brochure2.pdf |archive-date=13 June 2011|url-status=dead}}</ref> Most of the magnox builds suffered time overruns and cost escalation.<ref name=rcs-2011/>

For the initial start up of the reactor neutron sources were located within the core to provide sufficient neutrons to initiate the nuclear reaction. Other aspects of the design included the use of flux shaping or flattening bars or controls rods to even out (to some extent) the neutron flux density across the core. If not used, the flux in the centre would be very high relative to the outer areas leading to excessive central temperatures and lower power output limited by the temperature of the central areas. Each fuel channel would have several elements stacked one upon another to form a ''stringer''. This required the presence of a latching mechanism to allow the stack to be withdrawn and handled. This caused some problems as the [[Nimonic]] springs used contained cobalt, which became irradiated giving high gamma level when removed from the reactor. Additionally, thermocouples were attached to some elements and needed to be removed on fuel discharge from the reactor.